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Know your enemy: Combating whooping cough requires informed vaccine booster schedules

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In order to win any battle, it is important that you know your enemy. When it comes to fighting off diseases like the dreaded whooping cough, the big obstacle which the researchers realize today is an incomplete understanding of its underlying causes. This was stated by a University of Michigan population ecologist. The root causes of the resurgence of whooping cough remains a mystery which needs to be unraveled. However, public health officials in many countries”including the United States”have now introduced whooping cough booster shots into childhood immunization schedules.

In a paper published recently in the Proceedings of the National Academy of Sciences, Pejman Rohan, co-author of the study state that if the newly introduced booster schedules are based on a wrong diagnosis of the problem, the shots are likely to be “epidemiologically ineffective and economically costly. The first author the study is Maria Riolo. She is a former U-M doctoral student and presently is a postdoctoral researcher at the University of Chicago.

Rohani is a professor of ecology and evolutionary biology, a professor of complex systems, and a professor of epidemiology. He strongly thinks that childhood vaccinations are very important. In the U.S a series of five shots for whooping cough, also known as pertussis, for children under 7 is recommended by the Centers for Disease Control and Prevention. For older children and some adults, additional shots are recommended.

For their research, Riolo and Rohani made use of a mathematical model that identifies pertussis booster schedules which could result in the greatest reduction in disease for the lowest economic cost. Several potential causes of the pertussis resurgence were explored separately, even though they know that in the real world more than one of the mechanisms are likely to be at play simultaneously.

The mathematical model they used is based on a “genetic algorithm” that mimics how evolution by natural selection would operate on a population of diverse booster schedules. It was found that depending on the resurgence mechanisms, even the most effective booster schedules varied. Four possible mechanisms were explored which included: insufficient vaccine coverage, low vaccine efficacy, waning immunity and a leaky vaccine.

Insufficient vaccine coverage is when even a large-scale vaccination campaigns is unable to reach enough children. Low efficacy is the one that sometimes fails to provide adequate protection. In waning vaccine immunity, the vaccine is initially protective but the protection wanes over a period of time. For this particular vaccine scenario in their study, they studied booster strategies in cases where a pertussis vaccine protects for an average of 5, 15, 25, 35 or 45 years.

In case of a “leaky” vaccine the risk that a previously vaccinated person becomes infected is reduced but is not eliminated entirely. There is a possibility of this happening when all the microbial strains circulating in the general population are not used in the vaccine. It was found by Riolo and Rohani that simulated pertussis resurgence was primarily due to low coverage or low vaccine efficacy.  They also came to the conclusion that the most effective booster strategy was a single booster shot before children enter kindergarten.

In the case of waning immunity, it was found that the most successful schedules were the ones that administered a series of boosters between the age 5 and 45 to maintain population immunity.  The frequency of the vaccine actually depends on the duration of immunity provided by the vaccine. However, if the cause of the resurgence was leaky immunity, the team’s genetic algorithm found that no booster schedules could reduce the incidence of pertussis. Riolo said that in case a vaccine is to leaky, the pathogen can still continue to circulate in a fully vaccinated population, and that vaccine alone would not be effective in eliminating the pathogens.

There was a time when pertussis seemed under control because of the widespread childhood vaccination. However, since the 1980s, this bacterial illness is making a comeback in the United States and in some developed countries.