Pertussis antibodies in postpartum women and their newborns

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Abstract
To (1) determine the proportion of mothers and infants who had levels of IgG antibody to pertussis antigens predicted to be potentially protective at delivery; (2) evaluate the efficiency of maternal–infant antibody transport; (3) extrapolate infant antibody titers at 6 weeks; and (4) identify maternal factors associated with potentially protective infant antibodies. Sera from mother–infant pairs from February 2006 through to April 2007 were tested for antibody to pertussis antigens by standardized ELISA (enzyme-linked immunosorbent assay). Potentially protective antibody levels were defined as >5 ELISA units (EU) for pertussis toxin (PT), and >10 EU for fimbriae (FIM) and pertactin (PRN). Serological evidence of previous maternal infection was defined from antibody to four antigens by k-means cluster analysis. In total, 21% (17/81) of mothers and 26% (21/81) of infants had potentially protective antibody levels at delivery. Mean infant–maternal antibody ratios for PT, FIM and PRN were 1.26, 1.36 and 1.31, respectively. At 6 weeks, 11% (9/81) of infants were predicted to have potentially protective antibody levels. Using cluster analysis, 9% (7/81) of mothers had evidence of previous pertussis infection. Infants born to these mothers were predicted to be more likely to have potentially protective antibodies at 6 weeks (43%) than those born to mothers without previous infection (8%) (P=0.03). Approximately 75% of infants were born with pertussis antibody levels lower than the modest levels associated with potential protection. Despite effective antibody transfer, nearly 90% of infants were predicted to have little antibody by 6 weeks. Maternal immunization before or during pregnancy might simulate previous pertussis infection and help protect infants through the first months of life.