Serum antibodies after vaccination with Haemophilus influenzae type b capsular polysaccharide and responses to reimmunization: no evidence of immunologic tolerance or memory.

Haemophilus influenzae type b capsular polysaccharide vaccine was given in Finland in 1974 to approximately 50,000 infants and children, whose serum anti-H influenzae type b capsular polysaccharide levels have been followed for 3 1/2 years. The serum antibodies induced by the vaccination proved short-lived (less than 6 months) in the infants younger than 18 months. Elevated serum antibody levels were detectable for 1 1/2 years but less than 3 1/2 years in the children who were vaccinated when 18 to 35 months old. In the children who were 3 to 5 years old when vaccinated, the elevated anti-H influenzae type b capsular polysaccharide levels persisted for at least 3 1/2 years. Therefore, children vaccinated at the age of 18 months may need a new dose of vaccine 1 to 1 1/2 years after the first dose in order to be protected for the period of high susceptibility, until the age of approximately 7 years. Some of the vaccinated children were reimmunized 3 1/2 years after the first dose, and the anti-H influenzae type b capsular polysaccharide levels in their sera were studied in a similar manner. At no time did the anti-H influenzae type b capsular polysaccharide levels after the reimmunization differ from the anti-H influenzae type b capsular polysaccharide levels seen after the first vaccination in children of the same age.(ABSTRACT TRUNCATED AT 250 WORDS)