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.