Researchers at the Karolinska Institutet in Sweden found that people with mild or no symptoms had developed “T-cell” immunity — despite testing negative for antibodies that fight the novel disease.
T-cells are a type of white blood cell that recognise infected cells and are an essential part of the immune system.
The researchers also found that T-cells could be a source of immunity for twice as many people as Covid-19 antibodies.
“Advanced analyses have now enabled us to map in detail the T-cell response during and after a Covid-19 infection,” said Marcus Buggert, assistant professor at the Centre for Infectious Medicine, Karolinska Institutet, and one of the paper’s main authors.
“Our results indicate that roughly twice as many people have developed T-cell immunity compared with those who we can detect antibodies in.”
Scientists conducted immunity tests on samples from over 200 people between March and May 2020. This included Covid-19 inpatients at Karolinska University Hospital in Stockholm and their asymptomatic family members.
The team also analysed the blood of donors who had not been infected with the disease. According to the researchers, the findings suggest that levels of immunity in the wider-public could be much higher than previously thought.
However, there is no evidence to suggest that those who had developed T-cell immunity in the testing would not get infected in the future.
“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 Soo Aleman, a consultant at the Karolinska Institutet.
“Moreover, roughly 30 per cent 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.”
Professor Hans-Gustaf Ljunggren, co-senior author, added: “Our results indicate that public immunity to Covid-19 is probably significantly higher than antibody tests have suggested. If this is the case, it is of course very good news from a public health perspective.”
The study, published in the bioRxiv portal ahead of peer review, found that T-cell response was consistent with measurements taken after approved vaccinations for other viruses such as flu.
More studies on both T-cells and antibody production in the Covid-19 context are needed to understand how long-lasting immunity is, the researchers added.
Professor Danny Altmann, British Society for Immunology spokesperson and Professor of Immunology at Imperial College London, said: “Among the many studies of cellular (T cell) immunity to SARS-CoV-2 that have appeared in the past few months, this is one of the most robust, impressive and thorough in the approaches used.
“It adds to the growing body of evidence that many people who were antibody-negative actually have a specific immune response as measured in T cell assays, confirming that antibody testing alone under-estimates immunity.
“However, the big unknown for the moment is which parameters of immunity offer the most faithful indicator of true, protective immunity from future infection.”
He added: “So far, there is a sense from some studies that functional, virus-neutralising antibody is one such correlate of protection. We urgently need experimental studies to help confirm whether T cell immunity alone can give protection.”
A UK study published by the Office for National Statistics last month suggested that one in 20 people in the UK (5 per cent) had been infected with Covid-19. For London, the figure was one in six (17 per cent).
Experts are still unsure as to what level of immunity is given to a person who has recovered from the illness and how long that may last. The UK has reported more than 300,000 cases of coronavirus and recorded over 43,000 deaths, according to official figures.
Earlier this week, Leicester was placed on lockdown following a surge in infections, with experts warning that more local shutdowns could be enforced if people flouted social distancing guidelines.
Sweden, one of the few countries in the world not to impose a lockdown, has recorded more than 68,000 cases but less than 6,000 deaths.
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