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The rapidly fading effectiveness of the pertussis booster vaccine may help explain recent widespread outbreaks of whooping cough.

The United States stopped using a whole-cell pertussis vaccine in the 1990s and began using an acellular version called DTaP. Five vaccinations are given during childhood, and a booster vaccine, called the Tdap, is given to adolescents and adults.

Researchers looked at 1,207 pertussis cases among children who had had the acellular vaccine in childhood. The study, in Pediatrics, found that when these children got the Tdap booster, it was 69 percent effective after the first year, then dropped to less than 9 percent two to three years later.

A new, more effective vaccine against whooping cough is needed, but according to the lead author, Dr. Nicola P. Klein, co-director of the Kaiser Permanente Vaccine Study Center, a change in schedule might be effective until one is developed.

“We need to think about whether we should have a targeted routine of vaccines instead of an age-based method,” she said. “There are a number of ways to do this. We’ve seen epidemics every four years in California, so maybe every four years would work. Or we could vaccinate whenever there is an outbreak.”

Dr. Klein added that the vaccination of pregnant women is effective in preventing pertussis in newborns, and that all pregnant women should get the Tdap vaccine in the third trimester of pregnancy.