The whooping cough vaccine administered to thousands of San Diego children appears to be failing, according to a recent study.
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New research shows the vaccine is not lasting as long as scientists expected.
Two years ago, 22-month-old Matthew Bryce came down with pertussis, or whooping cough.
"The moment that I heard it, I was just thinking the worst because you have heard the news about the babies that have passed away," Matthew's father, Marlon Bryce, said in 2010.
Ten California babies died from whooping cough in 2010, and 9,000 people were sickened, according to data from the California Department of Public Health. It was the largest outbreak in the state in 60 years.
While nine of the newborns who died had not been vaccinated, in San Diego County, more than 80 percent of those who got sick had been immunized.
"We're seeing waning immunity over time," said Dr. Mark Sawyer, a pediatric infectious disease specialist at Rady Children's Hospital. Sawyer is on the Centers for Disease Control's Advisory Committee on Immunization Practices.
A recent study published in the medical journal Clinical Infectious Diseases found current pertussis vaccine doses are "insufficient to prevent outbreaks."
Part of the problems, according to researchers, is the newer vaccine the U.S. switched to in 1997 -- a vaccine considered safer, but also apparently weaker.
Scientists said the vaccine works well during the first two years, but trouble seems to start after that. The study shows infants under three months have the highest risk. Whooping cough cases area also spiking during ages 7-10 and 13-14, according to the study.
"It would suggest that we may need more boosters in order to keep elevating the immune system until we come up with a vaccine that has longer protection," said Sawyer.
Children in California are currently required to have six doses. The unknown is whether adding a booster could add side effects.
Knowing the risks, Marlon Bryce, a military reservist now in Germany, counts his blessings.
"The sad thing is there are some parents out there, their kids are not here today. I'm overall lucky, blessed to have Matthew here with us," Bryce said.
Whooping cough cases in California are down in 2012 so far.
Doctors said if a child comes down with it, the current vaccine will lessen the severity.
Unvaccinated children, researchers said, are eight times more likely to get sick.
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