Finding defies belief that resistance wears thin much sooner.

FRIDAY, Oct. 30 (HealthCare News) --Exposure to whooping cough will provide immunity for an average of three decades, new research suggests.

Doctors had previously thought that immunity lasted for much less time. But the new study, by researchers based at the University of Michigan and the University of New Mexico, rebuts that assumption.

Whooping cough, also known as pertussis, has become more common in the United States and elsewhere since the 1980s. Some health experts have thought that immunity is wearing off for people who'd been vaccinated or had been infected by the disease.

For the study, researchers used medical data from England and Wales from before a vaccine was available (1945-1957) as well as later (1958-1972). They created a mathematical model to determine how long immunity lasted after people were exposed naturally to the disease.

They found that immunity after natural infection lasts for at least three decades, on average, and maybe even as long as 70 years. The study suggests that people who lose some of their immunity might still have some protection and even gain more immunity when they're exposed again to whooping cough.

"This is surprising because clinical epidemiologists currently believe the duration of pertussis immunity is somewhere between four and 20 years," study co-author Pejman Rohani, of the University of Georgia, said in a news release from the publisher of PLoS Pathogens. The findings are published online Oct. 30 in the journal.

But there are caveats. "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."