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

BACKGROUND: 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. METHODS: A retrospective observational study was conducted in infants ≤6 months of age 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. RESULTS: 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%). CONCLUSIONS: 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.