TUESDAY, Nov. 27, 2012 (HealthDay News) -- Vaccination does safeguard children against whooping cough, but its protective effect seems to lessen over time, new research finds.
The 2010 outbreak of whooping cough (pertussis) in California, which sickened more than 9,000 people and left 10 infants dead, prompted an examination of the current vaccine's effectiveness. That study concluded that the vaccine is effective but loses power over the years, leaving children 7 to 10 years old particularly susceptible.
"The pertussis vaccine is our best protection against disease," said the study's lead author, Lara Misegades, an epidemiologist at the U.S. Centers for Disease Control and Prevention in Atlanta. "We found that unvaccinated children were eight times more likely to be a pertussis case than vaccinated children. Parents should ensure children complete the childhood series and make sure your children get the adolescent booster too."
In the United States this year, more than 36,000 whooping cough cases have been reported, including 16 deaths -- most in infants younger than 3 months old, according to the CDC.
Because the vaccine's protective shield diminishes over time, health experts have suggested that the current vaccine dosing schedule may need reevaluation.
"We're continuing to evaluate the changing epidemiology [of pertussis], but it's too early to speculate if there's a need for an additional booster," Misegades said.
Results of the study appear in the Nov. 28 issue of the Journal of the American Medical Association.
The vaccine, commonly referred to as the DTaP vaccine, also includes immunizations for diphtheria and tetanus. It is given in a five-dose series at 2, 4 and 6 months; at 15 to 18 months; and between 4 and 6 years. An adolescent booster is recommended between age 11 and 12.
The current study was designed to evaluate how long it had been between the last DTaP vaccination and the development of pertussis in patients involved in the California outbreak, the state's largest in 60 years.
The study included data from 15 California counties with a high incidence of whooping cough cases. A total of 682 children aged 4 to 10 years who had pertussis were included, and for each of these children three kids of the same age from the same physician practice who did not have pertussis were selected for comparison.
The researchers found that, compared to the controls, children who had contracted whooping cough were 89 percent less likely to have received all five doses of the vaccine. Children with the disease were more likely to be unvaccinated compared to those without the disease -- 7.8 percent versus 0.9 percent, according to the study.
The researchers also found that the more time that had passed since vaccination, the less effective the vaccine appeared to be.
"Within the first few years, the vaccine's efficacy was around 98 percent," Misegades said. "Five or more years out, the vaccine effectiveness had dropped to about 71 percent."
Seven- to 10-year-old children had the highest incidence of disease, and incidence increased with age. About 3 percent of the cases occurred in 4-year-olds, while 31 percent of the cases were in 10-year-olds.
The current vaccine is an acellular vaccine, and it doesn't contain whole cells of the bacterium responsible for pertussis infections. The previous vaccine did contain whole cells of the bacterium, but was more likely to cause side effects, according to the study.
The switch to the acellular vaccine may partly explain the resurgence of pertussis, according to an accompanying journal editorial, but other factors may also be involved. Better diagnosis may also play a role, Dr. Eugene Shapiro, a professor of pediatrics and epidemiology at the Yale University School of Medicine, wrote in the editorial.
Dr. Kenneth Bromberg, director of the Vaccine Study Center at the Brooklyn Hospital Center in New York City, agreed that the whole-cell vaccine may have been more effective than the acellular vaccine, although he pointed out that the whole-cell vaccine sometimes had production issues and one batch of vaccine wasn't always as effective as another. And, he said, the components of that vaccine that may have increased effectiveness may be the very ones that also increased the risk of side effects.
The takeaway message for parents, Bromberg said, is that "the vaccine works. It's effective, so make sure everyone is appropriately vaccinated according to the current schedule."
"We'll have to wait for further study to [determine if the current dosing schedule needs change]," he said. "Parents should stay tuned as to whether we recommend additional vaccinations for pertussis."
Learn more about pertussis vaccines from the U.S. Centers for Disease Control and Prevention.