Pertussis vaccine, although effective, offers less protection over time

The pertussis vaccine is effective, but its protection decreases over time, and coverage has been incomplete, according to researchers.

The findings contradict suspicions that a resurgence of the disease in the United States and Western Europe has occurred because the vaccine is simply ineffective, they wrote in Science Translational Medicine.

“Conventional wisdom is that the current vaccine is the problem, but that’s not consistent with what we see,” researcher Aaron A. King, PhD, a professor of ecology and evolutionary biology at the University of Michigan, said in a news release. “This resurgence is the predictable consequence of rolling out a vaccine that isn’t quite perfect and not hitting everybody in the population with that vaccine.”

Pertussis was once very common among children in the U.S., most of whom were exposed to its causative agent, Bordetella pertussis. The first vaccine against it was introduced in the late 1940s, and routine vaccination reduced incidence by 100-fold.

But in recent years, pertussis has come surging back in high-income countries where experts once thought it might soon be eradicated. These include the U.S., where about 16,000 cases, including six infant deaths from the disease, were reported in 2016, researcher Matthieu Domenech de Cellès, PhD, also of the University of Michigan department of ecology and evolutionary biology, said in an interview.

The researchers said that, following a period of high vaccine efficacy, older adults with natural immunity to pertussis through prior infection began dying off. At the same time, vaccine protection in younger generations began to wane. Those factors helped bring about recent resurgences.

In their study, de Cellès and colleagues assessed data on pertussis cases between 1990 and 2005 from the Massachusetts Department of Public Health. They found that epidemics became larger and more frequent over that period, and overall pertussis incidence increased fourfold. More than half of cases occurred in people aged between 10 and 20 years, and cases among those aged 20 years or older increased 10-fold.

To explore causes for the resurgence of pertussis, the researchers created models portraying hypothetical vaccine outcomes and applied them to the Massachusetts data. Models ranged from the vaccine providing perfect and enduring protection, to the vaccine providing imperfect and waning protection.

The researchers found that there was a 10% risk that protection from the vaccine would wane to nothing in 10 years, and a 55% chance of protection enduring lifelong.

The vaccine reduces B. pertussis circulating in the population, they said, but routine vaccination alone will not eradicate it. However, additional vaccination among certain age groups can further improve outcomes, they added.

The researchers found that primary school children and teenagers were what they called the “core transmission group” for pertussis. The researchers said a booster for children aged 5 years to 10 years, or one for those aged 10 years to 20 years, would reduce infant cases of pertussis by about 25%.

“The overwhelming amount of transmission is happening in those age groups,” King stressed. “So, we have to make sure that kids are getting vaccinated before they go to school. That’s going to have the biggest impact.” – by Joe Green

Disclosures: The authors report no relevant financial disclosures.