Waning Vaccine Protection Contributes to Whooping Cough Comeback

Waning vaccine protection and incomplete vaccinations may be behind a resurgence of whooping cough in the U.S. | Semevent / Pixabay

A resurgence in whooping cough cases in the U.S. is likely due to incomplete vaccine coverage in the past combined with waning protection from the vaccine, according to a new study published in the March 28 issue of Science Translational Medicine.

The findings shed light on the effectiveness of whooping cough vaccines in the U.S. — a subject of intense scientific debate — and may help guide future vaccination campaigns and public health strategies.

Whooping cough, also called pertussis, is a contagious disease caused by Bordetella pertussis bacteria. It infects the respiratory airways and often results in a prolonged cough that produces a characteristic "whooping" sound in patients.

Although the clinical course of pertussis is often milder in adults, it can cause severe symptoms and complications in young children and infants. Worldwide, the disease is responsible for approximately 195,000 infant deaths mostly in low-income countries, according to the new study.

The illness once plagued infants and children in the U.S., but the number of reported cases dramatically decreased after routine vaccination began in the 1940s. However, pertussis cases have been steadily rising again for several decades, with an estimated 15,700 cases and six infant deaths in the U.S. in 2016, according to a CDC surveillance report .

"Given the spectacular success of [past] vaccination programs, epidemiologists considered pertussis a candidate for elimination," said Matthieu Domenech de Cellès, a researcher at the Pasteur Institute in Paris and co-author of the new study. "However, since the mid-1970s pertussis has made a surprising comeback in the U.S. and the number of cases has steadily risen."

"This may be referred to as the 'pertussis paradox': an increasingly large disease burden, despite consistently high vaccination coverage," he said.

Alarmed by the resurgence of cases, health authorities and researchers have proposed several possible explanations. Some have argued that the switch from the older cellular vaccine, which uses whole bacteria, to the newer acellular vaccine, which was introduced in 1997 and contains components of the bacteria, resulted in weaker immunity among children and teenagers.

Domenech de Cellès and colleagues set out to explain the pertussis paradox and gain a better understanding of the efficacy of pertussis vaccines — particularly the new generation of acellular vaccines. The research team first gathered and analyzed data on pertussis incidence among recipients in Massachusetts from 1990 to 2005.

Despite the waning of vaccine-derived immunity, the authors still believe the pertussis vaccine is highly effective. Nevertheless, future control efforts may require implementing additional booster shots because of the disease's high transmission rate, said Domenech de Cellès.

They then used mathematical models to test three possible theories to explain the state's pertussis cases. The vaccine might be failing to protect certain individuals, the vaccine's protection might be waning over time, or it might be providing imperfect protection as the pertussis bacteria has evolved.

Their models revealed the resurgence of pertussis in Massachusetts was best explained by the vaccine's waning protection.

"We estimated that vaccine protection wanes over time, but slowly, with about 85 percent of children still protected 10 years after vaccination," said Domenech de Cellès.

In addition, a small part of the population in Massachusetts escaped vaccination as children, and then matured without developing immunity. As these individuals aged, the number of people susceptible to pertussis gradually increased, setting the stage for the disease's comeback.

The researchers concluded the rise in whooping cough cases represents "the end-of-honeymoon" — a predictable consequence of incomplete vaccination coverage of infants combined with effective, but imperfect, vaccines.

Domenech de Cellès stressed the resurgence of whooping cough isn't necessarily due to recent changes in pertussis transmission. "Rather, they may be interpreted as a legacy of past immunization practices, with long-to-manifest effects," he said.

Despite the waning of vaccine-derived immunity, the authors still believe the pertussis vaccine is highly effective. Nevertheless, future control efforts may require implementing additional booster shots because of the disease's high transmission rate, said Domenech de Cellès.

There was little evidence that switching from the cellular to the acellular pertussis vaccine had a significant effect on the spread of pertussis in Massachusetts. Instead, the study found the acellular vaccine confers long-term protection, a finding that runs counter to a widespread view in the medical community.

The study also showed the core transmission group of pertussis is school-aged children, with adults having little impact on pertussis spread. This finding suggests schoolchildren should be the main target of vaccination campaigns instead of adults, according to Domenech de Cellès.

The group's models could also help interpret data from future clinical trials that evaluate the effectiveness of booster shots in schoolchildren, "as well as predict the potential impact of new vaccination strategies," he said.

The research team is planning a new study to further examine the acellular vaccine's effectiveness. They also wish to test their models to determine whether the "end-of-honeymoon" effect explains the resurgence of pertussis in other countries.