Prevalence of Salivary IgA Reacting with SARS-CoV-2 among Japanese People Unexposed to the Virus

While the COVID-19 pandemic caused by SARS-CoV-2 has posed a threat to public health as the number of cases and COVID-19-related deaths are increasing worldwide, the incidence of the virus infection are extremely low in Japan compared with many other countries. To explore the reason for this strange phenomenon, we hypothesized the high prevalence of “natural” secretory IgA in saliva as mucosal IgA reacting with SARS-CoV-2, and thus surveyed the positivity for, as well as levels of, such reactive salivary IgA in a cohort of Japanese people of a wide range of age. The major findings were that 95/180 (52.78 %) of overall individuals who had not been exposed to SARS-CoV-2 were positive for salivary IgA with the levels ranging from 0.002 to 3.272 ng/ml, and that there may be a negative trend in positivity for salivary IgA according to age. These results suggest a role of mucosal IgA in host defense against SARS-CoV-2 infection.