Large numbers of Britons were already protected from coronavirus before the pandemic began because of previous exposure to common colds, a groundbreaking new study suggests.
Researchers at Imperial College found that half of people living with an infected person in the second wave – before vaccines were available – were carrying high levels of memory T-cells from colds that may have stopped them picking up the virus.
The study helps explain why some people never get infected, and also sheds light on why older people – who are less likely to pick up colds – are more susceptible, while children – who suffer many colds every year – are more protected.
The T-cells target proteins found deep within the core of many coronaviruses and rarely change even when the virus mutates on the outside.
The researchers believe that these core proteins could be used to create a universal vaccine that would fight all new variants. Current vaccines are based on the spike protein which sits on the outside of cells, and which is prone to mutating.
Prof Ajit Lalvani, senior author of the study and director of the NIHR Respiratory Infections Health Protection Research Unit at Imperial, told The Telegraph that the protective T-cells were likely to be present in a large number of people.
“It’s a good proportion of the population, a third in our study,” he said: “It explains the good outcomes or resistance to infection for some people."
He added: “It’s been a fundamental question since the start of the pandemic, why is there such a wide spectrum of outcomes in a naive population, some people in intensive care and dying and others not even getting infected?
“So it was postulated that exposure to common colds may leave memory T-cells that would protect people even though they’ve never seen Sars-CoV-2."
The immunity enigma
Early on in the pandemic, studies showed that some people carried immune cells that could recognise Covid-19 even though they had never been infected.
University College London even discovered that blood samples taken before the pandemic carried this immunity, but it was not known if it was actually stopping people picking up the virus.
To answer the question, Imperial recruited several hundred households in London last winter before the vaccine programme began, and took blood samples to measure levels of memory T-cells from colds before waiting to see who caught the virus.
During the study period 52 people ended up living with someone who tested positive for coronavirus.
The researchers found that there were significantly higher levels of cross-reactive T-cells from colds in the 26 people who did not become infected, compared with the 26 people who did become infected.
‘We’ve reached the Holy Grail’
Prof Lalvani added: “We’ve finally reached the Holy Grail which shows that those who were exposed but didn’t get infected had these types of T-cells. These are people who would have picked up a cold, one, two, three years before and their immune cells were in a memory state.
“Coronaviruses are a wide tribe, and these common cold viruses are quite far apart, yet even the T-cells targeting these distant relatives can cross-protect, so imagine if you took one of the core proteins of Sars-Cov-2 and put in a vaccine. The T-cells would cross-recognise variants which opens the door to a universal vaccine that will protect you against all current and future mutant strains as they arise.
“Such a vaccine would not need to be given as frequently because the memory T-cells last longer than the antibodies. We’re in this awful position where we need boosters every three months, but with T-cells you could cut it down to every year or two years.”
How many people have been vaccinated?
The study was carried out before omicron was circulating in Britain and before the vaccination programme began, meaning that most immunity is now likely to be from vaccine and past infection.
It is also likely that memory T-cells from common colds have now dropped in the population because of many months of lockdown, social distancing and mask-wearing, which has seen respiratory viruses plummet.
“You would expect if people have not recently seen the common cold virus they wouldn’t have had their usual bout of cold they would they would be more susceptible to more Covid,” Prof Lalvani added.
Scientists also warned that not all colds are caused by coronaviruses, so people should not think they are necessarily protected if they have recently had a cold.
Dr Simon Clarke, associate professor in cellular microbiology at the University of Reading, said: “It seems unlikely that everyone who has died or had a more serious infection, has never had a cold caused by a coronavirus.
“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 per cent of colds.”
Commenting on the research, Mala Maini, professor of viral immunology at University College London, added: "T-cells recognise viral fragments once they have got into cells, rather than blocking infection as antibodies can, so it is likely that the household members testing negative in this study had a transient abortive infection that didn’t get picked up by PCR tests rather than completely resisting infection.
“Just because somebody didn’t get infected once doesn’t mean their cross-reactive T-cells can protect them against more infectious variants so future vaccines that boost more robust and flexible immune responses are the key going forward.
The research was published in the journal Nature Communications.