A quarter of people may already be immune to coronavirus even though many of them have never been infected, a new study by Public Health England (PHE) suggests.

Over the past few months, researchers have followed nearly 2,850 key workers from the police, fire and health services to gauge levels of immunity to the virus.

They discovered that, by June, one in four 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.

Researchers believe they probably picked up immunity from similar coronaviruses such as those that cause the common cold. In the four months of follow-up, nobody with a high T-cell count became infected with Covid, suggesting they were protected against it.

Dr Peter Wrighton-Smith, the CEO of Oxford Immunotec, the company that developed the T-cell test for trial, said it showed antibody testing alone may underestimate the number of people already immune to the virus.

"Here we are talking about people on the front line, so 25 per cent may be a bit high, but this suggests we are not seeing a true picture through antibody surveillance surveys and that many more people have T-cell immunity," he said. "It also suggests that models predicting the outcome of the pandemic are unduly thinking more people are going to get it than really are.

"In this data, there is a significant cohort of people who have T-cells without antibodies. Clearly some of this may be because those antibodies have waned over time, but some is probably immunity from other infections. There has been growing speculation that there is a phenomenon of cross-reactive immunity in which people who have been exposed to a common cold virus will also be protected from Covid."

The rise and fall of coronavirus in the body

At least six studies have now reported T-cell activity against coronavirus in 20 to 50 per cent of people with no known exposure to the virus.

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

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

Currently, the MRC Biostatistics Unit Covid-19 Working Group at Cambridge University estimates that around 7.4 million people in England have had coronavirus – about 13 per cent. But the study suggests the true number of people who are immune could be closer to 14 million, or even higher, because more people have been infected since June.

The study also found that, among those who had never had coronavirus, the under-30s were more likely to have high levels of T-cells compared to the over-60s, which could help explain why young people are at less risk. 

Professor Karol Sikora, Dean of Medicine at the University of Buckingham, carried out tests early on in the pandemic and discovered that many young people who had been infected with coronavirus did not develop antibodies, leading him to believe T-cells were important for protection. 

Commenting on the data, he said: "The importance of these findings can’t be overstated. Estimates of remaining susceptible people drawn from antibody testing have to be disregarded. Most significant is the stark age difference. Does this suggest there is more immunity in a younger London, for example, hence a milder second wave? 

"Also, perhaps countries that have been hit less hard have more residual immunity from old coronaviruses. Huge questions have been asked, and Government scientists need to be open with the answers."

An update on the study was published by PHE on Wednesday, with the new data on cross-reactivity added to the "What we have discovered so far" section.

"About half the people with high levels of T-cells in their blood have not had Covid-19, as far as we could tell – the cells were probably there because of previous infection with coronaviruses other than SARS-CoV-2," the summary said.

The study is also the first to show that T-cells offer protection against coronavirus.

Dr David Wyllie, a consultant microbiologist at Public Health England and the lead author of the study, said: "Four months into the study, 20 participants with lower T-cell responses had developed Covid-19, compared with none among individuals with higher T-cell responses.

"This suggests individuals with higher numbers of T-cells recognising SARS-CoV-2 may have some level of protection from Covid-19, although more research is required to confirm this."