Long-term Follow-up of Swedish Children Vaccinated With Acellular Pertussis Vaccines at 3, 5, and 12 Months of Age Indicates the Need for a Booster Dose at 5 to 7 Years of Age

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Abstract
OBJECTIVES. The purpose of this work was to evaluate the long-term effectiveness of vaccination with acellular pertussis vaccines at 3, 5, and 12 months of age. METHODS. Clinical follow-up of reported culture- and polymerase chain reaction–confirmed cases of pertussis was initiated during October 1997 in most of Sweden (except Gothenburg and environs). The study population included 90% of Swedish children born during 1996 or later (ie, who received diphtheria-tetanus-acellular pertussis vaccines at 3, 5, and 12 months of age) and children who had participated in a large pertussis vaccine trial in 1993–1996. Age-specific incidences were estimated using reported culture- or polymerase chain reaction–confirmed pertussis from October 1997 to September 2004 in areas covered by enhanced surveillance. In addition, annual overall and age-specific incidences of pertussis throughout Sweden before and after introduction of acellular pertussis vaccines were estimated. RESULTS. The overall incidence of notified culture- and polymerase chain reaction–confirmed pertussis dropped from 113 to 150 per 100 000 during 1992–1995 to 11 to 16 per 100 000 during 2001–2004. In areas of enhanced surveillance, the incidence of pertussis was 31 per 100 000 person-years after 2 doses and 19 per 100 000 person-years after the third dose at 12 months of age. The age-specific incidence remained low for ∼5 years after the third dose but increased in children aged 6 to 8 years, becoming 32 and 48 per 100 000 person-years, respectively. The highest incidence occurred among infants who were unvaccinated or had received only 1 dose of diphtheria-tetanus-acellular pertussis vaccine. CONCLUSIONS. The increased incidence among 7- to 8-year-olds (ie, mainly acellular pertussis vaccine–vaccinated children) suggests waning of vaccine-induced protection from pertussis. Along with a concomitant increase in incidence among infants, most likely infected by older siblings, these data suggest a booster dose of acellular pertussis vaccine is warranted from 5 to 7 years of age.