Unexpectedly Limited Durability of Immunity Following Acellular Pertussis Vaccination in Preadolescents in a North American Outbreak

Background. Despite widespread childhood vaccination against Bordetella pertussis, disease remains prevalent. It has been suggested that acellular vaccine may be less effective than previously believed. During a large outbreak, we examined the incidence of pertussis and effectiveness of vaccination in a well-vaccinated, well-defined community. Methods. Our center provides care to 135000 patients, 40% of the population of Marin County, California. A total of 171 patients tested positive for B. pertussis from 1 March to 31 October 2010 by polymerase chain reaction (PCR). Electronic medical records were reviewed for demographic characteristics and vaccination status. Results. We identified 171 cases of clinical pertussis, 132 of which were in pediatric patients. There was a notable increase in cases among patients aged 8-12 years. The rate of testing peaked among infants but remained relatively constant across ages until 12 years. The rate of positive tests was low for ages 0-6 years and increased among preadolescents, peaking among those aged 12 years. The vaccination rate among PCR-positive preadolescents were approximately equal to that of controls. The vaccine effectiveness was 41%, 24%, and 79% for children aged 2-7 years, 8-12 years, 13-18 years, respectively. Conclusions. Our data suggests that the current schedule of acellular pertussis vaccine doses is insufficient to prevent outbreaks of pertussis. We noted a markedly increased rate of disease from ages 8-12 years, proportionate to the interval since the last scheduled vaccine. Stable rates of testing ruled out selection bias. The possibility of earlier or more numerous booster doses of acellular pertussis vaccine either as part of routine immunization or for outbreak control should be entertained.