Seasonal variations in maternal serum and mammary immunity to RS virus.

We have recorded the systemic and mammary/mucosal immune responses of women following natural infection with RS virus during the second and third trimesters of pregnancy. Anti-RS virus IgG antibody levels in the sera of women collected in the first trimester of pregnancy showed a bimodal distribution with high and low antibody groups. Antibody levels increased after exposure to the winter RS virus epidemic in the second trimester of pregnancy, probably as a result of infection but only for women in the low antibody group. Despite the increases, antibody levels for these women remained well below those of the high antibody group. There was no rise in mean antibody levels after exposure in the third trimester, even among women with low antibody, suggesting a degree of immunosuppression in late pregnancy. There was no evidence that infection during pregnancy was associated with adverse consequences for the infant. Exposure to RS virus in the first two trimesters, but not the third, was associated with high colostral IgA antibody levels that were maintained in the milk throughout the first 7 weeks of lactation. There was a significant correlation between colostral and maternal nasal IgA antibody levels at delivery. Levels of blood or colostral lymphocyte transformation responses at delivery were unaffected by exposure to RS virus in pregnancy. These observations upon natural infection suggest that vaccination during pregnancy is likely to achieve only marginal effects upon serum antibody levels but boost maternal mammary/mucosal immunity.