If Covid-19 is not spreading in the community, volunteers will find it difficult to catch, meaning scientists can’t prove whether the vaccine actually makes any difference.

Some 10,000 people are being recruited to test the jab over the coming weeks, but Prof Hill said he expected fewer than 50 of those to catch the virus, and the results could be deemed useless if fewer than 20 test positive.

‘We said earlier in the year that there was an 80 per cent chance of developing an effective vaccine by September,’ he told the paper.

‘But at the moment, there’s a 50 per cent chance that we get no result at all.

‘We’re in the bizarre position of wanting Covid to stay, at least for a little while. But cases are declining.’