Impact of pre-existing SARS-CoV-2 reactive T cells in uninfected individuals on COVID-19 mortality in different countries

Several recent studies identified SARS-CoV-2 reactive T cells in people without exposure to the virus. However, pathophysiological implications of these findings remain unknown. Here, the potential impact of pre-existing T cell reactivity against SARS-CoV-2 in uninfected individuals on markedly different COVID-19 mortality levels in different countries has been investigated. The inverse correlation is documented between the prevalence of pre-existing SARS-CoV-2 reactive T cells in people without exposure to the virus and COVID-19 mortality rates in different countries. In countries with similar levels of pre-existing SARS-CoV-2 cross-reactive T cells in uninfected individuals, differences in COVID-19 mortality appear linked with the extend and consistency of implementations of social measures designed to limit the transmission of SARS-CoV-2 (lockdown; physical distancing; mask wearing). Collectively, these observations support the model that the level of pre-existing SARS-CoV-2 reactive T cells is one of the important determinants of the innate herd immunity against COVID-19. Together with the consistent social measures directed to limit the virus spread, high levels of pre-existing SARS-CoV-2 reactive T cells appear significant determinants diminishing the COVID-19 mortality. Observations reported in this contribution should have significant impact on definitions of the herd immunity threshold required to effectively stop the pandemic in different countries across the globe.