The biggest obstacle in the current fight against whooping cough resurgence is an incomplete understanding of its underlying causes, according to a University of Michigan population ecologist. Public health officials in several countries, including the United States, have introduced whooping cough booster shots into childhood immunization schedules.
Co-author of the study Pejman Rohani said, "If those booster schedules are based on a misdiagnosis of the problem, the shots are likely to be epidemiologically ineffective and economically costly."
Researchers used a mathematical model to identify pertussis booster schedules that result in the greatest reduction in disease for the lowest economic cost. They looked at several potential causes of the pertussis resurgence separately, though in the real world it is likely that more than one of the mechanisms are simultaneously at play. The mathematical model uses a genetic algorithm that mimics how evolution by natural selection would operate on a population of diverse booster schedules. Researchers found that the most effective booster schedules varied depending on the resurgence mechanisms. They looked at four possible mechanisms-
1. Insufficient vaccine coverage- It can occur when large-scale vaccination campaigns fail to reach enough children or when parents opt out of a vaccination program.
2. Low vaccine efficacy- The vaccine sometimes fails to provide protection
3. Waning immunity- The vaccine is initially protective, but protection wears off over time
4. Leaky vaccine.
Researchers found that when a simulated pertussis resurgence was primarily due to low coverage or low vaccine efficacy, the most effective booster strategy was a single booster shot before children enter kindergarten. For waning immunity, the most successful schedules called for a series of boosters between 5 and 45 years to maintain population immunity. The frequency of the boosters depends on the duration of immunity given by the vaccine. In case of leaky vaccine, the team's genetic algorithm found no booster schedules that further reduced the incidence of pertussis. Researcher Maria Riolo said, "If a vaccine is too leaky, the pathogen can continue to circulate in a fully vaccinated population, and you won't be able to get elimination using that vaccine alone. You can still get a large reduction from pre-vaccine levels of disease, but the leakiness limits how far you can get."
The study is published online in Proceeding of the National Academy of Sciences.