Risk of whooping cough (pertussis) is highest in infants too young to have completed their primary immunization series (6 months old or younger) and they are at highest risk of developing life-threatening complications. Immunizing pregnant women with tetanus, diphtheria and acellular pertussis (Tdap) vaccine can create enough maternal antibodies to protect infants, but the best time to immunize mothers to maximize concentrations of neonatal pertussis toxin antibodies is unknown.
This observational study included 626 pregnancies and compared pertussis antibody concentrations in umbilical cord blood among newborns whose mothers received Tdap immunization in the third trimester, as is recommended by the U.S. Centers for Disease Control and Prevention, with those whose mothers didn't receive the Tdap vaccine during pregnancy. Maternal immunization with the Tdap vaccine during the third trimester of pregnancy was associated with higher concentrations of pertussis antibodies in infants at birth, with immunization early in the third trimester associated with the highest concentrations of antibodies. Definitive antibody concentration levels for immunity from whooping cough for infants are unknown. The study design doesn't allow for cause-and-effect interpretations of the findings.