Researchers said that headache, sore throat and runny nose are presently the most commonly reported symptoms linked to Covid infection in the UK.
Professor Tim Spector, who runs the Zoe Covid Symptom study, said catching the Delta variant can feel "more like a bad cold" for younger people, reports the BBC.
Although one may not feel very ill, they could be contagious and put others at risk; anyone who thinks they may have Covid should take a test, Spector suggested.
The classic Covid symptoms people should look out for, UK's National Health Service said are - cough, fever and loss of smell or taste.
However, Spector said these are now less common, based on the data the Zoe team has been receiving from thousands of people who have logged their symptoms on an app.
"Since the start of May, we have been looking at the top symptoms in the app users - and they are not the same as they were," he said.
The change appears linked to the rise in the Delta variant, first identified in India and now accounting for 90% of Covid cases in the UK.
Fever remains quite common but loss of smell no longer appears in the top 10 symptoms, Spector said.
"This variant seems to be working slightly differently," he remaked.
"People might think they've just got some sort of seasonal cold and they still go out to parties and they might spread around to six other people. We think this is fuelling a lot of the problem. The message here is that if you are young, you are going to get milder symptoms anyway," Spector said.
"It might just feel like a bad cold or some funny 'off' feeling - but do stay at home and do get a test."