But although they may not feel very ill, they could be contagious and put others at risk.
“It's more like a bad cold in this younger population and people don’t realise that and it hasn’t come across in any of the government information,” said Professor Tim Spector, who runs the Zoe Covid study.
“This means that people might think they’ve got some sort of seasonal cold and they still go out to parties and might spread around to six other people and we think this is fuelling a lot of the problem.”
He added that the top symptoms reported to the app were now different from what they once were.
“The number one symptom is headache, followed by runny nose, sore throat and fever. Not the old classic symptoms.
“We don’t see loss of smell in the top ten anymore, this variant seems to be working slightly differently.”
“The UK really does now have a problem and we’ll probably be seeing, in a week, 20,000 cases and by 21st June well in excess of that number,” he said.
“Most of these infections are occurring in unvaccinated people. We’re only seeing slight increases in the vaccinated group and most of those in the single vaccinated group,” he said.
He advised people who were feeling unwell to stay at home for a few days, use lateral flow tests with a confirmation PCR test if they get a positive result.
It comes as the Delta variant now makes up 90 per cent of Covid cases in the UK.
On Sunday, the UK recorded 7,490 new Covid cases and eight more deaths - the fifth day in a row that new infections have exceeded 7,000.