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:

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.


2 thoughts on “Causality & The Vaccine Adverse Event Reporting System

  1. I don’t think it’s possible to determine causality in a particular case – only that a particular problem could plausibly have been caused by the vaccine.
    Researchers can find an association between a vaccine and a side effect. But in a particular case, they still wouldn’t know if a problem was caused by the vaccine.


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