Polysaccharide vaccines of group A Neisseria meningtitidis and Haemophilus influenzae type b: a field trial in Finland.

During an epidemic caused by group A Neisseria meningitidis, 98,272 children (three months to five years of age) were vaccinated in three provinces of Finland. Meningococcal group A and Haemophilus influenzae type b polysaccharide vaccines were used in a double-blind fashion. Mlild side reactions were common, especially after group A meningococcal vaccine, which also caused high fever in 1.8% of the children. Antibody responses were measured by radioimmunoassay. The response to the group A meningococcal vaccine was consistently good in children older than 18 months of age and was fair in infants as young as six months of age. Good secondary responses were obtained at all ages (three to 17 months) when a booster dose was given. The response to H. influenzac type b vaccine was poor in children below one year of age and was good only in those above 18 months of age; booster doses did not appear to have any effect. Clinical efficacy of the group A N. meningitidis vaccine was good at all ages; protection seemed to extend to the entire age range and resulted in an age-specific decrease in the incidence of the disease. The H. influenzae type b vaccine afforded good protection in children older than 14 months of age at the time of vaccination but no protection in those below that age; this result correlates well with the age-related immunogenicity of the H. influenzae type b vaccine.