Serum Antibodies After Vaccination with Haemophilus influenzae Type b Capsular Polysaccharide and Responses to Reimmunization: No Evidence of Immunologic Tolerance or Memory

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Abstract
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 3½ 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 1½ years but less than 3½ 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 3½ years. Therefore, children vaccinated at the age of 18 months may need a new dose of vaccine 1 to 1½ 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 3½ 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. In addition, the children who had received their first dose of vaccine when younger than 18 months and therefore were not responding, responded now. Thus, there was no evidence of immunologic tolerance.