Sept. 20, 2011 (Chicago) -- The protection provided by the vaccine against whooping cough may wane after only about three years, a preliminary study suggests.
The findings come from a survey of about 15,000 children in Marin County, Calif., where an outbreak of the highly contagious bacterial disease killed 11 infants and sickened about 9,100 people in 2010.
In 2006, there were only about 13,300 new whooping cough cases in the entire country, according to the CDC.
"The attack rate is the highest in California in 50 years," says researcher David Witt, MD, chief of infectious diseases at Kaiser Permanente Medical Center in San Rafael, Calif.
The bulk of the cases occurred among "fully vaccinated children" aged 8 to 12, he tells WebMD.
"That was a surprise to us," as it was thought most cases would be among unvaccinated children, Witt says.
The childhood vaccine schedule for whooping cough calls for shots at ages 2 months, 4 months, and 6 months; 15-18 months; 4-6 years; and 11-12 years.
For the study, any child with prolonged cough was screened for pertussis, the medical term for whooping cough.
Testing confirmed 171 cases in all age groups. There were 103 cases among 8- to 12-year-olds; they had gone at least three years since their last shot.
These children were 10 to 20 times more likely to get sick than those whose last booster injection was more recent, Witt says.
For example, the disease rate was 3,600 cases per 100,000 children aged 8 to 12, compared with 350 cases per 100,000 children aged 4 to 5. Illness in older children declined following the 12-year vaccination.
More than 80% of the children who developed whooping cough in the study were fully vaccinated.
The findings were presented here at the 51st Interscience Conference on Antimicrobial Agents and Chemotherapy.
The outbreak eased after younger children were vaccinated, Witt says.
Revising the Immunization Schedule
A revised immunization schedule, with more frequent boosters, might be necessary to vaccinate children, he says.
But a preventive medicine expert tells WebMD the study may have overestimated the number of children with whooping cough.