Pre-pandemic cross-reactive T-cell responses are associated with subsequent protection against SARS-CoV-2 infection, but not against symptomatic illness

For reasons that are not well understood, the severity of SARS-CoV-2 infections was lower in sub-Saharan Africa during the first wave of the COVID-19 pandemic. We thought that immune responses from past seasonal coronavirus infections may provide some protection against COVID-19. To see if this was true, we studied a group of female sex workers from Nairobi, Kenya who had blood samples taken in mid-2019 (pre-pandemic) and again 1 year later, shortly after the pandemic started. We checked for evidence of SARS-CoV-2 infection in their second blood sample, and looked at whether pre-pandemic T-cell responses (a type of immune response) against the SARS-CoV-2 virus protected against infection. We also tested for antibodies against the four seasonally circulating coronaviruses in their pre-pandemic samples. Our study includes 100 people who got infected with SARS-CoV-2 (cases) and 100 people who did not get infected (controls) during the first wave of the pandemic, matched for age and date of follow up. We found that individuals with stronger T-cell responses against SARS-CoV-2 prior to the start of the pandemic were less likely to be infected by SARS-CoV-2; however, among those who did become infected these prior immune responses did not lead to milder symptoms. Antibodies against the seasonal coronaviruses were found in all participants, suggesting that the SARS-CoV-2 immune responses found prior to the pandemic were induced by previous exposures to these seasonal coronaviruses.