By Genevra Pittman
Nov 27 (Reuters Health) - Unvaccinated youngsters were nine times more likely to contract whooping cough during a recent outbreak in California than those who had received the entire five-shot series, researchers found.
However, the effectiveness of the vaccine waned as more time elapsed since a child's final dose of the so-called DTaP vaccine for whooping cough, also known as pertussis.
"Pertussis vaccines are still our best tool to prevent pertussis," said Lara Misegades, the lead researcher on the study at the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention (CDC). But the study, she said, "reinforces the importance of getting the adolescent DTaP booster."
A DTaP booster was recently added to the vaccine schedule for 11-to-12-year-olds, on top of the five doses traditionally given between the ages of 2 months and 6 years. The combination vaccine also protects against diphtheria and tetanus.
During the 2010 whooping cough outbreak in California, more than 9,000 cases were reported and 10 infants died, the researchers wrote on Tuesday in the Journal of the American Medical Association.
The new study included 682 children aged 4 to 10 in California with whooping cough and another 2,016 children who never came down with the disease.
According to their medical records and immunization registries, 7.8 percent of the kids who developed whooping cough had not received any DTaP vaccines, compared with 0.9 percent of their pertussis-free peers.
Children who recently had finished their vaccine series were the least likely to become infected. Just 2.8 percent of kids who became ill had received their fifth DTaP dose in the past year, compared with 17.6 percent of those who did not come down with whooping cough.
And with each year that passed since a child's last vaccine dose, the odds of that child developing pertussis rose.
"The message should clearly not be, 'Don't get the vaccine because it doesn't work,'" said Dr Eugene Shapiro, a pediatrician and infectious diseases researcher from the Yale School of Medicine in New Haven, Connecticut.
"It works, but we need to continue to work to improve it," said Shapiro, who wrote an editorial accompanying the new study. "The problem is we don't really know what should be done."
He said one strategy might be to give an earlier DTaP booster dose, or to consider a different type of pertussis vaccine. Each of those ideas, however, will require more research.
To prevent the most serious whooping cough cases including deaths, both researchers emphasized the importance of vaccinating pregnant women and anyone living with an infant. Babies are the most likely to become seriously ill in a whooping cough outbreak, and "cocooning" could prevent them from being exposed in the first place.
Misegades said the CDC also recommends catch-up vaccination for everyone who has not received the entire DTaP series. Whooping cough cases for this year have already topped 36,000 in the United States, she added.
Journal of the American Medical Association, online Nov. 27, 2012. (Reporting by Genevra Pittman in New York; Editing by Ivan Oransky and Matthew Lewis)