Dismissing Anecdotes – Are Scientists Jerks?


When sharing an article on social media that is critical of a particular “alternative” treatment, I often get responses from friends and family that appeal to anecdotes in order to argue that the treatment is effective. These may include stories they have heard from others or a story of their own personal experience with the treatment.

For example, if I point out that a thorough review of over 3,000 scientific studies shows that acupuncture is an elaborate placebo, someone may chime in and say “well, it worked for me” or “it worked for a friend of mine,” or “it has worked for many people. How can all of those people be wrong?”

In some cases, the objections are defensive in tone. After all, who are scientists to dismiss the personal experiences of others? Do scientists think these people are untrustworthy? Do they think that these people are lying? Do they think that these people are stupid? Are scientists just arrogant and pretentious jerks who dismiss these claims because they don’t want to be told they’re wrong?

These defensive objections are certainly understandable. The experience of feeling better after a treatment can be incredibly compelling – to the point that it feels completely obvious that the treatment was effective. Hence, it seems only natural to take offense when scientists refer to these anecdotes in a pejorative manner.

Why do scientists react this way?

To understand the reason for this, it helps to first understand the logical flaws in the typical arguments such as “it worked for me.”

I admit, the formal logic and arguments that I’m about to discuss may seem dry and unpersuasive to the reader who is convinced they benefited from a treatment. After all, why should one be persuaded by such dispassionate arguments to change a belief that emotionally feels very real and true? To these individuals, I would ask the following questions:

  • Do you want to believe things that are most likely to be true?
  • If you currently hold a belief that isn’t true, would you want to discover that?

If your answers to either of these questions is “no”, then stop reading right now.

Logically Fallacious

There are two problems with the logic used in the above arguments. First, consider the argument “how could so many people be wrong?” This argument appeals to the popularity of a belief, arguing that if a large number of individuals hold the belief, then that belief is more likely to be true. In fact, this is untrue. The popularity of a belief is not a justifiable reason to conclude that the belief is true. This is a logical fallacy known as the appeal to popularity. There are countless examples in history where a large number of people believed something that turned out to be untrue.

More importantly, the bigger problem has to do with the logical fallacy known as post hoc ergo propter hoc, which I discussed in a previous post. In short, the anecdotes claiming that alternative treatments are effective are largely based on the fact that a positive outcome was observed following the administration of the treatment. As such, the fallacy here is the conclusion that the positive outcome must have been caused by the treatment because it followed the treatment.

Apart from logical fallacies, there are a number of reasons why scientists consider anecdotes to be unreliable.

Regression to the Mean

Regression to the mean is the scientific way of saying that when things move toward one extreme or another, that over time, they tend to trend back toward the average. For example, imagine you are sick with a cold but you feel better again after 10 days without taking any medication. You moved toward a state of being sick, and then trended back toward the average of feeling normal. Consider an alternate scenario: you are sick for the same amount of time, but you decide to take some cold medicine on the 9th day and you feel better the next day. Did the cold medicine cause you to feel better, or would you have gotten better anyway? There is simply no way to know based on this evidence alone. Therefore, this anecdote is not a reliable way to believe what is true.


To the person making the claim that an alternative treatment helped them, it may seem like a cop-out to say that it was a coincidence. However, a basic consideration of statistics and probability tells us that with large numbers of random events, some coincidences are expected.

Humans are generally terrible at comprehending large numbers and intuitively understanding randomness. For example, think of a random sequence of ten digits between 1 and 10. Chances are that the sequence you thought of is not truly random. Experiments of this sort have shown that we tend to underestimate how often two or more identical numbers will follow each other in a truly random sequence such as this one.

A good example is the case of vaccines and autism. Given the many millions of vaccine doses administered and the high frequency of autism, statistics tells us that we would expect a pretty significant number of children to begin showing signs of autism around the time a vaccine is given. It would be incredibly odd if there were no coincidences. Based on this information alone, there is no reliable way to determine whether or not there is a causal connection. This is why scientists are not persuaded by the anecdotes of parents who claim vaccines caused their kids’ autism. Unfortunately, many parents lack an understanding of statistics and they come to the erroneous conclusion that scientists are heartless shills.

Selection Bias

Humans tend to focus on unusual, interesting events while ignoring mundane ones. This is why we don’t see headlines such as “Man drives to work without getting into a car accident” and “Local couple flies across the country without incident.” Thus, anecdotes tend to focus more on these unusual events and the result is that studies have shown that we tend to overestimate how common these events actually are.

Let’s say that 100,000 people have a particular type of terminal cancer and someone invents an untested alternative treatment for this cancer. All 100,000 people try the treatment, 4,000 people go into remission, and 96,000 pass away. Following these events, we now have 4,000 people providing testimonials that this alternative treatment cured their cancer. Should we accept these testimonials? Well, consider that diagnostic tests for cancer are not perfect, and some percentage of patients will be a “false positive,” i.e. diagnosed with cancer even though they don’t have cancer. In this example, if the false positive rate for this cancer was 4%, we would expect 4,000 people to not have cancer in the first place – the same number of people who are now providing testimonials.

The lesson here is not that the claims of anecdotes are necessarily wrong, but simply that anecdotes alone are not a reliable way to determine whether or not a treatment is effective.

Confounding Factors

If someone takes multiple treatments at the same time, it is not possible to know which treatment (or combination thereof), if any, caused the positive outcome based solely on an anecdote.

For example, let’s say someone has a condition and they begin taking three different supplements at once. Then, their condition gets better and they become convinced that a particular pill caused the improvement. They then begin to tell their friends, so convinced that this one pill made them better that they omit the fact that they also took other supplements. Based on their anecdote, there is no way to be sure that other factors weren’t involved.

This phenomenon has been observed in some cancer patients who claimed that an alternative treatment cured them. Upon further investigation it was then revealed that the patient was taking other conventional treatments at the same time, any of which may have been the cause of the remission.

The Placebo Effect

Experiments have shown that believing or not believing in the effectiveness of a treatment can have real physiological effects in the body and that the more invasive a treatment is perceived to be, the stronger this effect. For example, placebo experiments have shown that two placebo pills are more effective than a single placebo pill, that placebo injections are more effective than placebo pills, and that placebo (sham) surgeries are more effective than injections or pills. This may seem completely counter intuitive, but it is a real, demonstrated effect that scientists are attempting to understand.

Therefore, when you feel like an alternative treatment worked for you or a friend, it may very well be due to the placebo effect, and not a product of the treatment itself. Hence, there is no way to know if the treatment itself actually worked based solely on an anecdote.


Understanding the limitations of anecdotal evidence is one of the greatest challenges that our society faces today. The only way to overcome this challenge is to think more critically by asking questions and considering alternative explanations. When faced with anecdotes, we need to critically evaluate their reliability in order to determine what is most likely to be true. In most cases, anecdotes alone are insufficient to determine what is true with any reliability, and in these cases we need to come to the sobering conclusion of “I don’t know.”

When a scientist dismisses your anecdote, it’s not because they think you are stupid, untrustworthy, or uneducated. It is because they are astutely aware of the limitations of anecdotal evidence, and they know that more rigorous, high quality studies that control for confounding factors and biases are required in order to verify a claim. They are simply doing what they have been trained to do, and it is likely not a personal judgment about you.

If you want to believe true things, be skeptical. The next time you’re tempted to come to a firm conclusion based on a personal experience or a friend’s anecdote, ask yourself how reliable that anecdote really is.

Mercury in Vaccines: Are We Really STILL Talking About This?


One basic principle of toxicology is that the dose makes the poison. This means that a substance has observable adverse effects in the body only once a certain dose is achieved. This not only applies to substances that are widely considered “harmful”, but also to substances that are generally considered “safe.”

For example, water is a molecule that is vital to the existence of life, yet it can cause serious problems if large quantities are consumed within a short period of time. While it is generally considered to be the least toxic chemical compound, it is possible to drink enough water such that the normal balance of electrolytes in the body is thrown off, which in rare cases can lead to death. Cases of water intoxication have been recorded in marathon runners as well as in participants of water drinking contests.

This may seem like an extreme example, because it is. However, there are plenty of other examples to illustrate this concept:

  • Snake venom can be toxic and lethal, however there is a dose below which there is no detectable toxic effect
  • Drugs such as acetaminophen and ibuprofen are usually benign when taken as directed but can cause serious harm in large quantities
  • Acetic acid can cause severe burns, but we regularly consume smaller, harmless doses in vinegar
  • Oxalic acid is naturally found in vegetables, including broccoli, garlic, beets, onions, carrots, celery, cucumbers, peas, tomatoes, and potatoes, yet in larger doses it causes a variety of problems
  • Nicotine, a drug found in cigarettes, is found in small quantities in plants such as tomatoes, eggplants and peppers
  • Vitamin A is an essential nutrient, yet large quantities of it can cause liver damage
  • Our bodies require magnesium, however large quantities can lead to diarrhea and even death
  • Consumption of alcohol in large quantities can lead to alcohol poisoning, while much smaller amounts cannot
  • Solanine is a compound found naturally in potatoes, apples, blueberries, bell peppers and tomatoes, yet in large quantities it can cause gastrointestinal symptoms, hallucinations, paralysis, and death

Anti-vaxxers claim that certain substances have no safe level of exposure, even though this violates basic toxicology. It is not enough to say that a substance is toxic. The question that matters is “at what dose are adverse events observed?”

Thankfully, scientific studies can answer this question. Animal studies can be performed to measure the lethal dose and toxicity of substances. Researchers can measure the LD50 (a measure of acute toxicity) which is also known as the “median lethal dose.” This is the dose of a given substance that is required to kill 50% of the test population. The lower the LD50, the higher the toxicity.

Another quantity is the “no-observed-adverse-effect-level,” or the NOAEL, which is the highest studied dose at which no adverse effects were observed in the test population. For example, if researchers were studying substance X and divided animals into four groups each receiving different doses, say 25, 50, 100 and 200 milligrams (mg), and adverse effects were observed only in the 200 mg group, the NOAEL for substance X would be 100 mg.

Scientists can also study large human populations to determine whether people exposed to a given substance experience more adverse effects compared to those that have not been exposed to it, or they can look for correlations that may be indicative of adverse events.

Chemistry is Important

Elemental (pure) sodium will explode when it comes into contact with water. Chlorine can be used to create a biological weapon and it can also be used to clean your swimming pool. Intuitively, we might be tempted to demonize any compound that contains one of these molecules, but basic chemistry shows that this fear is unfounded. Sodium and chlorine combine to form sodium chloride, otherwise known as table salt – a substance that does not produce a volatile explosion in water, does not serve as a chemical weapon or effective pool water cleaner, and has different effects on the body compared to pure sodium or chlorine.

Basic principles of chemistry tell us that it is erroneous to characterize a compound as dangerous based on the properties of its constituent molecules.

Thimerosal in Vaccines

In the late 19th and early 20th centuries, bacterial contamination in vaccines was a serious problem for health professionals. Vaccines that were delivered in multi-dose vials were sporadically prone to bacterial contamination resulting from multiple uses of the same vial. This contamination was less of a problem with single-dose vials, however the disadvantage of single-dose vials is that they are more costly to produce compared to multi dose vials.

In 1916, a tainted batch of typhoid vaccines caused 68 severe reactions and 4 deaths in South Carolina. In 1928, 12 children died after being administered a diphtheria vaccine that was contaminated.

Health professionals and researchers sought to fix this problem of bacterial contamination by adding a substance called a germicide to the vaccines. I won’t go into the finer details here, but the end result was that they discovered that ethylmercury (the principle ingredient of thimerosal) was a very effective germicide and that it could be administered in high doses (up to 20 mg per kg of body weight) without any adverse effects. Thus, beginning in the 1930’s, thimerosal was added to vaccines to prevent this bacterial contamination and save lives.

Why Fear Mercury?

Mercury is widely considered to be dangerous for humans – and with good reason. The Minamata Bay incident in Japan in the 1950’s is a good example of how mercury poisoning can be frightening and even deadly. The Chisso Corporation had a factory in the town of Minamata that produced acetaldehyde, a chemical used to make plastics. For many years, the company dumped its waste water into Minamata Bay. Eventually, there were stories of cats “dancing” and dying in the street, and strange symptoms soon appeared in humans – which in some cases proved to be fatal. Scientists eventually traced the cause of these symptoms to methylmercury poisoning. It turns out that significant quantities of methylmercury were being dumped into the bay by the Chisso Corporation. Unfortunately, at that time, seafood from the bay was the community’s primary source of protein.

Today, human exposure to methylmercury is achieved via consumption of fish and other aquatic species. Methylmercury is not efficiently excreted (eliminated) from the body and it therefore tends to accumulate up the food chain because it remains absorbed by tissues for a significant period of time.

Mercury and Autism?

Concern about thimerosal in vaccines began to take hold in the late 90’s, around the time Andrew Wakefield published his study raising concerns about the MMR vaccine. (Ironically, the MMR vaccine does not and has never contained thimerosal.) After many scientific studies showed that there was no link between autism and the MMR vaccine, anti-vaxxers then committed the informal fallacy of moving the goal posts by turning their attention to thimerosal.

In the anti-vaxxers’ eyes, thimerosal in vaccines was causing autism. This idea was championed by journalist David Kirby in his book Evidence of Harm, and by celebrities such as Robert F.Kennedy Jr. and Jim Carrey.


Problems with their Argument

Our intuition might tell us that it is reasonable to be fearful of thimerosal in vaccines. Is this true? No, here’s why.

First, we must go back to our discussion above which explained that chemicals with different structures behave differently. Readers with a keen eye (or those already familiar with this topic) will have noticed that in discussing mercury toxicity, I made reference to methylmercury, while I described thimerosal as containing ethylmercury. The difference is important and cannot be overstated. Somewhat similar to my example of sodium and chlorine behaving differently from sodium chloride, it is true that methylmercury and ethylmercury behave quite differently. Methylmercury, found in seafood, is absorbed by the body’s tissues for quite some time, while ethylmercury is efficiently eliminated from the body over a short period of time. Therefore, it is wrong to speak of methylmercury (found in seafood) and assume that its effects will be the same as that of ethylmercury (found in vaccines). Anyone who does this is demonstrating a lack of knowledge of basic chemistry. (This is also similar to methanol vs. ethanol: methanol is unfit for consumption, while ethanol in wine is enjoyed safely by many.)

Second, consider the other basic principle I covered earlier: the dose makes the poison. Anti-vaxxers may argue that the differences between methyl and ethyl mercury don’t matter, and that exposure to any dose of mercury is toxic. Of course, we know this is not true as this argument violates a basic principle of toxicology.

But what if ethylmercury is toxic at the dose given in vaccines? To support this assertion, someone once sent me a study showing that thimerosal administered to mice can cause autoimmune problems. Now that we know that the dose makes the poison, a perfectly reasonable and rational question is: what dose of thimerosal was administered to the mice? How many vaccines must a human receive to achieve a dose equivalent to that given in the study?

The lowest dose given to the mice that produced adverse effects was 147 micrograms per kg of body weight, per day. Doing some math, we can determine that to get the equivalent dose in a 150 lb human, we would need to administer approximately 10 mg of thimerosal. The vaccine with the highest dose of thimerosal is currently the flu shot, which comes in at 0.025 mg of thimerosal per vaccine dose. A 150 lb human would need to receive 400 flu shots to achieve this dose.

Further, we must account for the fact that ethylmercury is excreted by the body. Therefore, the span of time over which the vaccines are administered matters as well. Given that this dose of thimerosal was administered to mice over 70 days, the 400 vaccines for a 150 lb human would also have to be administered within a span of 70 days. In reality, childhood vaccines are spread out over many years. For example, the publicly funded vaccine schedule in the province of Ontario recommends 18 vaccines staggered over several years of childhood.

The two paragraphs above don’t even take into account two important points: many vaccines don’t contain thimerosal, and observations made in animals are not directly translatable to humans. These points aside, using this mouse study to argue that thimerosal is toxic is akin to arguing that a cup of water is toxic because drinking 24 cups of water at once can kill you. (The LD50 for water is roughly 6 litres)

What About Humans?

If thimerosal was somehow connected to the rise in incidence of autism, then we would expect autism rates to go down if the dose of thimerosal given to children were reduced. This occurred in 2001 in the United States when the CDC removed thimerosal from the childhood vaccination schedule. Proponents of the thimerosal-autism connection, such as David Kirby, predicted that the incidence of autism would plummet. They were wrong, as autism rates continued to rise even after the removal of thimerosal. California passed a law in 2006 removing thimerosal from all of their vaccines, yet the state’s autism rates continued to rise.

Epidemiological studies in humans have extensively studied the alleged connection between thimerosal and autism. A review of the scientific evidence published in 2010 concluded:

[…] studies have consistently failed to identify a cause-effect relationship between thimerosal and autism. In addition, the prevalence of autism has increased despite a decrease in the thimerosal content of vaccines; […] Despite failure to demonstrate an association, certain states continue to mandate that vaccines given to children contain no more than trace amounts of thimerosal. Epidemiologic studies continue to provide evidence that there is no association between thimerosal exposure and autism.

Of course, these findings should not come as a surprise given all of the science that has been done to investigate the toxicity of thimerosal.


Despite the wealth of epidemiological studies and thimerosal’s removal from most vaccines having no effect on autism rates, anti vaccine proponents and celebrities continue to stoke a fire that should have been extinguished over 10 years ago.

The case of thimerosal is a good example of why basic tenets and evidence from epidemiology, toxicology and chemistry need to be effectively communicated to the public.

“Whoop” There It Is – An Overview of Pertussis Vaccines

Pertussis (whooping cough) is a contagious respiratory disease characterized by uncontrollable, violent coughing which can make it difficult to breathe. The name “whooping cough” comes from the many deep breaths that are needed in order to regain the air that was missed after a fit of coughing. The disease can be fatal and infants less than 1 year of age are most vulnerable.

The childhood vaccine for pertussis was known as DTP or DTwP (diphtheria, tetanus and pertussis). The ‘w’ stands for ‘whole cell.’ The whole cell pertussis vaccine was introduced in the US in the late 1940’s and significantly decreased the incidence of pertussis.

pertussis rates

(Note: Tdap is a version of the DTaP vaccine that contains reduced doses of the diphteria and pertussis vaccines. It is meant as a booster shot for individuals 11 and up, after immunity from the DTaP vaccine given around 4 to 6 years of age has waned.)

Despite its effectiveness, there was serious concern over adverse reactions, which were believed to include febrile seizures and infantile spasms. A 1982 review discussed these in detail. Febrile seizures were found to be increased in vaccinated individuals and while the symptoms are extremely alarming to parents, they are ultimately benign. It is unclear whether the other reported adverse events were due to the vaccine:

“The problems attached to exploration of the epidemiology of alleged serious neurologic complications of pertussis immunization are magnified by the extreme rarity of all of the above conditions and the fact that no specific post-pertussis vaccine neurologic syndrome has been identified. […] Pertussis vaccine is normally given early in the first year of life at the very stage when a wide range of unexplained neurologic degenerative diseases tend to present. Some of these diseases present with neurologic symptoms before other diagnostic clues (e.g., tuberous sclerosis, tumors, leukodystrophies, ataxia telangiectasia, Leigh’s and Kawasaki’s diseases). Thus, it is necessary to follow up children with claimed postvaccination syndromes for many years before alternative diagnoses can be eliminated.”

The decrease in pertussis incidence and the resulting lack of observable cases made many groups begin to question the use of the DTwP vaccine, especially given the concern over the adverse events. Several countries stopped administering the vaccine and as a result saw a resurgence of pertussis within five years. For example, Sweden ceased its pertussis immunizations in 1979 and incidence of the disease slowly increased. In 1981 there were 700 cases per 100,000 people and by 1985 that number had increased to 3,200 per 100,000.

These events led researchers to attempt development of an acellular pertussis vaccine (DTaP) which was just as effective but without the adverse events associated with the whole cell vaccine. Researchers were successful in one aspect; the DTaP vaccine was associated with fewer adverse events. However, the DTaP vaccine was found to be not as effective as the DTwP vaccine.  In addition, the immunity provided by this new DTaP vaccine wanes over time. For this reason, booster shots of Tdap vaccine are recommended. This vaccine is similar to DTaP but contains lower doses of the diphtheria and pertussis vaccines. Despite its lower effectiveness, there is still irrefutable evidence that the DTaP vaccine greatly reduces the risk of pertussis symptoms.

Tdap immunization is recommended during pregnancy. A randomized clinical trial found no evidence of adverse events in mothers that were vaccinated and also provided newborns with a higher concentration of pertussis antibodies during the first 2 months of life, when the infant would otherwise be vulnerable.

Outbreaks of whooping cough have been cropping up recently in Alberta, California, Michigan, and the list goes on. The rise in outbreaks is attributed to a decline in vaccination rates. For example, in an outbreak in Huron County, Ontario, most cases were found in unvaccinated individuals in a Mennonite community. The city of Nelson, British Columbia saw an outbreak of pertussis, where one third of parents refuse to vaccinate their children – the lowest vaccine compliance in the province. In Kanata, Ontario, a family of seven children all contracted the disease. Ironically, their parents had just changed their stance on vaccines and had already made doctor’s appointments for them to get vaccinated, but it was too late.

The outbreaks of pertussis pose a real danger to infants too young to be vaccinated. The Facebook page Light for Riley was set up by parents of an infant named Riley who contracted whooping cough and passed away as a result of the disease.

A recent study hints at a possible explanation for the resurgence of pertussis that we are seeing. The study examined the transmission of pertussis in baboons who were immunized with the TDaP vaccine. They found that while vaccinated baboons did not exhibit any of the symptoms of pertussis, they were still able to contract the disease and pass it on to others. (For an in depth discussion of the study, check out Tara Haelle’s summary here.)

Pertussis incidence and # of cases in Canada. (Source: Canada Communicable Diseases Report)

Pertussis incidence and # of cases in Canada. (Source: Canada Communicable Diseases Report)

The study provides a possible explanation for the resurgence of pertussis. It could be that unvaccinated individuals could catch the disease from a vaccinated person even though the latter never experienced any symptoms of the disease. (It’s important to point out that this transmission would be occurring even without any vaccines. Some have argued that the vaccine is “causing” these outbreaks in pertussis, but based on the way the TDaP vaccine is made, this is impossible.)

While the study provides a potential explanation for the outbreaks, it is still too early to be sure that this is happening because an effect observed in baboons may not necessarily be observed in humans. Until a study in humans is done, we won’t know for sure. Still, if true, it will be more important for people to get vaccinated because it will no longer be possible to rely on herd immunity. The most effective way to safeguard against the potentially lethal symptoms of pertussis is to get vaccinated. Given that young infants cannot be vaccinated, it is even more important for pregnant women to get vaccinated so that they can impart some protection to their children during this vulnerable stage of their infancy.

It should come as no surprise that the pertussis vaccines have been a target of the anti-vaccine campaigners. One of the logical fallacies that is pervasive among anti-vaxxers is the nirvana fallacy (aka the “Perfect Solution” fallacy), which says that if an action is not 100% safe and/or effective then it is better to take no action at all. The problem here is that any examination of the benefits of said action are not even considered. This is essentially what happened with the whole cell pertussis vaccine; all of the adverse events of the vaccine were trumpeted by the anti-vaccine community while any consideration of the benefits was absent.

Failure to discuss the benefit vs. risks of replacing the acellular pertussis vaccine with the whole cell vaccine was not without consequences. As Seth Mnookin points out:

“Because of the near-impossibility of having an honest discourse about vaccine side effects, there were few conversations about whether the advantages of the acellular pertussis vaccine outweighed its disadvantages — or even what those disadvantages were.

It’s looking increasingly like we’re in the midst of learning the consequences of failing to have those tough conversations two decades ago. For the past several years, the United States has had a series of unusually robust pertussis outbreaks. (Typically, outbreaks go in multi-year cycles, with peaks and troughs. That hasn’t been happening as of late: There were 27,550 cases in 2010, and there have already been 26,146 so far this year.) One theory has been that the acellular vaccine doesn’t confer as lengthy immunity as the whole-cell vaccine did — and a new study published in The New England Journal of Medicine (abstract, PDF) provides strong evidence that that is, indeed, the case. “[O]ur evaluation of data from a large pertussis outbreak in California [in 2010],” the authors write, “showed that protection from disease after a fifth dose of DTaP among children who had received only DTaP vaccines was relatively short-lived and waned substantially each year. Our findings highlight the need to develop new pertussis-containing vaccines that will provide long-lasting immunity.”

There are those who would point out that we actually know of a pertussis-containing vaccine that provides long-lasting immunity — but the chances of returning to the whole-cell DPT vaccine are next to nil.”

The end result has been the widespread adoption of an acellular pertussis vaccine that, while effective, provides more rapidly waning immunity, and may possibly be ineffective at preventing transmission of the disease to unvaccinated individuals. Ironically, the nirvana fallacy then rears its ugly head once again among anti-vaxxers, who are now citing this lower efficacy as a reason to completely forego vaccination.

The story of the pertussis vaccines underscores the need for more widespread teaching of critical thinking skills to the general public.

And remember: vaccination for pertussis still protects against the potentially lethal symptoms of pertussis.

Causality & The Vaccine Adverse Event Reporting System

I spend more time than I care to admit having online debates with people. Recently, I was having a discussion with a mother about the Vaccine Adverse Event Reporting System (VAERS). She was of the opinion that vaccines cause a wide range of things such as autism and death. She argued that all of the reports of adverse events in VAERS were a cause for concern and she seemed to be under the impression that VAERS is a reliable way to establish a causal connection between the vaccine and the reported adverse event. Her logic was that since the CDC (and the FDA) manages VAERS, they are vetting the submitted reports based on quality and hence the reports in its database are reliable. In particular, she was concerned about the Gardasil HPV vaccine and wanted to delay her daughter’s vaccination.

To determine whether VAERS is reliable for determining causality, let’s look at what it is. Why does VAERS exist? In 1986, the United States passed the National Childhood Vaccine Injury Act, which requires health professionals to report any adverse event that may be suspected of being caused by an administered vaccine. In response to this bill, the CDC and the FDA established VAERS in 1990. This is a good thing and certainly favorable compared to having no system in place to report potential adverse events.

VAERS is a passive reporting system. This means that anyone can go online and submit a report, be it doctors, nurses, patients or parents. In fact, when I say anyone, I mean literally anyone. This page of data from the VAERS database shows deaths occurring after administration of the Gardasil HPV vaccine. A quick perusal of the data shows that some of the reports are not being made by people who have witnessed the adverse event, but rather by people who have read about it on the internet:

A consumer reported that he/she read an [sic] internet concerning a female who on an unspecified date was vaccinated with a dose of GARDASIL (lot # , dose and route not reported). On an unspecified date, the patient died of “clot blood” eight hours after vaccination. This is one of several reports from the same source. No further information is available.

There are other sad stories within the data, including a girl dying on a lacrosse field from cardiac failure and a son who committed suicide by hanging himself.

These reports are quite grave and it’s understandable that if someone believes these are legitimate reports investigated by the CDC for quality and causality, that person would certainly be inclined to be concerned about the Gardasil vaccine. Therefore, it’s important to determine whether the CDC is actually determining causality from these reports.

The VAERS database is fully accessible to the public. It turns out the answer to our question is not difficult to find, as there is a full page disclaimer about the reliability of the VAERS database that the user must acknowledge before being given access to the data. I encourage you to read the entire page, but in case you don’t, here are some excerpts:

When evaluating data from VAERS, it is important to note that for any reported event, no cause-and-effect relationship has been established. […] VAERS collects data on any adverse event following vaccination, be it coincidental or truly caused by a vaccine. The report of an adverse event to VAERS is not documentation that a vaccine caused the event. […] VAERS reports can be submitted voluntarily by anyone, including healthcare providers, patients, or family members. Reports vary in quality and completeness. They often lack details and sometimes can have information that contains errors. […] A report to VAERS generally does not prove that the identified vaccine(s) caused the adverse event described.  It only confirms that the reported event occurred sometime after vaccine was given. No proof that the event was caused by the vaccine is required in order for VAERS to accept the report. VAERS accepts all reports without judging whether the event was caused by the vaccine.


Anti-vaccine advocates often cite VAERS data as evidence that vaccines cause autism. The amount of cognitive dissonance that is required in order for them to click this button must be astronomical.

I suspect the person with whom I was debating never tried to access the data from the VAERS website. If she did, she must have not read or ignored this disclaimer.

Whenever VAERS evidence is cited to “prove” some sort of adverse event, that person is falling prey to the logical fallacy known as post hoc ergo propter hoc. (I covered this in a previous post.) In short, an adverse event occurring after vaccination does not prove that the vaccine caused the event. This fallacy seems lost on many anti-vaccine advocates, as they cite VAERS as a reliable way to determine that vaccines are dangerous.

From Sherri Tenpenny’s “Tenpenny on Vaccines” Facebook page:

>20,000 VAERS adverse event reports have been filed about Gardasil. There were 129 new reports just since 10/1, including 2 deaths. Vaccines are what need to be feared, NOT the “fear” of a temporary illness. You will never question your decision to NOT vaccinate again if you spend some time on this website: www.VAERS.hhs.gov

I could list thousands of other instances of anti-vaxxers invoking the VAERS database as reliable evidence, but it would take an inordinate amount of time and I think you get the idea. In short, the data in VAERS is not reliable for determining causality. The fact that some individuals continue to view it as reliable is a result of confirmation bias combined with post hoc ergo propter hoc.

It should be noted that another argument used by anti-vaccine advocates to make the VAERS data appear more alarming is to point out that VAERS suffers from under-reporting. This is true with any passive reporting system. For example, many adverse events such as swelling at the injection site are likely to not be reported. However, this argument is a non-sequitur, i.e. an argument in which its conclusion does not follow from its premises. The fact that some adverse events are not reported is completely irrelevant when it comes to determining causality. If all of the events were reported, we would simply be left with a larger sample of events from which we cannot reliably determine causality.

So why bother having VAERS when it is not a reliable way to determine causality? The answer is that VAERS is a kind of early warning system. If there are many reports of an adverse event, then researchers can conduct higher quality studies in which confounding factors are controlled for in order to determine whether the event is actually being caused by the vaccine. This makes sense – remember that to avoid falling prey to the post hoc ergo propter hoc fallacy, higher quality evidence must be considered.

In this regard, VAERS has been successful in the past. For example, in 1999, there were many reports in VAERS of intussusception occurring after administration of the rotavirus vaccine RotaShield. Further studies were conducted which confirmed the increased risk and this data led to the subsequent removal of the vaccine from the US market. In another instance, VAERS determined that there was a high incidence of Guillain-Barré syndrome occurring as a result of the meningococcal conjugate vaccine, Menactra, and further controlled studies are underway to investigate the issue.

Finally, the inconsistent logic of anti-vaccine advocates must be highlighted. These individuals often dismiss any information about vaccine safety provided by the CDC because they believe the CDC to have ties to pharmaceutical companies. (The Shill gambit.) Yet they don’t hesitate to cite reports from VAERS as reliable evidence, despite the fact that VAERS is sponsored by the CDC. Basically, they ignore the CDC information that goes against their beliefs and deem CDC information that supports their beliefs to be irrefutable. Once again, confirmation bias reigns supreme.

Don't be fooled by memes such as this. VAERS data cannot reliably tell us this. (Also, change the zero on the left to a one, since an immune-compromised woman recently died from pneumonia caused by the measles (autopsy confirmed.))

Don’t be fooled by memes such as this. VAERS data cannot reliably tell us this. (Also, change the zero on the left to a one, since an immune-compromised woman recently died from pneumonia caused by the measles – this was confirmed by the autopsy.)

So whenever you hear a passionate argument from an anti-vaccine advocate who cites VAERS as evidence that a vaccine caused an adverse event, remember the limitations of the VAERS data and recognize that we need higher quality evidence in order to come to this conclusion.

Experience with Dr. Sherri Tenpenny on Facebook

Recently, I was perusing some anti-vaccination pages on Facebook. I came across the page of Dr. Sherri Tenpenny, quite popular in the anti-vaccine movement. She holds a Bachelor of Arts and a specialization in osteopathic medicine.

Tenpenny posted a link to an article by Sayer Ji of greenmedinfo. The headline proclaims “Study Calls Into Question Primary Justification for Vaccines.” Tenpenny shared the article, saying “Did you know that the basic premise of vaccine effectiveness has been called into question?” The article states:

newly published research has revealed that in some cases no antibodies are required for immunity against some viruses.


This view, however, has been called into question by the new study:  “Although this concept may apply to other viral infections, our findings with VSV turn this view upside down, indicating that during a primary infection with this cytopathic virus, innate immunity can be sterilizing without adaptive immune contributions.”

Does this strike a mortal blow to the antibody theory which underlies vaccinology, and constitutes the primary justification for the CDC’s focus on using vaccines to “boost” immunity?

The remainder of the article goes on to make a case that scientists have gotten it wrong, that vaccines are largely ineffective at providing immunity to diseases and that the entire basis for how vaccines are supposed to work is utterly wrong.

I must confess, I don’t have a background in biology and therefore I was not sure whether the arguments made by Sayer Ji had any validity. Being a good skeptic, I decided to dig up the original study and read it myself to see whether the representations made by Ji’s article were accurate. Again, my lack of expertise in this area made it difficult to understand the contents of the study itself.

Having no other recourse, I decided to contact the corresponding author of the study. This felt like a long shot; based on my own past experience, authors are sometimes difficult to get in touch with. In some cases, the author has moved on to another institution and ceased using their email address in the process.

The corresponding author for this study is Ulrich H. von Andrian, M.D. and Professor of Immunopathology at Harvard Medical School. The message I sent was the following:


I am a blogger and I stumbled upon a recent anti-vaccine article on greenmedinfo here:


The article is saying that your study “B Cell Maintenance of Subcapsular Sinus Macrophages Protects against a Fatal Viral Infection Independent of Adaptive Immunity” calls into question the entire premise upon which vaccines are founded. I am highly skeptical of course, but I confess that I am a physicist and the content of the paper is outside of my knowledge.

I was wondering if you could offer a general comment regarding the article on greenmedinfo.


I was pleasantly surprised when I received a response within two hours of this correspondence:

Hi Adam — The online article you referenced below misrepresents our paper.  Our work in no way calls into question the utility of vaccines, which in my personal view are among the most impactful and cost-effective accomplishments of modern medicine.  Our study had examined the immune response to a viral infection of mice that were immunologically naive, i.e. they had never ‘seen’ the virus before.  The animals were able to survive the infection and clear the virus without requiring T or B cells. This per se is not a huge surprise; there are countless invertebrate species that don’t have T or B cells and survive infections every day.  Our observation merely suggests that at least in some settings mammals can do the same.
A key point is that although the mice in our study were able to deal with the infection, they did get infected (i.e. they got ‘sick’ so to speak).  The purpose of vaccines is to prevent infections from occurring in the first place.  Had we vaccinated our mice prior to using the virus, the infection would never have occurrred even if we exposed the animals to a lethal dose of the pathogen.
I hope this is helpful.

Best, Uli

Basically, Sayer Ji’s article misrepresented the original paper and was being used to scare well-meaning parents that happened to stumble upon the article.

Given this information, I decided that it was my duty to share this response with the followers of Tenpenny’s Facebook page. Surely, if an article is said by the authors to misrepresent their study, this must be reliable.

What followed was a very revealing indication regarding the unreliability of Sherri Tenpenny’s page. I posted the authors’ response to the article.


Within 24 hours, my reply was deleted and I was banned from commenting on Tenpenny’s Facebook page. This begs the question: how reliable is this page as a source of information when all opposing viewpoints, no matter how valid, are deleted?

More comical was that some people still defended the article as if they somehow knew better than the study’s authors. Others accused me of making up the story and showed their inconsistent logic in the process. How do they know that Sayer Ji’s article was truthful? This is an example of confirmation bias, in which an individual favors a source of information that agrees with their own belief and they discredit other sources that go against their belief. In this case, Sayer Ji’s article is being favored as an accurate depiction of the research study and my contacting the authors is being discredited. If a third party wanted to know the truth, both Ji’s and my accounts should be taken with a grain of salt, since it is possible to get the truth directly from the study’s authors.

This exercise serves to highlight the unreliability of the information peddled by Sherri Tenpenny. This is a prime example of why skepticism is so important. Without it, claims are taken at face value and beliefs are formed based on inaccurate information.

Anecdotes of “Vaccine-Injured Children” – What Can They Really Tell Us?

If you’re familiar with the information provided by anti-vaccine websites, then you have probably encountered some anecdotal (word of mouth) accounts of children getting injured by vaccines. The story usually goes something like this: a child receives one or more vaccines and then within a short time frame after vaccination, the child experiences an adverse event. Examples of some of these stories include sudden infant death syndromeautism, and others.

My heart goes out to the parents of these children. To some, this may come across as a disingenuous sentiment from an internet blogger. However, the truth is that a member of my immediate family has autism and I have witnessed firsthand, for over two decades, some of the challenges that parents may face following such a diagnosis. I have also experienced a variety of challenges as well.

These anecdotal stories can be incredibly alarming to the well-meaning parent that is trying to decide whether to vaccinate their own child, or to the parent trying to determine whether a vaccine was the cause of their child’s ailments. The end result is that many parents could come away with the impression that vaccines are unsafe and potentially life-threatening.


It is certainly understandable that parents might associate an adverse event with a vaccine if these two occur around the same time. What is crucially important, however, is how reliable this temporal (timing) correlation of events is in terms of informing us as to what is true. Can this evidence reliably tell us whether there is a causal relationship between the two events?

Let’s consider the example of the measles, mumps, rubella (MMR) vaccine and autism. Although there is a lot of other evidence on this topic, for now, I am only interested in looking at the temporal coincidence of events and determining what this specific evidence can tell us.

Every year, roughly 4 million children are born in the US. It is estimated that the current rate of autism spectrum disorder (ASD) is about 1 in 68. So every year there are over 58,000 children diagnosed with ASD, which amounts to an average of approximately 160 children diagnosed with ASD every single day.

On average, children are diagnosed with ASD around the age of 4, but signs of ASD can be noticed by parents as early as before the 1st birthday. The vaccination schedule in the US recommends a first dose of MMR at 1 year of age and a second dose at 4-6 years.

Armed with this information, imagine a 6-year calendar (or 6 normal calendars linked together) that represents the first 6 year span of children’s lives. Next, populate that calendar with dots representing the time at which children are diagnosed with ASD. As mentioned earlier, the average rate is 160 per day, so each day on the calendar needs on average 160 dots. Now, children aren’t very likely to be diagnosed in the first few months, so move those dots to a later time in the calendar. For simplicity, let’s assume that all the kids’ parents follow the recommended schedule and give the first dose of MMR on their first birthday and the second dose on their 4th birthday.


Snapshot of a 2 week span in the calendar. One dot represents one ASD diagnosis on the given day. Is it possible to determine which dots are coincidental and which are not?

With this calendar in mind (marked with dots representing ASD diagnoses and two vaccine dates marked), is it possible that there are dots that happen to fall on the vaccination days, or within one week of vaccine administration? Given the large number of dots, basic statistical probability would say that it is extremely likely that some dots will fall after vaccine administration. In other words, it would be extremely unlikely if there were NO dots following the vaccination days. Given this, it is very likely that we will have some coincidences in timing between some ASD diagnoses and vaccination. This doesn’t mean that those dots are absolutely coincidences, but the simple truth is that based solely on this evidence, there is simply no way to determine which dots are coincidental, if any, and which dots might be due to the vaccines, if any.

This may seem like a somewhat contrived example, but the truth is that one can set it up however they like and the main conclusion is unchanged. We could add or remove vaccines, increase or decrease the number of diagnoses, change the length of the calendar, change the vaccine type, change the adverse event from autism to death or something else. We could even change the distribution of dots so that more dots are clustered around one time compared to others. No matter what, there is just no reliable way, based solely on this evidence, to be certain as to whether the adverse event is causally associated with the vaccine. We cannot separate the coincidental events from the causally associated ones. In order to do this, we need more evidence. We also need more evidence in order to answer questions such as: How many children that received the MMR were diagnosed? How many children that did NOT receive the MMR were diagnosed? Is there a significant difference between these two groups?

Unfortunately, instead of being skeptical and asking themselves these kinds of questions, many parents will understandably fall victim to a logical fallacy when faced with these kinds of vaccine injury stories. They may also fail to recognize that any emotional attachment to their belief, while being completely normal, has no influence on the truth.

Post Hoc Ergo Propter Hoc

Concluding a causal relationship based on succession of events is known in philosophy as a logical fallacy called post hoc ergo propter hocor post hoc fallacy for short. It is a logical fallacy that states “Since event Y followed event X, event Y must have been caused by event X.” For more on this, Paul Henne, a philosophy graduate student at Duke University, has a great video with further explanation.

Inconsistent Logic

Consider two cases:

1) You read a blog post about a child that received a vaccine and subsequently developed autism
2) You read a blog post about a child that received a vaccine and did not subsequently develop autism

If case 1 is sufficient evidence to assume causation, then case 2 must be sufficient as well. Clearly, this logic is inconsistent because it leads us to contradictory conclusions when both cases are considered.


Sadly, many well-meaning parents in the anti-vaccine movement fall prey to this logical fallacy. So from now on, if you hear about someone referring to their “vaccine-injured” child, remember that they are likely coming to this conclusion based on a succession of events and that in order to determine what is really true, we need to be skeptical and consider higher quality evidence.