Methods: PT-specific IgG was quantified by enzyme-linked immunosorbant assay (ELISA) in umbilical cord sera of infants born at ≥ 37 weeks gestation. Geometric mean concentrations (GMCs) with 95% confidence intervals (C.I.) were calculated. PT-specific IgG levels (IU/ml) in sera of infants born to Tdap immunized women (Tdap+) were compared to levels in sera from infants born to women not immunized during pregnancy (Tdap-).
Results: Infant cord sera from 626 singleton pregnancies were included in the study (312 Tdap+ and 314 Tdap-). Mean maternal age was 29.7 years (range 15.5-46.2); self-reported race/ethnicity was 41.2% white, 27.2% Hispanic, 26% Black, 4.8% Asian and 0.7% other. Mean gestation at delivery was 39.4 weeks (37-42.4). Tdap+ mothers were older than Tdap- (30.8 vs 28.7 years: P < 0.001), more likely to be white (48% vs 34%; P < 0.001) and less likely to be black (17% vs 35%; P < 0.001). Tdap+ women received Tdap at mean gestation of 31.2 weeks (range 27.3-36.4) and a mean 57 days (range 21-90) prior to delivery. The GMC of PT-specific IgG was 3.7-fold higher in sera of infants whose mothers received Tdap (47.3 IU/ml; 95% C.I. 42.10-53.15) compared to sera of infants of mothers not immunized (12.93 IU/ml; 95% C.I. 11.8-14.17) (P < 0.001). More infants in the Tdap+ group had anti-PT antibodies above the lower limit of quantitation of the assay (15 IU/ml) than those in the Tdap- group (86% vs 14%; P < 0.001). 45% of Tdap+ infants had PT-specific IgG levels estimated to persist at > 20 IU/ml through 2 months of age.
Conclusion: Maternal immunization with Tdap results in significantly higher PT-specific IgG in infants. Although serological correlates of protection in infants are unknown, these levels are likely sufficient to protect infants from pertussis infection through the start of the infant immunization series.
C. M. Healy,
Pfizer Inc: Scientific Advisor , Consulting fee
M. Rench, None
M. Harrison, None
M. Martin, None
C. J. Baker, Pfizer, Inc.: Consultant , Consulting fee