by Kevin Crowe, Brooke Williams and Joanne Faryon | inewsource

On the tail of the biggest whooping cough outbreak in California in more than 60 years, federal health officials said Monday that while the vaccine is highly effective, immunity may be waning for children ages 8 through 10.

The federal Centers for Disease Control and Prevention (CDC) worked with the California Department of Public Health to study whooping cough cases in 15 counties, including San Diego, where two babies died from the disease and more than a thousand were treated for it.

Officials from the CDC are presenting preliminary results of the studies this week at a national immunization conference in Washington, D.C.

“In looking at the breakdown of cases by age and vaccine status, there was a higher number in the 7 to 10 year-olds that had been fully immunized,” said Lara Misegades, an epidemic intelligence service officer with the CDC, in an interview.

A KPBS and inewsource investigation published and aired in December raised questions about the efficacy of the current whooping cough vaccine as well as the possibility of a more virulent strain of the disease, also known as pertussis. Global experts on the disease agree waning immunity is a problem, but they disagree about how long vaccines are effective.

For the current studies, epidemiologists examined the vaccine’s effectiveness in three age groups: 4-5 year olds, 6-7 year olds and 8-10 year olds. They found that while the vaccine was 95 percent effective for the first group, effectiveness dropped to 84 percent for the second group and then to 81 percent for the last group.

“It’s that drop we’re going to be looking at more closely,” Misegades said.

Misegades stressed the findings of the study are preliminary and will change as the agency continues to work with the data it gathered in California.

More than 9,000 people in California were diagnosed with whooping cough in 2010, about 1,300 of whom were 8 to 10 years old. Of those diagnosed in that age group, about 52 percent were up to date with their vaccinations, according a Watchdog Institute analysis of data from the California Department of Public Health.

“Vaccine effectiveness is good overall, but relative effectiveness in the (age groups) suggests that immunity is waning,” Misegades told attendees.

Misegades said that while pertussis cycles through the population in California every three to five years, officials still aren’t sure what caused the high case counts in 2010. California is the only state in the nation to declare the pertussis outbreak and epidemic, but other states, including Ohio, Indiana and Texas, reported high numbers of cases last year.

“Kids can be susceptible to pertussis even if they are completely vaccinated, and that’s something that was reaffirmed in our assessment as well,” Misegades said.

CDC officials have maintained that the pertussis vaccine is 85 percent effective. What officials have not been able to address so far is the increasing rate of disease, which went from less than 1 case per 100,000 population nationally in the 1970s to more than 24 cases per 100,000 last year in California.

Still, health officials agree that the current vaccine is the best way to fight the disease.

Unvaccinated children are still at higher risk than those who have been vaccinated, Misegades said.

“So, with an effective vaccine, it’s important to get your kids vaccinated.”

The CDC will continue to examine the data it collected in California, including information on vaccine lot numbers and how health care providers stored the vaccine.

The National Immunization Conference, organized by the CDC, lasts through Thursday in Washington, D.C. Officials from both the CDC and California Department of Public Health are scheduled to speak about the whooping cough epidemic in California and around the country.

Whooping Cough Cases in California by County

Click on a county to view the number of cases of pertussis reported and rate of incidence for 2010.

The California Department of Public Health included pertussis cases with onset from January 1 through December 31, 2010 as reported to the department by local health jurisdictions.

Darker colors on the map indicate higher rates per capita.

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