Miracle workers that they are, no vaccine is perfect. Aside from the occasional side-effects and uncomfortable delivery methods (even those nasal sprays sting a bit), vaccines don’t always prevent the disease they’re intended to protect against.
Often, like with the influenza virus, this happens because the germ in question changes its appearance enough to sneak past the vaccine’s training of our immune system. Other times, the vaccine’s protective effect slowly loses its punch as we become older. In either case, these known shortcomings require us to routinely go back to the doctor’s office for a booster shot.
The latter reason has been largely blamed for the resurgence of the infectious disease pertussis, also known as whooping cough. Pertussis causes its sufferers to have frightening coughing fits, among other symptoms, and can be rarely fatal, especially in the very young. Though once considered a relic of the past, rates of pertussis have been steadily climbing throughout the 1990’s, with states like California experiencing their largest outbreaks of the bacterial malady in more than 60 years (anti-vaccination sentiment, though present, is seen as a distant second cause for its return). Last year alone, according to the Centers for Disease Control (CDC), the United States saw close to 30,000 cases of pertussis, which, while a drop from the 48,000 seen in 2012, is still more than triple the average amount of cases reported in past decades.
Public health experts have traced the rise in cases to the replacement of the whole-cell pertussis vaccine (DTwP) by an acellular version (DTaP) in the early 1990’s — one instead composed of small fragments of the bacteria (both versions are packaged with a tetanus and diphtheria vaccine, hence the acronym). Though the latter carries less side-effects, the tradeoff has seemingly been less effectiveness. According to a new study in BMC Medicinethough, the vaccine may not only lose its potency quicker than the original, it could be turning the vaccinated into healthy but infectious carriers of pertussis.
Noting that an earlier 2014 study of vaccinated baboons found a carrier effect, the authors created an epidemiological model of human pertussis transmission. Mapping over the reported data of pertussis cases by the CDC, the authors came to the conclusion that the vaccine’s waning effect wouldn’t entirely explain the increase of pertussis. There had to be transmission occurring from vaccinated individuals to the unvaccinated or too lightly protected for the current number of pertussis cases to make sense.
“There could be millions of people out there with just a minor cough or no cough spreading this potentially fatal disease without knowing it,” said study author Ben Althouse, a Omidyar fellow at the Santa Fe Institute, in a press statement. “The public health community should act now to better assess the true burden of pertussis infection.”
Their analysis, disturbing in its implications as it is, isn’t a reason to stop vaccinating against pertussis, the authors say. If anything, it should only prompt people to be more vigilant about obtaining their shots. “It’s the symptoms of pertussis infection that kill people,” co-author Sam Scarpino says, “and the existing vaccine prevents the most debilitating effects of whooping cough.”
The DTaP vaccine and its five booster shots are currently given to children starting at the age of 15 to 18 months, but teens and adults are recommended to receive their own version (Tdap) as well. Even with a less effective vaccine, the greater the coverage, the lower the risk of transmission to especially vulnerable populations like infants who can’t yet get their shots.
“There are lots of people out there who may be transmitting pertussis unknowingly,” Scarpino said. “Not vaccinating your own child puts her or him at increased risk of severe disease, even death.”