Fri 17 Jul 2020
T-cell responses may be as important as antibody driven immunity
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Researchers at the Karolinska Institute in Sweden have shown there is a much higher proportion of people with cell-mediated immunity to SARS-CoV-2 than expected. In the study, the researchers performed immunological analyses of samples from over 200 people, many of whom had mild or no symptoms of COVID-19 as well as healthy controls.
The researchers carried out extensive testing of SARS-CoV-2-specific T cell responses in a large group of unexposed individuals as well as exposed family members and individuals with acute or convalescent COVID-19. People with acute phase SARS-CoV-2-specific T cells displayed a highly activated set of T-cell attributes that correlated with various clinical markers of disease severity. After recovery people had SARS-CoV-2-specific T cells that displayed a stem-like memory pattern. Importantly, SARS-CoV-2-specific T cells were detectable in antibody-seronegative family members and individuals with a history of asymptomatic or mild COVID-19.
Marcus Buggert, assistant professor at the Center for Infectious Medicine, Karolinska Institutet, and one of the paper's main authors said, "Advanced analyses have now enabled us to map in detail the T-cell response during and after a COVID-19 infection. Our results indicate that roughly twice as many people have developed T-cell immunity compared with those who we can detect antibodies in." They also found that approximately 30 per cent of the Swedish blood donors who’d given blood in May 2020 had COVID-19-specific T cells, a figure that’s much higher than previous antibody tests have shown. This is potentially good news as it might mean that the “herd immunity” threshold where enough people have immunity to give protection to those who are not immune could be reached with or without a vaccine. The exact threshold is currently unknown but estimates range from 50-80% of the population need to have immunity.
Unfortunately, this type of testing of T-cell responses is very complex, time-consuming and expensive and is unsuitable for routine clinical use. It is possible that further work may uncover a simple test suitable for routine use that correlates well with other markers of T-cell immunity but this is not assured.
The Karolinska Institute stated, “Our collective dataset shows that SARS-CoV-2 elicits robust memory T cell responses akin to those observed in the context of successful vaccines, suggesting that natural exposure or infection may prevent recurrent episodes of severe COVID-19 also in seronegative individuals. SARS-CoV-2-specific memory T cells will likely prove critical for long-term immune protection against COVID-19.”
Karolinska Institute press release
Full study (not yet peer-reviewed)