“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.”