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. David Witt, MD, 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 this week 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.