Using mathematical models, researchers at the University of New Mexico and University of Michigan have revealed that whooping cough immunity can last at least 30 years and perhaps as long as 70 years after natural infection. Once thought to be under control, whooping cough (pertussis) has been on the rise since the 1980s in the United States and several other countries with high rates of routine pediatric immunization.
Photo: Assistant Professor Helen Wearing
The increase has led to concerns over the possible reemergence of pertussis, the effectiveness of current vaccination practices and whether or not whooping cough can ever be eradicated.
“There’s been a lot of interest in the United States and other countries about the resurgence of whooping cough cases,” said Helen Wearing, UNM biology and mathematics assistant professor and lead author on the paper published Oct. 23 in the open-access journal PLoS Pathogens. “Using mathematical models, we discovered that immunity from natural infection can be long lasting. ”
"This is surprising because clinical epidemiologists currently believe the duration of pertussis immunity is somewhere between four and 20 years," said UM’s Pejman Rohani, professor of ecology and evolutionary biology.
Several explanations have been proposed for the surprising increase in cases, and one leading idea is that the immunity enjoyed by vaccinated or previously exposed people is simply wearing off. It's been documented that in some individuals immunity has waned over time, but the exact details of how long protection typically lasts and how its waning affects disease transmission have not been clear.
Wearing and Rohani took a mathematical approach, using models to explore various scenarios and comparing the predictions generated by those models to data on whooping cough incidence. The researchers constructed two models, a basic model and an immune-boosting model, with different assumptions about what happens when a person whose immunity has lapsed is exposed to pertussis and how much that person contributes to transmission.
The basic model assumes individuals may experience repeat infections in a manner similar to primary infections. In the immune-boosting model, re-exposure may boost an individual's immunity whereupon they revert to being temporarily protected from natural infection.
The researchers then 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.
“We found the best agreement between data and model predictions when immunity lasts, on average, at least 30 years, and possibly much longer. Our analysis also suggests that the duration of natural immunity is inherently variable, so that some people quickly lose immunity while others experience lifelong protection after an infection,” Wearing said.
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 infectious. In fact, their immunity to the disease may be boosted by re-exposure, the study suggests.
If correct, the results represent encouraging news, Rohani said. "They suggest that loss of immunity may be playing a less significant role than is currently thought. And at least in these historical data, vaccination seems to interrupt transmission substantially."
Still, the researchers are cautious about drawing conclusions about current day vaccination practices from their study of historical data.
"It's worth pointing out that in the past 20 years or so, the nature of the vaccines that have been used has changed quite fundamentally," Rohani said. "The data we're using are from a time when a whole-cell vaccine was in use; now an acellular vaccine, which stimulates a different part of the immune system, is typically used, especially in North America."
“Ultimately, to understand recent trends in whooping cough incidence we should also examine variation in vaccination coverage, age structure and population movements,” Wearing said. “Using more recent data from the U.S., our mathematical framework can be modified to investigate the importance of these other factors.”
The paper is available at: PLoS Pathogens.
For more information Helen Wearing and her research visit: Wearing's Research Group.
Media Contacts: UNM, Steve Carr, (505) 277-1821; e-mail: scarr@unm.edu or UM, Nancy Ross-Flanigan, (734) 647-1853; e-mail: rossflan@umich.edu
Posted by scarr at October 30, 2009 09:34 AM