But even among children who were fully vaccinated, the longer it had been since their final dose of the so-called DTaP vaccine, the higher their risk of coming down with whooping cough, also known as pertussis.

The lead researcher on the study from the Centers for Disease Control and Prevention (CDC) said the effectiveness of many childhood vaccines is known to decrease over the years after vaccination.

Because of such waning immunity, 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 age 2 months and 6 years. The combination vaccine also protects against diphtheria and tetanus.

"Pertussis vaccines are still our best tool to prevent pertussis," said Lara Misegades, from the National Center for Immunization and Respiratory Diseases at the CDC.

"The first couple of years after children complete their vaccine series, vaccine effectiveness is still high." Her team's study, she told Reuters Health, "Reinforces the importance of getting the adolescent DTaP booster."

During the 2010 whooping cough outbreak in California, more than 9,000 cases were reported and 10 infants died, the researchers wrote Tuesday in the Journal of the American Medical Association.

The new study included 682 kids aged 4 to 10 with whooping cough across the state and another 2,016 comparison children who never got the disease.

According to their medical records and immunization registries, 7.8 percent of the kids who got whooping cough hadn't received any DTaP vaccines, compared to 0.9 percent of their pertussis-free peers.

Children who'd finished their vaccine series recently were the least likely to become infected. For example, just 2.8 percent of kids who got sick had received their fifth DTaP dose in the past year, compared to 17.6 percent of those who didn't come down with whooping cough.

And with each year that passed since children's last vaccine dose, their odds of getting 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," Shapiro, who wrote an editorial accompanying the new study, told Reuters Health. "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 - but each of those ideas 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 get seriously ill in a whooping cough outbreak, and "cocooning" could prevent them from being exposed in the first place (see Reuters Health story of October 18, 2012: reut.rs/TyhQ6N).

Misegades said the CDC also recommends catch-up vaccination for everyone who hasn't gotten their entire DTaP series. Whooping cough cases for this year have already topped 36,000 in the U.S., she added.

"This is still an ongoing pertussis epidemic that we're seeing in many areas, and not just California," she said.

SOURCE: bit.ly/JjFzqx Journal of the American Medical Association, online November 27, 2012.

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