Pertussis vaccine is imperfect but effective, new report concludes

The pertussis vaccine might not be perfect, but it does work, concludes a new report that sought to address an upward trend in cases of the disease.

The study, published in Science Translational Medicine, sought to find the reason behind a resurgence in pertussis cases. A vaccine was developed for pertussis in the 1940s, but the CDC has tracked an upward trend in cases since the 1970s with a total of 15,808 cases reported in 2017-and nearly 22 percent of those cases occurred in adults.

Clinicians and epidemiologists have been at odds over the reason for the rise in cases, said Matthieu Domenech de Cellès, PhD, of the Institut Pasteur at the University of Versailles and lead author of the report. He said the new study is significant because it reveals that while the pertussis vaccine is highly effective, it is also not perfect. He also cautioned that occasional failures of the vaccine is not necessarily a cause for widespread alarm.

"Consequently, infections in previously vaccinated individuals are expected to occur. Cleary, such vaccine failures may cause alarm at the clinical and individual level," Domenech de Cellès said. "However, a population-level perspective is needed to correctly interpret the frequency of such failures and to assess the impact of pertussis vaccines. Therefore, when clinicians observe a vaccinated case of pertussis, they should not automatically conclude that the vaccines are bad."

To find out why cases were trending up, researchers from the University of Michigan, the University of Georgia, Queen’s University in Canada, and the University of Versailles St-Quentin-en-Yvelines studied 16 years worth of data from Massachusetts.

Massachusetts was chosen due to its strong disease surveillance and vaccination tracking system, according to the report. This data was used to develop a mathematical disease transmission model which suggested that the reason for the upward trend in pertussis cases was waning efficacy of the vaccine over time. The report revealed that 10 percent of patients have no immunity a decade after vaccination, and 55 percent have immunity that lasts a lifetime. Researchers also found no differences in transmission rates in patients who had been vaccinated and contracted pertussis compared to those who had not been vaccinated.

While children remain the most likely transmitters of the disease, Domenech de Cellès said adults can still be at risk depending on their immunity status. Resurgence of pertussis has been most pronounced in adults, he added.

"This is because of an accumulation of adults that remained susceptible to contract the disease, after having escaped both vaccination and natural infection as children," Domenech de Cellès said. "Again, this effect is expected and need to be interpreted through an epidemiological lens."

CDC estimates from 2016 place vaccination coverage for pertussis boosters in adolescents at around 90 percent, he said, compared to 25 percent in adults. Pregnant women are also now advised to receive the pertussis booster in order to pass immunity onto their babies upon birth.

While the vaccine remains imperfect, Domenech de Cellès said there is no reason not to continue recommending it to patients.

"The current vaccines are safe and highly efficacious, but admittedly imperfect and insufficient to eliminate the disease," Domenech de Cellès said. "New vaccines are in development and may provide better immunity and better protection in the future. In the meantime, however, booster doses of the current vaccines will remain an essential part of control efforts."

He said he hopes the report will put to rest any disagreements over the value of the pertussis vaccine, despite its imperfections.

Public health officials should consider the waning efficacy of the vaccine when designing vaccination schedules, according to the report. Researchers also advised that targeted booster campaigns would be the only way to completely eradicate the disease.