Analysis of an outbreak of the disease in California shows that some vaccinated children are still vulnerable.
A study found that the vaccine for whooping cough can lose its effectiveness after three years.
This may explain a recent outbreak of the disease in Marin County, Calif.
Children age 8-12 appear to be most vulnerable among the vaccinated.
The vaccine for whooping cough commonly administered to young children loses its effectiveness after three years, according to the preliminary results of a new U.S. study.the
The results released on Monday come from a survey of 15,000 children in Marin County, Calif., where an outbreak of the bacterial disease killed 11 infants and infected more than 8,000 people in 2010.
"When we first started having a pertussis outbreak, we assumed that this would be primarily in the unvaccinated population," Dr David Witt, of Kaiser Permanente Medical Center in San Rafael, California told a conference in Chicago, using the scientific name for the once-common childhood disease.
"What we pretty quickly identify is that the bulk of the outbreak was in fully vaccinated children" in the 8-12 age group, he said.
"Older kids and younger kids seemed to be pretty well protected but the age of eight to 12 was the vast bulk of the cases. And when we examined that, it was correlated to being more than three years from the last vaccine booster dose."dose
He added that a lot of the children did not seek care because the vaccine mitigated the symptoms.
He cautioned that more research was necessary as he presented the preliminary results at the Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) organized by the American Society of Microbiology..
In the group of 15,000 children surveyed, doctors found 171 confirmed cases of whooping cough across all age groups, including 103 in the group of eight to 12-year-olds.
These children were up to 20 times more likely to become infected than those whose last booster injection was more recent, Witt said.
He insisted, however, that parents should get their kids vaccinated, saying the vaccine provided good protection for the first two to three years and appeared to mitigate the symptoms of those who became infected.
The infection causes coughing fits lasting up to 10 weeks and can be fatal, especially in infants. It is commonly treated with antibiotics, which are used to reduce symptoms and prevent the spread of the disease.
The disease infects 30-50 million people worldwide each year and kills some 300,000, according to the Center for Disease Control (CDC).