Antibody responses of human infants to three doses of group A Neisseria meningitidis polysaccharide vaccine administered at two, four, and six months of age.

Infants were immunized with group A Neisseria meningitidis polysaccharide vaccine at two, four, and six months of age. Two lots of group A vaccine that differed in molecular weight were used; lot no. 1980A was of significantly larger molecular size than lot no. A-7. No significant adverse reactions occurred. The geometric mean concentration of serum antibody to group A N. meningitidis one month after the third dose of lot no. 1980A was 0.89 microgram/ml, nearly twice the concentration induced by lot no. A-7 (0.48 microgram/ml). When the infants were 12 months of age, concentrations of antibody in both groups had declined to similar levels, which were still greater than the mean concentration of antibody in unimmunized children. By 18 months of age, the antibody levels of immunized and unimmunized infants were the same. The three-dose schedule resulted in significantly lower concentrations of antibody than previously studied schedules of two doses given three to four months apart.