Immunity to COVID-19 Higher Than Serological Tests Suggest

A new study shows that many people with mild or asymptomatic COVID-19 demonstrate a T-cell-mediated immunity, even if they have not tested positively for antibodies.

For the study, researchers at Karolinska Institutet (Solna, Sweden) and Karolinska University Hospital (KUH; Stockholm, Sweden) performed immunological analyses of samples from over 200 inpatients (many of whom had mild or no symptoms of COVID-19) at KUH, as well as their exposed asymptomatic family members who returned to Stockholm after holidaying in the Alps in March 2020. Healthy blood donors who gave blood during 2020 and 2019 (the control group) were also included.

The researchers then mapped the functional and phenotypic landscape of T-cell responses. They found that SARS-CoV-2-specific T cells were detectable in seronegative family members and individuals with a history of asymptomatic or mild COVID-19. The collective dataset showed 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 in seronegative individuals as well. The study was published on June 29, 2020, in in bioRxiv.

“T cells are a type of white blood cells that are specialized in recognizing virus-infected cells, and are an essential part of the immune system. Advanced analyses have now enabled us to map in detail the T-cell response during and after a COVID-19 infection,” said senior author Marcus Buggert, PhD, of KI. “Our results indicate that roughly twice as many people have developed T-cell immunity compared with those who we can detect antibodies in.”

“One interesting observation was that it wasn't just individuals with verified COVID-19 who showed T-cell immunity, but also many of their exposed asymptomatic family members,” said study co-author Soo Aleman, PhD, of KUH. “Moreover, roughly thirty percent of the 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.”

T cells are lymphocytes that originate in the bone marrow and mature in the thymus. They determine the specificity of immune response to antigens. They can be distinguished from other lymphocytes by the presence of a T-cell receptor on the cell surface. One of the more important T-cells are CD4+ helper cells which send signals to other types of immune cells, including CD8 killer cells, which then destroy the infectious particle.

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