A third of those who remained asymptomatic through their coronavirus infection could have developed immunity to Covid-19, new research suggests.
A study led by the Karolinska Institutet in Sweden found that public Covid-19 immunity levels could be as high as 30 per cent.
The findings showed that people with mild or no symptoms may have developed 'T-cell' immunity, despite testing negative for the antibodies usually needed to fight the disease.
Professor Hans-Gustaf Ljunggren, co-senior author of the paper, said: “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.”
Here's everything you need to know:
What are T-cells?
T-cells play a central role in the immune response.
Their important role in controlling and shaping the immune response comes through their provision of a variety of immune-related functions, such as ‘immune-mediated cell death’.
Certain types of T-cells – also known as 'killer cells' – are able to directly kill virus-infected cells, and can even ‘recruit’ other cells when mounting an immune response.
Others – affectionately known as 'helper cells' – can indirectly kill cells identified as foreign by determining if and how other parts of the immune system respond to threats.
How do they affect coronavirus patients?
Research undertaken by London's Francis Crick Institute in May 2020 found those with severe forms of Covid-19 had extremely low T-cell number, and that the disease may be able to 'overpower' them.
The Insitute's Professor Adrian Hayday said it was a "great surprise" to see what was happening with the immune cells.
"They're trying to protect us,” Prof Adrian told the BBC, “but the virus seems to be doing something that's pulling the rug from under them, because their numbers have declined dramatically.”
Scientists looked at immune cells in the blood of 60 Covid-19 patients and found an apparent crash in the numbers of T-cells.
Normal healthy adults have between 2,000 and 4,000 T-cells per microlitre (0.001ml) of blood; the Covid patients the team tested had around 200 – 1,200.
"The virus that has caused this completely Earth-changing emergency is unique - it's different. It is something unprecedented,” said Prof Adrian.
"The exact reason for this disruption - the spanner in the works of the T-cell system - is not at all clear to us.
"This virus is really doing something distinct and future research... needs to find out the mechanism by which this virus is having these effects."
What does the new research mean?
Despite findings that Covid-19 may be able to bypass T-cells in patients with low numbers of them, the Karolinska Institutet's research found the cells could be a source of immunity for twice as many people as Covid-19 antibodies.
Marcus Buggert, assistant professor at the Centre for Infectious Medicine, Karolinska Institutet, and one of the paper’s main authors, said: “Advanced analyses have now enabled us to map in detail the T-cell response during and after a Covid-19 infection.
“Our results indicate that roughly twice as many people have developed T-cell immunity compared with those who we can detect antibodies in.”
But there is still no evidence to suggest those with T-cell immunity cannot go on to become infected in the future.
Soo Aleman, a consultant at the Karolinska Institutet, said: “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.”
So this is good, right?
While the findings certainly represent significant progress in the understanding of the disease, the researchers say further study is needed to understand how long-lasting immunity is.
Professor Danny Altmann, British Society for Immunology spokesperson and Professor of Immunology at Imperial College London, described the Karolinska Institutet's findings as "the most robust, impressive and thorough."
But he said, "the big unknown for the moment is which parameters of immunity offer the most faithful indicator of true, protective immunity from future infection.”