Natural defences against a common cold could offer some protection against Covid-19, too, research suggests.
The small-scale study, published in Nature Communication, involved 52 individuals who lived with someone who had just caught Covid-19.
Those who had developed a “memory bank” of specific immune cells after a cold – to help prevent future attacks – appeared less likely to get Covid-19.
Experts say no-one should rely on this defence alone, and vaccines remain key.
But they believe their findings could provide useful insight into how a body’s defence system fights the virus.
Covid-19 is caused by a type of coronavirus, and some colds are caused by other coronaviruses – so scientists have wondered whether immunity against one might help with the other.
But the experts caution that it would be a “grave mistake” to think that anyone who had recently had a cold was automatically protected against Covid-19 – as not all are caused by coronaviruses.
The Imperial College London team wanted to understand better why some people catch Covid-19 after being exposed to the virus and others do not.
They focused their study on a crucial part of the body’ s immune system – T-cells.
Some of these T-cells kill any cells infected by a specific threat – for example, a cold virus.
And, once the cold has gone, some T-cells remain in the body as a memory bank, ready to mount a defence when they next encounter the virus.
In September 2020, researchers studied 52 people who had not yet been vaccinated but who lived with people who had just tested positive for Covid-19.
Half the group went on to get Covid-19 during the 28-day study period and half did not.
A third of the people who did not catch Covid-19 were found to have high levels of specific memory T-cells in their blood.
These were likely to have been created when the body had been infected with another closely-related human coronavirus – most frequently, a common cold, they say.
Researchers accept other variables – such as ventilation and how infectious their household contact was – would have an impact on whether people caught the virus, too.
Dr Simon Clarke, at the University of Reading, said although this was a relatively small study, it added to the understanding of how our immune system fights the virus and could help with future vaccines.
He added: “These data should not be over-interpreted. It seems unlikely that everyone who has died or had a more serious infection, has never had a cold caused by a coronavirus.
“And it could be a grave mistake to think that anyone who has recently had a cold is protected against Covid-19, as coronaviruses only account for 10-15% of colds.”
Professor Ajit Lalvani, senior author of the study, agreed vaccines were key to protection.
He added: “Learning from what the body does right could help inform the design of new vaccines.”
Current vaccines specifically target spike proteins that sit on the outside of the virus, but those spike proteins can change with new variants.
But the body’s T-cells target internal virus proteins, which do not change as much from variant to variant, meaning vaccines harnessing the work of T-cells more closely could provide broader, longer-lasting protection against Covid-19, he said.