As a result, we shouldn’t be surprised that some are more prone to catching viruses than others. We can see this happening in real-time in the laboratory. Researchers at Oxford University and Imperial College London are currently carrying out “challenge studies”, where volunteers are deliberately exposed to Covid, usually through a liquid solution sniffed into their nose, then kept in isolation and observed for two weeks.

All volunteers have received the same number of vaccines, and all are exposed to exactly the same quantity of Sars-Cov-2 (the virus that causes Covid), in exactly the same way. And yet, if it’s anything like previous challenge studies, scientists expect volunteers will mount notably different immune responses. Some will see their antibody and T-cells burst into action; others will not.

We can also see this playing out in hospitals. At the beginning of the pandemic, researchers at University College London recruited a large cohort of London-based healthcare staff for their COVIDSortium study. All of the volunteers were probably exposed to Sars-Cov-2 during their jobs. Their test results were monitored thoroughly. At the end of the trial, about 20 per cent of the healthcare staff showed signs of a clear-cut Covid infection, whilst 65 per cent had clearly not been infected.

But most interesting was the remaining 15 per cent. Members of this third group appeared to have experienced low-level “abortive infections”, not picked up on PCR tests. They didn’t have Covid antibodies in their blood, but they had a much higher T-cell count than average, with particularly high levels of the specific T-cell known to combat Covid. Essentially, their T-cells had nipped the virus in the bud before it ever got the chance to set up camp inside their bodies. It looked as though their immune systems already knew how to fight Covid, even though it was still the early days of the pandemic.

“They didn’t completely resist the infection, but they eliminated it so rapidly that it couldn’t be picked up by the standard test,” says Mala Maini, a professor of viral immunology at University College London, and lead author of the study.

Here was clear evidence that some people may be naturally immune to Covid. Prof Altmann, who was not involved in the study, says the results look “convincing”.

But what explains this natural immunity? The most likely theory is that these people’s immune systems have already been exposed to similar viruses, years or decades earlier. Sars-Cov-2 is one of a family of seven human coronaviruses, most of which cause the common cold. All of these viruses look fairly similar. When your T-cells learn how to fight one, they get better at fighting them all, it is thought.