Baboons vaccinated against whooping cough could still carry the illness in their throats and spread it, research published in a science journal on Monday has found. The surprising new finding has not been replicated in people, but scientists say it may provide an important clue to a puzzling spike in the incidence of whooping cough across the country, which reached a 50-year high last year.

The whooping cough vaccines now in use were introduced in the 1990s after an older version, which offered longer-lasting protection, was found to have side effects. But over the years, scientists have determined that the new vaccines began to lose effectiveness after about five years, a significant problem that many researchers believe has contributed to the significant rise in whooping cough cases.

The new study, published on Monday in Proceedings of the National Academy of Sciences, offers another explanation. Using baboons, the researchers found that recently vaccinated animals continued to carry the infection in their throats. Even though those baboons did not get sick from it, they spread the infection to others that were not vaccinated.