The resurgence of whooping cough in the United States could be the result of waning pertussis immunity combined with incomplete historical coverage, researchers said.

In the March 28, 2018, edition of Science Translational Medicine , researchers reported on a study that used different models of transmission to explore what might be the cause of the steady increase in pertussis infections since the mid-1970s.

The three modes of vaccine failure modeled in the study were primary vaccine failure in a fraction of the population; waning of vaccine-induced protection over time; and failure in the degree of protection offered by the vaccine, perhaps caused by antigenic evolution in the pertussis bacteria.

Using 16 years’ worth of detailed, age-stratified incidence data from Massachusetts, researchers found that the model which assumed a gradual waning in protection was the best fit for the observed patterns of pertussis incidence across the population.

This model suggested significant variability in how the level of protection changes over time, with a 10% risk of vaccine protection waning to zero within 10 years of completing routine vaccination and a 55% chance that the vaccine would confer lifelong protection.

“Crucially, we find that the vaccine is effective at reducing pathogen circulation but not so effective that eradication of this highly contagious bacterium should be possible without targeted booster campaigns,” wrote Dr. Matthieu Domenech de Cellès, PhD, of the Institut Pasteur at the University of Versailles (France) and his coauthors.

The model also considered the possibility that the whole-cell and acellular pertussis vaccines might show differences in immunity, which had been suggested as one explanation for the resurgence of the disease. However, the authors found little evidence of a marked epidemiological switch from the whole-cell to acellular vaccines, although their results did suggest the acellular vaccine has a moderately reduced efficacy.

“Our results suggest that the train of events leading to the resurgence of pertussis was set in motion well before the shift to the DTaP vaccine,” Dr. Domenech de Cellès and his associates said.

The model also pointed to big shifts in the age-specific immunological profile caused by introduction of vaccination, which led to a reduction in transmission and also a reduction in natural infections both in vaccinated and unvaccinated individuals.

This meant individuals who either did not get vaccinated as children or who did not gain immunity from vaccination were growing to adulthood without ever being exposed to natural infection.

“Concurrently, older cohorts, with their long-lived immunity derived from natural infections experienced during the prevaccine period, were gradually dying out,” the authors said. “The resulting rise in the number of susceptible adults sets the stage for the pertussis resurgence, especially among adults.”

Two authors were supported by the National Institutes of Health and by Models of Infectious Disease Agent Study–National Institute of General Medical Sciences. No conflicts of interest were declared.

SOURCE: Domenech de Cellès M et al. Sci Transl Med. 2018 Mar 28;10:eaaj1748 .