Seroprevalence of diphtheria and pertussis IgG antibodies among pregnant women in North India

Abstract Background Childhood vaccination provides immunity against diphtheria and pertussis, but antibody levels wane by adulthood. Understanding immunity in pregnant women is crucial because low antibody levels leave them susceptible to infection and increases the risk of transmission to newborns. We aimed to assess diphtheria and pertussis seroprevalence among pregnant women attending antenatal care (ANC) clinics in North India. Methods The leftover serum samples collected under HIV Sentinel Surveillance 2023 round from ANC clinic attendees were analyzed. A total of 1695 randomly selected samples from six North Indian states were tested for serum IgG antibody levels against diphtheria and pertussis using ELISA (Serion). Protective immunity for diphtheria was defined as IgG >1 IU/mL (safe protection), 0.1-1 IU/mL (minimal), and <0.1 IU/mL (insufficient). For pertussis, IgG <40 IU/mL indicated susceptibility, 40-100 IU/mL was borderline, and ≥100 IU/mL indicated recent infection or recent immunization. Results About 58% of the participants had safe protection against diphtheria, while 42% showed minimal protection, requiring booster for long term immunity. None were found to have insufficient protection. Nearly two-thirds of the pregnant women were susceptible to pertussis infection and 11% had high titres (>100 IU/mL) suggesting recent infection. The protective antibody titres were low in rural areas as compared to urban areas. Conclusions Findings emphasize that childhood immunization alone does not ensure lifetime protection, as many pregnant women lacked pertussis immunity putting them and their newborns at risk. This indicates the need for revisiting the antenatal immunization policies and considering tetanus-diphtheria-acellular pertussis (Tdap) vaccine instead of tetanus-diphtheria (Td) vaccine. There is a need for strengthening immunization in rural areas and conducting periodical serosurveys to assist immunization coverage surveys. Key messages • About two-third of pregnant women lacked pertussis antibodies and nearly half had minimal diphtheria protection, illustrating that childhood vaccine protection often wanes by adulthood. • Low maternal immunity puts newborns at risk of pertussis infection; findings support routine antenatal Tdap vaccine (instead of Td) to protect mothers and newborns.