Protective Role of Maternal Antibodies Against SARS-CoV-2 in Infants Six Months or Younger: An Observational Study.

Publication date: Dec 17, 2025

Maternal humoral immunity appears to confer protection to infants against SARS-CoV-2. However, data comparing the protective effect of naturally acquired versus vaccine-induced maternal immunity in infants remain limited. A retrospective observational study was conducted in infants aged ≤6 months with suspected infection between January 2021 and September 2024. The sample was divided into those with confirmed SARS-CoV-2 infection and those with a negative microbiological result for the virus. The association between maternal serology (anti-S1-RBD IgG), the origin of maternal immunity and symptomatic SARS-CoV-2 infection in infants was analyzed, adjusting for infant age and month of infection. A total of 244 infants were included, of whom 22% had confirmed SARS-CoV-2 infection. Anti-S1-RBD IgG antibodies were detected in 76. 2% of mothers, and their presence was associated with a lower risk of infection in infants [relative risks (RR): 0. 459; 95% confidence interval (95% CI): 0. 284-0. 744; P = 0. 002]. Naturally acquired immunity showed a stronger protective effect (RR: 0. 397; 95% CI: 0. 213-0. 744; P = 0. 004) compared with vaccine-induced immunity (RR: 0. 558; 95% CI: 0. 325-0. 958; P = 0. 034). Maternal antibody titers demonstrated limited predictive capacity for infant protection (area under the curve = 0. 699), with an optimal threshold of 109. 50 BAU/mL (sensitivity 68. 5%, specificity 73%). The presence of maternal anti-S1-RBD IgG antibodies was associated with a lower risk of SARS-CoV-2 infection in infants ≤6 months old, with a more pronounced effect from natural maternal immunity. However, its predictive value was limited, suggesting that additional immunological factors may contribute to infant susceptibility.

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