Immunity to COVID-19 is probably higher than tests have shown

New research from the Karolinska Institute and Karolinska University Hospital shows 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 positive for the antibody test. According to the researchers, this means that public immunity is probably higher than antibody tests suggest. The article is freely available on the bioRxiv server and has been submitted for publication in a scientific journal.

“T cells are a type of white blood cell that specialize in recognizing virus-infected cells and are an essential part of the immune system,” says Marcus Buggert, assistant professor at the Center for Infectious Medicine, Karolinska Institute, and one of the leading authors of the article. “Advanced analyzes have now allowed us to map the T-cell response in detail during and after a COVID-19 infection. Our results indicate that approximately twice as many people have developed T-cell immunity compared to those in whom we can detect antibodies. ” “

In the current study, the researchers performed immunological analyzes on samples from more than 200 people, many of whom had mild symptoms or no symptoms of COVID-19. The study included inpatients at Karolinska University Hospital and other patients and their exposed asymptomatic relatives who returned to Stockholm after their vacation in the Alps in March. Healthy blood donors who donated blood during 2020 and 2019 (control group) were also included.

T-cell immunity in asymptomatic individuals

The consultant Soo Aleman and her colleagues at the infection clinic of the Karolinska University Hospital have monitored and evaluated patients and their families since the illness period.

“An interesting observation was that not only people with verified COVID-19 showed T-cell immunity but also many of their exposed asymptomatic family members,” says Soo Aleman. “Additionally, approximately 30 percent of blood donors who had donated blood in May 2020 had COVID-19-specific T cells, a figure that is much higher than previous antibody tests have shown.”

T cell response was consistent with measurements taken after vaccination with vaccines approved 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.

Very good news from a public health perspective.

“Our results indicate that public immunity to COVID-19 is probably significantly higher than antibody testing has suggested,” says Professor Hans-Gustaf Ljunggren of the Center for Infectious Medicine, Karolinska Institute, and co-author. “If this is the case, it is very good news from a public health perspective.”

T-cell tests are more complicated to perform than antibody tests, and therefore are currently only performed in specialized laboratories, such as the Karolinska Institute Center for Infectious Medicine.

“Longer and more longitudinal studies on both T cells and antibodies now need to be done to understand how long-lasting immunity is and how these different components of COVID-19 immunity are related,” says Marcus Buggert.

The results are so new that they have not yet undergone a peer review before being published in a scientific journal. Pending such review, the article has been published on a prepress server, bioRxiv.