Common colds may prime the immune system against Covid, scientists believe, after finding that some people never develop an infection despite repeated exposure to the virus.

Researchers at University College London (UCL) have discovered that some people have natural protection against Covid and seem to fight off an infection using pre-existing memory T-cells.

T-cells remember past infections and circulate in the body looking for invaders. They can directly kill cells or activate other parts of the immune system.

The scientists studied a cohort of 129 healthcare workers at high risk of an infection for 16 weeks and found that 57 never tested positive for the virus. 

When they looked to see what their immune systems were doing, they discovered they had mounted a "robust" T-cell response to Covid, suggesting they had encountered the virus but managed to shake it off.

Crucially, the T-cells had ramped up targeted areas of the genetic code shared by both colds and Covid. The T-cells were so effective in clearing out the infection that the healthcare workers never developed antibodies to the disease.

Francois Balloux, Professor of Computational Systems Biology at UCL, said: "The evidence points to protection originating from prior exposure to endemic common cold coronaviruses [HCoVs]. Latent infection to low levels of SARS-CoV-2 might also have played a role.

"There may indeed be such a thing as ‘immunological dark matter’ against SARS-CoV-2, at least in some people who were extensively exposed to HCoVs pre-pandemic."

Prevelance of common cold in UK

Scientists have previously speculated that the reason older people are far more affected by Covid is because T-cells diminish in later life. Children also pick up far more colds than adults, which may also be one reason they are at less risk.

Last year, a study by Public Health England (PHE) found that one in four healthcare workers had high levels of T-cells which recognised Covid, suggesting they had some level of protection against the virus – but nearly half had never been infected.

In the four months of follow-up, nobody with a high T-cell count became infected with Covid, suggesting they were protected against it. However, it was unclear until now what was triggering the natural immunity. The latest research suggests T-cells that have encountered common colds also recognise Covid. 

Imperial College is also working on similar research and is expected to publish findings shortly.

Prof Ajit Lalvani, the chair of infectious diseases at Imperial, told a recent discussion at the Science Museum: "In the vast majority of cases, the infection is asymptomatic or the illness is mild. 

"This tells us that the majority of people actually have quite effective natural immunity to the virus – somehow their own immune responses temper proliferation of the virus and limit the symptoms, like a protective break, if you will. 

"But additionally we also know that many people are exposed to the virus and the majority don’t even get infected. So they have a kind of pre-existing natural immunity that acts like a protective shield, and allows them somehow to resist infection altogether."

Several studies had hinted at prior immunity. In blood donor samples taken in the US between 2015 and 2018, around half showed some kind of immune resistance to Covid even though they were taken years before the virus emerged. 

Likewise in the Netherlands, Covid-fighting T-cells were found in two of 10 people who had not been exposed to the virus. 

During the Swine Flu outbreak, scientists discovered that people had prior immunity to the H1N1 strain, probably through earlier exposure to flu.

The new results suggest that the current estimates of the number of people immune to Covid through vaccination or previous infection may be too low, because scientists are only measuring antibodies. 

Prof Balloux said:  "The results suggest that serological testing [antibody tests] might be missing a proportion of natural immunity."

The research is yet to be peer-reviewed, but is published as a preprint on the MedRxiv website.