Immunogenicity of the reduced-antigen-content dTpa vaccine (Boostrix®) in adults 55 years of age and over: A sub-analysis of four trials

BackgroundOlder adults, especially those over 65 years, are at risk of more severe morbidity from diphtheria, tetanus and pertussis and may transmit pertussis to unvaccinated or not yet fully vaccinated infants, but data on their response to reduced-antigen-content tetanus, diphtheria and acellular pertussis (dTpa) vaccines are lacking.MethodsA sub-analysis pooled immunogenicity results in 293 adults aged 55+ years (mean age 64.4 years) from four randomised, controlled clinical trials of dTpa vaccine (Boostrix®, GlaxoSmithKline Biologicals) with or without IPV co-administration, or dTpa-IPV (Boostrix® IPV).ResultsSeroprotective antibody levels were achieved by 82.8% for diphtheria and 94.5% for tetanus. For pertussis antigens, the booster response rate, defined as initially seronegative subjects [<5 EU/mL] reaching ≥5 EU/mL; or a ≥2-fold increase in antibody concentration if initially seropositive was 89.2% for pertussis toxoid, 95.8% for filamentous haemagglutinin and 94.5% for pertactin. Post-booster geometric mean concentrations (GMC) increased for all antigens. Post-booster anti-tetanus and anti-PRN GMCs tended to be higher in 55- to 64-year olds than in those aged 65+.ConclusionLarger numbers of subjects over 75 years are needed to better define responses in advanced age, but these data suggest that a single booster dose of dTpa or dTpa-IPV induces good immunological responses in most, and that these vaccines could be readily integrated into existing programmes.