Immunity to whooping cough lasts at least 30 years on average, a new study by researchers at the University of Michigan and the University of New Mexico has shown. The figure turn out to be much higher than previously thought.
During the study, Pejman Rohani (based at the University of Georgia during completion of this study) and Helen Wearing used mathematical models to explore various scenarios and compared the predictions generated by those models to data on whooping cough incidence.
The researchers constructed two different models based on assumptions of the effects of pertussis exposure on a person whose immunity has lapsed and that person's relative contribution to transmission.
Then they compared the models' predictions to whooping cough incidence data from England and Wales from both the pre-vaccine era (1945-1957) and the vaccine era (1958-1972).
In particular, Rohani and Wearing looked for matches in two key measures: the number of years between big outbreaks and the frequency of "extinctions"---periods of time when no whooping cough cases were reported in the population.
The analysis revealed that, on average, whooping cough immunity lasts at least 30 years and perhaps as long as 70 years after natural infection.
"This is surprising because clinical epidemiologists currently believe the duration of pertussis immunity is somewhere between four and 20 years," said Rohani.
In addition, repeat infections appear to contribute relatively little to the transmission cycle, the researchers found. And when people whose immunity has waned are re-exposed to whooping cough, they rarely become infected.
In fact, their immunity to the disease may be boosted by re-exposure, the study suggests. Still, the researchers are cautious about drawing conclusions about current day vaccination practices from their study of historical data.
The study has been published in the open-access journal PLoS Pathogens.