VACCINE FAILURES AFTER PRIMARY IMMUNIZATION WITH HAEMOPHILUS-INFLUENZAE TYPE-B CONJUGATE VACCINE WITHOUT BOOSTER

Background Diseases of early childhood associated with Haemophilus inf luenzae type b (Hib) can now be prevented by vaccination. The rapid im plementation of routine infant vaccination with Hib polysaccharide-tet anus protein conjugate (PRP-T) vaccine has allowed us to assess whethe r an accelerated 2, 3, and 4 month schedule can protect in the longer term without a booster dose and whether carrier priming influences pro tective efficacy. The degree of protection afforded by a catch-up prog ramme with Hib oligosaccharide conjugate (HbOC) for older children was also assessed. Methods Paediatricians and microbiologists in the UK w ere asked to report all cases of invasive H influenzae infection in ch ildren who had received at least one dose of Hib-conjugate vaccine. Se rum samples from convalescent children were obtained and the isolate w as verified. Efficacy was estimated by comparing observed rates of Hib disease in those who had been vaccinated with rates predicted by age adjustment of disease rates from the prevaccine era. Findings Of 164 r eports of invasive infection between Oct 1, 1992, and Oct 1, 1995, 43 were considered true vaccine failures. The estimated overall efficacy for three doses of PRP-T was 98.1% (95% CI 97.3-98.7%). Efficacy in in fants aged 5-11 months was 99.1%, 12-23 months 97.3%, and 24-35 months 94.7%. In infants aged 3-11 months, who received their first dose of PRP-T after tetanus toroid vaccination, disease was unlikely from 1 we ek after one dose of PRP-T vaccine (88.6% protection in the second to fourth weeks [66.8-97.7%]). The disease rate in vaccinated infants age d 2 months has declined year on year. In children aged 13 months to 2 years given HbOC, as a catch-up vaccine, the estimated efficacy was 94 .0% (84.7-98.4%). Interpretation A high degree of efficacy has been ob served with PRP-T vaccine given as a three-dose schedule in infancy an d with HbOC as a single dose in older children. Efficacy of PRP-T appe ars to be enhanced by carrier priming. Although with increasing age th ere was a small decline in efficacy of PRP-T, Hib disease is now close to being eliminated in the UK, and we suggest that a booster is not n ecessary in the second year of life.