Importance of timing of maternal combined tetanus, diphtheria, and acellular pertussis (Tdap) immunization and protection of young infants.

BACKGROUND Pertussis booster vaccine (Tdap) recommendations assume that pertussis-specific antibodies in women immunized preconception, during, or after previous pregnancies persist at sufficient levels to protect newborn infants. METHODS Pertussis-specific immunoglobulin G (IgG) was measured by IgG-specific enzyme-linked immunosorbent assay (ELISA) in maternal-umbilical cord serum pairs where mothers received Tdap during the prior 2 years. Geometric mean concentrations (GMCs) of pertussis antibodies and cord-maternal GMC ratios were calculated. RESULTS One hundred five mothers (mean age, 25.3 years [range, 15.3-38.4 years]; mean gestation, 39 weeks [range, 37-43 weeks]) immunized with Tdap vaccine a mean of 13.7 months (range, 2.3-23.9 months) previously were included; 72 (69%) had received Tdap postpartum, 31 at a routine healthcare visit and 2 as contacts of another newborn. There was no difference in GMCs for pertussis-specific IgG in maternal delivery or infant cord sera for women immunized before (n = 86) or during (n = 19) early pregnancy. Placental transport of antibodies was 121%-186% from mothers immunized before and during pregnancy, respectively. Estimated GMC of IgG to pertussis toxin was <5 ELISA units (EU)/mL at infant age 2 months (start of infant immunization series). More infants of mothers immunized during pregnancy had pertussis toxin levels estimated to be higher than the lower limit of quantitation of the assay (4 EU/mL) through age 2 months (52% vs 38%; P = .34). CONCLUSIONS Infants of mothers immunized preconception or in early pregnancy have insufficient pertussis-specific antibodies to protect against infection. Maternal immunization during the third trimester, immunization of other infant contacts, and reimmunization during subsequent pregnancies may be necessary.