Three 'classic' Covid symptoms first identified at the start of the pandemic are less common in Omicron cases, according to an expert.
The three symptoms in question are fever, cough and loss of taste or smell.
UK epidemiologist and professor Tim Spector claims the 'virus has evolved', meaning the symptoms have changed with it.
Prof Spector said 'only half' of people on the ZOE Covid Symptom App are showing the three main Covid symptoms.
He is the co-creator of ZOE, which has been tracking symptoms of the virus since March 2020, the Daily Record reports.
Some 480 million reports on the ZOE app show a significant trend toward different symptoms than earlier in the pandemic.
"Importantly, we found that only half of people with COVID had any of the classic three symptoms of fever, cough or loss of sense of smell," Prof Spector wrote in a blog post on newsGP, a news site for Australian GPs.
He also listed the top 20 Omicron symptoms being reported.
ZOE, whose data analysis is done by King's College London, analysed symptoms of the Delta and Omicron variants. They found 'no significant' differences between Delta and Omicron symptoms.
However, there were 'clear differences' in how common some symptoms were in Omicron infections. None of the three classic symptoms made it into the top five, which are: runny nose, headache, any fatigue, sneezing and sore throat.
Persistent cough came in at number six, followed by fever at 10, altered smell at 13 and loss of smell at 17 - which used to be number 10 in October.
Prof Spector wrote: " Omicron appears to be continuing the trend set by Delta. It’s causing symptoms that are much more like a regular cold, particularly in people who’ve been vaccinated, and fewer general systemic symptoms, such as nausea, muscle pains, diarrhoea and skin rashes."
He added: "For example, anosmia (loss of smell) was in the top 10 in October but has fallen to 17th place. What was once a key indicator of COVID is now only seen in around one in five people testing positive. And according to our data, less than a third of people (29%) will ever experience a fever, which is also far less common than we’ve seen in the past."
Researchers compared reports from October when Delta was the dominant variant to data from December when Omicron spread in the UK.
They also analysed data from contributors who'd been told by the government that their PCR was suspected or confirmed Omicron.
