By Kevin Crowe, Watchdog Institute and Joanne Faryon, KPBS The vaccine for whooping cough, a highly contagious disease that reached epidemic levels in California last year, begins to lose effectiveness after three years, according to a preliminary study conducted in Marin County. The diminished protection was most evident in children ages eight through 12. Video Video unavailable. Read transcript below. Above: KPBS and the Watchdog Institute at SDSU spent four months investigating why whooping cough, a disease that was nearly extinct thirty years ago, has infected thousands of people in California and killed 10 babies. Just why it’s made such a vengeful comeback has two of the world’s leading whooping cough experts in disagreement. KPBS Reporter Joanne Faryon raises serious questions about how well the vaccine to prevent the disease works. Dr. David Witt, chief of infectious diseases at Kaiser Permanente Medical center in San Rafael and the lead on the study, told colleagues and reporters at a conference in Chicago on Monday that the vaccine still offers strong protection to those recently immunized. A KPBS-Watchdog Institute joint investigation last year questioned the efficacy of the whooping cough vaccine. The report found the majority of people diagnosed with whooping cough in San Diego County had been immunized. Statewide, many county health departments reported high numbers of cases of whooping cough in people who had been immunized. Witt, speaking on a panel at the Interscience Conference on Antimicrobial Agents and Chemotherapy, said he and his researchers expected to find many of the cases in his county in the unvaccinated population. But that was not the case. “What we identified pretty quickly was that the bulk of the outbreak was in vaccinated, fully vaccinated up-to-date children,” he said. (Story continues below) The bulk of the cases wound up in the 8- to 12-year-old age group, Witt said. “And when we examined that, it really correlated to being more than three years from your last vaccine booster dose.” Video Dr. David Witt Talks About the Return of Whooping Cough Whooping cough, also known as pertussis, sickened more than 9,100 people in 2010 and killed 10 infants. As of August of this year, the California Department of Public health reported 2,164 additional cases. Under a law that took effect this year, children entering 7th and 12th grades are required to show proof of a booster for pertussis before starting school. However, parents can opt out of the vaccine by signing a personal belief exemption form. San Diego County health officials issued a short statement in response to questions about the Marin County study: “This is a preliminary study that has not been fully reviewed by local and state health officials. Further, the County does not make recommendations about routine vaccine intervals. We follow recommendations that come from the CDC’s Advisory Committee on Immunization Practices.” The statement refers to the federal Centers for Disease Control and Prevention, which informs public health policy nationwide. Earlier this year, CDC researchers from the presented preliminary findings from a study of the California epidemic that showed immunity waning in 8- to 10-year-olds. Global experts on the disease agree waning immunity is a problem, but they disagree about how long vaccines are effective.