“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.

(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)

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.