Efficacy of Haemophilus influenzae type b capsular polysaccharide vaccine on the incidence of epiglottitis and meningitis.

48,997 children of 3 months to 5 years were vaccinated with Haemophilus influenzae type b (Hib) capsular polysaccharide vaccine in a double blind study carried out in Finland in 1974. A clear-cut effect was obtained in the incidence of epiglottitis. The number of patients decreased 58% for three years when compared with the three-year-period before vaccination (p less than 0.002). On the other hand, the number of cases during the three years following vaccination was 65% less than during the next three year period (p less than 0.001). The efficacy implies an exceptionally strong herd immunity, because only 37.7% of the children in the area were vaccinated. Regarding meningitis, no herd immunity was observed but vaccination induced a highly significant (p less than 0.001) individual protection (2 cases in the specifically vaccinated versus 20 in the control group in four years' time), provided the vaccine was given at the age of 18 months or later. No efficacy was obtained before this age limit. It was calculated that approximately 90 per cent of cases of epiglottitis and 50 per cent of meningitis would be prevented with the vaccine currently available. Taking also into account the strong herd immunity tendency, vaccination is likely to offer even larger benefits.