Mild cases of coronavirus disease 2019 (COVID-19) can trigger robust memory T cell responses, even in the absence of detectable virus-specific antibody responses, researchers report August 14 in the journal Cell. The authors say that memory T cell responses generated by natural exposure to or infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) the virus that causes COVID-19 may be a significant immune component to prevent recurrent episodes of severe disease.

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"We are currently facing the biggest global health emergency in decades," says senior author Marcus Buggert of the Karolinska Institutet. "In the absence of a protective vaccine, it is critical to determine if exposed or infected people, especially those with asymptomatic or very mild forms of the disease who likely act inadvertently as the major transmitters, develop robust adaptive immune responses against SARS-CoV-2."

To date, there is limited evidence of reinfection in humans with previously documented COVID-19. Most studies of immune protection against SARS-CoV-2 in humans have focused on the induction of neutralizing antibodies. But antibody responses tend to wane and are not detectable in all patients, especially those with less severe forms of COVID-19. Research in mice has shown that vaccine-induced memory T cell responses, which can persist for many years, protect against the related virus SARS-CoV-1, even in the absence of detectable antibodies. Until now, it was not clear how SARS-CoV-2-specific T cell responses relate to antibody responses or to the clinical course of COVID-19 in humans.