In a new study, researchers found that many people with mild or asymptomatic COVID-19 demonstrate so-called T-cell-mediated immunity to the new coronavirus, even if they have not tested positively for antibodies.
This means that public immunity is probably higher than antibody tests suggest.
The research was conducted by a team from Karolinska Institutet and Karolinska University Hospital.
T cells are a type of white blood cells that are specialized in recognizing virus-infected cells and is an essential part of the immune system.
Advanced analyses have now enabled the team to map in detail the T-cell response during and after a COVID-19 infection.
The results indicate that roughly twice as many people have developed T-cell immunity compared with those who we can detect antibodies in.
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.
The study included inpatients at Karolinska University Hospital and other patients and their exposed asymptomatic family members who returned to Stockholm after holidaying in the Alps in March.
Healthy blood donors who gave blood during 2020 and 2019 (control group) were also included.
The team found that it wasn’t just people with verified COVID-19 who showed T-cell immunity but also many of their exposed asymptomatic family members.
Moreover, roughly 30% 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.
The T-cell response was consistent with measurements taken after vaccination with approved vaccines for other viruses.
Patients with severe COVID-19 often developed a strong T-cell response and an antibody response; in those with milder symptoms it was not always possible to detect an antibody response, but despite this many still showed a marked T-cell response.
These results indicate that public immunity to COVID-19 is probably significantly higher than antibody tests have suggested.
If this is the case, it is very good news from a public health perspective.
The team says larger and more longitudinal studies must now be done on both T cells and antibodies to understand how long-lasting the immunity is and how these different components of COVID-19 immunity are related.
One author of the study is Marcus Buggert, an assistant professor at the Center for Infectious Medicine, Karolinska Institutet.
The article is freely available on the bioRxiv server and has been submitted for publication in a scientific journal.
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