Maternal and infant immunity against Bordetella pertussis, Norway, 2020 to 2023.

BACKGROUNDPertussis remains a serious threat to young infants. In Norway, infants receive an acellular pertussis vaccine (aP) according to a 2 + 1 schedule at 3, 5 and 12 months of age, delivered as a hexavalent vaccine.AIMWe aimed to study susceptibility to pertussis in mothers and infants to guide decisions regarding vaccination in pregnancy.METHODSIn this prospective observational study, we included 366 mother/infant pairs during 2020-2023, collecting blood samples from mothers in late pregnancy, cord blood at delivery and from infants before their first and after their third vaccine dose. We retrieved health registry data on vaccination and pregnancy-related information. IgG antibody levels against pertussis-antigens, diphtheria and tetanus were measured using a multiplex immunoassay.RESULTSOf the pregnant women, 48% (174/366) had low levels of antibodies against pertussis toxin (PT) defined as below 5 IU/mL. Maternal antibodies declined in infants from birth until first vaccination, leaving 72% (154/215) of infants with anti-PT IgG levels below 5 IU/mL. All infants responded well to vaccination and we found no evidence of blunting from high levels (> 40 IU/mL) of maternal antibodies against PT. Infants of mothers who received an aP-containing booster vaccine within 2 years before pregnancy displayed low anti-PT IgG levels, with 58% (15/26) having levels below 5 IU/mL.CONCLUSIONA high proportion of pregnant women and their infants under 3 months of age had low anti-PT antibody levels, indicating high susceptibility to pertussis. The results support the introduction of vaccination against pertussis during pregnancy in Norway.