Summary. Pneumococci are a leading cause of severe bacterial disease in infants and children world wide. A possible means of protecting infants in the first few months of life is immunisation of the mother during pregnancy. We prospectively assessed pneumococcal immunisation of pregnant women to determine the amount of pneumococcal antibody transmitted to the infants in serum and milk and the half-life of the passively acquired antibody. Healthy pregnant women in Dhaka, Bangladesh, were randomised to receive pneumococcal or meningococcal vaccine with routine prenatal tetanus immunisation at 30-34 weeks of gestation. Serum and breast milk specimens from the mothers and sera from infants were collected up to 22 weeks of age and assayed for specific serum IgG, IgG1, and IgG2 and for milk IgA antibodies to pneumococcal serotypes 6B and 19F. 55 mothers and 56 infants were followed from birth to five months. Women who received pneumococcal vaccine had geometric mean antibody increases of 2·6 and 3·4 to types 6B and 19F, respectively. The mean infant/maternal antibody ratios were 0·56 and 0·59 (range 0·11-1·46) for these serotypes. Infant cord antibody titres correlated with maternal titres. Infant/maternal IgG ratios correlated with the interval between immunisation and birth and were higher for specific IgG1 than for IgG2. Infants of pneumococcal vaccine recipients had geometric mean antibody concentrations of 6·8 and 7·5 μg/mL to serotypes 6B and 19F in cord blood; in cord blood and in all subsequent serum specimens the concentrations were 2-3 fold higher than in control infants. The median half-life of passive antibody was about 35 days; at five months of age 63-71% of infants of pneumococcal vaccine recipients had antibody concentrations greater than 0·15 μg/mL. Breast milk IgA antibodies for pneumococcal serotype 19F, but not for type 6B, were significantly higher in vaccine recipients up to five months after delivery. If maternal pneumococcal polysaccharide antibodies do not interfere with active immunisation of the infant with new glycoprotein conjugate pneumococcal vaccines, passive-active immunisation of infants can be a feasible strategy for developing regions.
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