American pertussis outbreak may have been caused by waning vaccine immunity

    North of San Francisco, Marin County in California, USA, has one of the highest rates of vaccine refusal in the state and possibly the country, according to data presented at the Interscience Conference on Antimicrobial Agents and Chemotherapy in Chicago.

 The index case for a 2010 pertussis outbreak was in an unvaccinated family; the infection spread rapidly through the county. David Witt, MD, of Kaiser Permanente Medical Center in San Rafael, California, and colleagues looked to the unvaccinated population as the source of the pertussis contagion. But he found that the bulk of the outbreak was in fully vaccinated children, particularly those in the 8-to-12 year group. In those children, it had been three years or more from their last booster.  

   From his institution’s database of approximately 15,000 children, he confirmed 171 cases of pertussis (103 in those under 12). The pertussis attack rate was the highest seen in 15 years (peaking at 3,600/100,000 person years in the 8-12 group). During the period of the outbreak, March through October, there was a high absentee rate from Marin County schools as well as much concern for the safety of the unvaccinated and the very young. Eleven infants in California died.     

   As his data suggest that young pre-adolescents are the main source of pertussis in infants, who are not fully protected until they are six months of age, Dr. Witt and colleagues went to the State of California to request that all children with severe, prolonged coughs be screened for pertussis. This was done.

   While the outbreak is over it has left lingering questions about the efficacy of the current vaccine schedule and the possible need to vaccinate eight- year-old children in order to prevent future outbreaks, he noted at a press briefing.