“So there is probably background T-cell immunity in people before they see the coronavirus, and that may be relevant that many people get a pretty asymptomatic disease.

“Those T-cells get a bit tired once you’re beyond the age of 65 and may not be as effective at removing a virus, so that may explain a number of different features of the disease.”

The vaccine being developed by Oxford University has been found not only to stimulate antibodies but also to boost T-cell response. But many more people may already have some protection, suggesting herd immunity will be easier and quicker to establish, the research suggests. 

Professor Sarah Gilbert, of the vaccine team, said: “It’s possible that we are underestimating natural or already acquired immunity to this virus, and we really need to keep an eye on it.

“There is certainly evidence that people who have been infected with Covid-19 have not developed antibodies but have developed a T-cell response, and that would be likely to protect them against another infection. I think you have to keep an open mind about whether you have a large number of people who have protective T-cells in the absence of antibodies.”

A recent study suggested children may be protected from coronavirus because they catch so many colds. 

Unlike other conditions, such as flu, children rarely develop a severe case of Covid-19 even though they appear to catch the disease as much as adults.

The common cold is caused by four different types of coronavirus which circulate in the community and are largely harmless. But while adults pick up a cold around two to four times a year, school age children catch an average of 12 colds annually, studies have shown, which may give them an advantage in battling off coronavirus.

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