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Unexposed people may have covid-19 specific memory T-cells

Date: 13 July 2020 Tags: Miscellaneous


At least five studies have shown that people unexposed to and not infected with novel coronavirus may still exhibit T cell responses specific to the virus. 



An overwhelming percentage of adults are exposed to four different coronaviruses that cause the common cold. It is thought that SARS-CoV-2-specific T cell responses seen in healthy people might arise from memory T cells derived from exposure to ‘common cold’ coronaviruses.



  • The studies found 20-50% of healthy people display novel coronavirus-specific memory T cells. Whether the presence of pre-existing immunity from memory T cells offers clinical relevance when exposed to novel coronavirus is not known.

  • The healthy people studied were those tested prior to the pandemic or have not been infected with the novel coronavirus.

  • A few studies used peripheral blood mononuclear cells and plasma samples collected from healthy donors years before the coronavirus outbreak occurred in Wuhan.

  • Pre-existing T cell immunity to SARS-CoV-2 could be relevant because it could influence COVID-19 disease severity.

  • It is plausible that people with a high level of pre-existing memory CD4+ T cells that recognize novel coronavirus could mount a faster and stronger immune response upon exposure to the virus and thereby limit disease severity.

  • It can potentially facilitate an increased and more rapid neutralizing antibody response against the virus.

  • One of the ways of ascertaining the role of pre-existing immunity in protecting against the SARS-CoV-2 virus is by measuring such immunity and correlating with infection and severity of disease in such people when they are exposed to the virus.

  • The relationship between infections by common cold coronavirus and age is not well established and so, too, the immunity caused by infections by the four coronaviruses that cause the common cold.

  • This becomes particularly important as children do not show severe symptoms, while older people often do. 

  • There is a possibility that pre-existing T cell memory might influence vaccination outcomes. Pre-existing immunity could help elicit better immune responses against novel coronavirus, and these responses can manifest faster.

  • The pre-existing immunity can reduce the immune responses that the vaccine causes through a mechanism called the “original antigenic sin”.

  • It can also lead to antibody-mediated disease enhancement, where antibodies present at sub-neutralising concentrations can actually augment virus infection and cause more severe disease. 

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