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.
Vaccine's Effectiveness Fades
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.
That's because the test used to screen for the disease shows only that a person is carrying the bug. That doesn't mean he or she has an active infection, says Michael Decker, MD, of Vanderbilt University in Nashville, Tenn.
The CDC says further research is needed.
"At this time, the data are too preliminary to warrant a change in timing of the adolescent booster dose, especially when it would impact the rest of the adolescent vaccine platform," says CDC spokeswoman Alison Patti.
Witt agrees the findings are preliminary and that further study is needed.
Still, Witt says, "The 11 deaths in California are a catastrophe. Our population has lost their fear of childhood illness."
The greatest fear is transmission to infants who cannot be fully vaccinated until they are 6 months of age, he says. "Families are at risk of transmitting to infants. We know the vaccine mitigates the impact, but it is still dangerous to younger siblings."
Witt recommends vaccination even while the optimal schedule is debated.
"We still find there was a higher risk among unvaccinated children in Marin County. Any group that is unprotected multiplies the risk to the rest of the community," he says.
Plus, vaccination appears to offer good protection for at least two to three years and may help symptoms to resolve more quickly, he notes.
About 8% of Marin County residents are not vaccinated against whooping cough, Witt says.
Whooping Cough in Adults
Illness can be severe, even in adults, according to Witt.
"We know there are ongoing outbreaks among adults. We know that adults represent a reservoir for the disease. We still have a large unimmunized herd that is a danger to young infants," he says.
The benefits of vaccinating beyond school age were questionable with the previous vaccine. But in the late 1990s, the nation started using a new type of whooping cough vaccine that has fewer side effects, making adult vaccination more feasible, he says.
Patti tells WebMD that preliminary results of a study of the new vaccine show "modest waning each year after, but still some protection even five years out."
Final results from that and another study of the vaccine will be presented at an infectious diseases meeting in October.
Whooping cough infects 30 million to 50 million people worldwide each year, killing about 300,000, according to the CDC.
The infection typically starts out with cold-like symptoms but progresses to a dry, hacking coughing fits that last up to 10 weeks. It can be fatal, especially in infants. It is typically treated with antibiotics, which help relieve symptoms and prevent spread.
These findings were presented at a medical conference. They should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.