Since May, I’ve closely followed research on T cell cross-reactivity to SARS-CoV-2 in unexposed individuals. This could hint at some degree of pre-existing immunity that’s potentially more widespread than previously believed, leading to a number of hopeful possibilities: Could herd immunity thresholds be lower than believed? Are seroprevalence studies underestimating the number of people infected (and thus overestimating infectional fatality rates)? Can we expect post-infection immunity to robust and long-lasting?
Two articles offer perspective on the emerging evidence.
“An intriguing new study of these memory T cells suggests they might protect some people newly infected with SARS-CoV-2 by remembering past encounters with other human coronaviruses. This might potentially explain why some people seem to fend off the virus and may be less susceptible to becoming severely ill with COVID-19.
… they looked at blood samples from 23 people who’d survived SARS. Their studies showed that those individuals still had lasting memory T cells today, 17 years after the outbreak. Those memory T cells, acquired in response to SARS-CoV-1, also recognized parts of SARS-CoV-2.
Finally, Bertoletti’s team looked for such T cells in blood samples from 37 healthy individuals with no history of either COVID-19 or SARS. To their surprise, more than half had T cells that recognize one or more of the SARS-CoV-2 proteins under study here. It’s still not clear if this acquired immunity stems from previous infection with coronaviruses that cause the common cold or perhaps from exposure to other as-yet unknown coronaviruses.
What’s clear from this study is our past experiences with coronavirus infections may have something important to tell us about COVID-19. Bertoletti’s team and others are pursuing this intriguing lead to see where it will lead—not only in explaining our varied responses to the virus, but also in designing new treatments and optimized vaccines.”
And from an editorial published in the BMJ:
“Preliminary studies from the US and Europe recently documented T cells specific to SARS-CoV-2 in people with acute covid-19 and in those recovering from infection. They report helper and killer T cells specific to SARS-CoV-2 in people with and without antibodies.6 More unexpectedly, they found specific T cells in people with no history of exposure to SARS-CoV-2—individuals who had repeatedly swabbed negative for the virus.4 These cells have even been found in stored blood taken before the pandemic (2015-18). Finally, the studies identified strong T cell memory responses in people recovering from covid-19. Memory cells are critical for protective and enduring immunity.
What are the implications of these early findings? Principally, these studies show that a good T cell immune response and immunological memory accompany natural exposure to or infection with SARS-CoV-2, that evidence of these responses is present in some people who have apparently never encountered the virus, and that T cell immune responses can exist in the absence of detectable antibodies.
That some “virus naive” participants in early studies had pre-existing memory helper (50% of participants) and killer T (20%) cells with potential activity against SARS-CoV-2 is intriguing. These cells might arise from cross reactions to other circulating coronaviruses, such as some common cold viruses, and might be a welcome hint of possible background immunity to covid-19 in populations at risk—even in the absence of antibodies.”