LAUSR

Objective Uganda has registered fewer COVID-19 cases and deaths per capita than Western countries. Lower numbers of cases and deaths might be due to pre-existing cross-immunity induced by circulating common cold human coronaviruses (HCoVs) before the COVID-19 pandemic. To investigate pre-existing mucosal antibodies against COVID-19, we compared IgA reactivity to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and HCoVs in milk of mothers collected in 2018. Methods Ugandan and US milk samples were run on enzyme-linked immunoadsorbent assay (ELISA) to measure specific IgA to SARS-CoV-2 and HCoVs NL63, OC43, HKU1, and 229E spike proteins. Pooled plasma from US COVID-19 positive and negative cases were positive and negative controls, respectively. Results One Ugandan mother had high milk IgA reactivity against all HCoVs and SARS-CoV-2 spike proteins. Ugandan mothers had significantly higher IgA reactivity against the betacoronavirus HCoV-OC43 than US mothers (p = 0.018). By contrast, US mo thers had significantly higher IgA reactivity against the alphacoronaviruses HCoV-229E and HCoV-NL63 than Ugandan mothers (p < 0.0001 and 0.035, respectively). Conclusion Some Ugandan mothers have pre-existing HCoV-induced IgA antibodies against SARS-CoV-2 which may be passed to infants via breastfeeding.