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. "When you're newly vaccinated you are an asymptomatic carrier, which is good for you, but not for the population," said Tod J. Merkel, the lead author of the study, who is a researcher in the Office of Vaccines Research and Review in the Food and Drug Administration. The current whooping cough vaccines were developed after a surge in concerns from parents that their children were getting fevers and having seizures after receiving the old vaccine. The new finding [suggest that] people recently vaccinated may be continuing to spread the infection without getting sick.