Scanning electron micrograph of a human T-cell
NIH/National Institute of Allergy and Infectious Diseases
Throughout the coronavirus pandemic there have been fierce debates over the science – when to lock down, whether face coverings help and whether children are less susceptible, for example. The latest row is over whether we have been ignoring a crucial part of our immune response to the virus: T-cells.
This matters because if people have more immunity to the virus than we thought, then perhaps we could abandon some covid-19 countermeasures. This was the case made by US President Donald Trump’s newest adviser on covid-19, Scott Atlas at Stanford University in California. It has also been championed by others who argue against lockdowns. Is there any truth to the idea?
The immune system has two main arms to fight off pathogens such as the coronavirus. The one we hear most about consists of antibodies, small molecules that can recognise specific pathogens and target them for destruction.
Antibodies against the coronavirus can be measured in the blood and so surveys quickly began to gauge the proportion of people who have them. But even in places hit hard by the pandemic, antibody levels aren’t high enough to give herd immunity, which occurs when enough people are immune to the virus that it can no longer easily spread.
Researchers have estimated that 65 per cent of a population would need to be immune to achieve herd immunity, based on how contagious the virus is. In London, antibody levels were about 10 per cent between 26 April to 9 August. For England as a whole, as with many European countries, it is in single digits.
These levels are widely taken as indicating how many people have been infected by the coronavirus. But the picture is more complex than this because of the second arm of our immune system, T-cells. Antibodies are sometimes seen as more important because they can stop viruses from entering the body. But once viruses make it inside, only T-cells can kill infected cells.
It takes a few days to obtain results for tests of T-cell activity against the coronavirus, compared with as little as 90 minutes for antibody tests, but a few groups have been testing on a small scale. They have found T-cells that react to the coronavirus in 10 to 50 per cent of people tested.
That doesn’t necessarily mean that up to half the population is immune to covid-19, says Alessandro Sette at the La Jolla Institute for Immunology in California. Some of these studies, including one by Sette and his colleagues, looked at blood donations given before the current pandemic, to test for pre-existing immunity to the coronavirus. Others examined samples from people with covid-19.
The most likely explanation is that the pre-pandemic blood samples that tested positive were from people who had previously caught milder coronaviruses, such as the ones that cause colds, and their T-cells are reacting to the one that causes covid-19. It is probable, although by no means definite, that such people would get less sick with covid-19, but they could still get infected – and pass it on to others, says Sette.
However, a Swedish study that tested about 200 people, including some known to have had covid-19 and their family members, found that those who had been sickest with covid-19 had more T-cell activity. This suggested it was directed against the current coronavirus, not old ones, says Marcus Buggert at the Karolinska Institute in Stockholm, who worked on the study. “But we can’t say every single T-cell was induced by this new virus,” he says.
As with antibodies, it is unclear how long T-cell immunity will last. “I have seen (people) using our data to say we should open up society. I definitely do not want that,” says Buggert.
T-cells could explain some puzzling anomalies in antibody testing. “We have had people with confirmed cases of covid-19. Their antibody tests have come back negative, but their T-cells tested positive. That suggests antibody tests are not telling us the whole picture,” says James Hindley at UK firm Indoor Biotechnologies, which has developed a relatively fast and simple T-cell test.
The firm’s work hasn’t yet been published, and its test has so far only been used on about 100 people. But Hindley’s team has found a few people testing positive for T-cell activity whose spouse had confirmed covid-19, yet they themselves somehow avoided it, as far as they know. “It raises the question of whether the T-cells kept the virus at bay,” says Hindley.
It is unlikely that questions such as these will be resolved until T-cell testing becomes much more common. Until then, says Hindley, the growing body of T-cell work should be seen as cause for hope – but not complacency.
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