During a whooping cough outbreak in California in 2010, immunized children between eight and 12 years old were more likely to catch the bacterial disease than kids of other ages, suggesting that the childhood vaccine wears off as kids get older, according to new research. Whooping cough, or pertussis, is caused by Bordetella pertussis bacteria. The pertussis vaccine, a five-shot series referred to as DTaP, is recommended for children at ages two-, four-, six- and 18-months, and at four to six years old. The CDC recommends that at age 11 or 12 kids get the booster shot called Tdap. In early 2010, a spike in cases appeared at Kaiser Permanente in San Rafael, and it was soon determined to be an outbreak of whooping cough -- the largest seen in California in more than 50 years. [Dr. David Witt, senior author of the study,] had expected to see the illnesses center around unvaccinated kids, knowing they are more vulnerable to the disease. We started dissecting the data. What was very surprising was the majority of cases were in fully vaccinated children. That's what started catching our attention," said Witt. Comparing the kids who got pertussis to the more than 22,000 kids in the medical center's database who didn't, Witt's group wrote in the journal Clinical Infectious Diseases that the vaccine is effective about half of the time for all kids, and just 24 percent of the time in the eight to 12 year old age group.