The return of whooping cough, also known as pertussis, continues to make news. As of August, there were more than 23,000 reported cases across the United States, including 13 deaths.

A new study suggests children who have had the complete set of five shots may lose their protection just a few years after the final dose, leaving them at risk for the illness. Reporting in the New England Journal of Medicine, researchers at Kaiser Permanente evaluated data from a large pertussis outbreak in California in 2010. Since the full series of whooping cough vaccines is mandatory for all school-aged children in California, researchers were looking for reasons behind the outbreak.

In this study, the risk of pertussis increased by 42% each year after the fifth dose of the vaccine was administered. The incidence of disease was highest among children aged 8 to 11. Researchers say these findings point to the need for new vaccines that will provide long-lasting immunity.