Symptoms associated with the omicron Covid variant could be similar to those that normally accompany a cold.

Experts are warning that the variant could be easily mistaken for a mild, everyday illness.

The ZOE Covid Study analyses thousands of Covid symptoms uploaded to an app by the British public.

Health officials are warning people to not underestimate the omicron variant.

Symptoms associated with the omicron Covid-19 variant could be similar to those that normally accompany a cold, but experts are warning people that they should not underestimate the risks posed by the more transmissible variant.

One British study has now suggested that omicron infections could be associated with symptoms that make it easy to mistake it for an everyday illness like a cold.

The ZOE Covid Study, which analyses thousands of Covid symptoms uploaded to an app by the British public, looked this week at symptoms associated with Covid cases in London that were recorded over two separate weeks in October and December, that is, before (as far as we know) and after omicron was spreading in the capital.

This initial analysis found similarities between the delta and the omicron variant, suggesting the latter hasn't mutated back into the more flu-like symptoms of previous Covid strains. The team said that the top five symptoms reported in the ZOE app in those two different weeks were:

Runny nose

Headache

Fatigue (either mild or severe)

Sneezing

Sore throat

London was selected for the ZOE analysis due to the higher prevalence of omicron compared to other regions. The omicron variant is already the dominant variant in the capital and will soon account for nearly all infections in the capital and wider U.K.

Experts predict this phenomenon is likely to be repeated across other countries around the globe. This time, with the omicron variant, cases could be harder to spot, however.

Professor Tim Spector, lead scientist on the ZOE Covid Study app, said that there was a risk that potential omicron cases could well be mistaken for minor colds.

"As our latest data shows, omicron symptoms are predominantly cold symptoms, runny nose, headache, sore throat and sneezing, so people should stay at home as it might well be Covid," Spector said in ZOE's latest report Thursday.

"Hopefully people now recognise the cold-like symptoms which appear to be the predominant feature of omicron," he added.

Spector noted, as have other British experts on Covid, that the omicron looks set to be the dominant strain in the U.K. by Christmas, with many people now questioning whether the U.K. could go into a lockdown in the new year.

"In the New Year cases could hit a peak higher than anything we've ever seen before," Spector noted, although he hoped that there could be some reversal of a rise in cases in London as people are encouraged by Prime Minister Boris Johnson and leading health experts to curtail their social mixing, work from home and wear face masks.

What we know of omicron

It would be a big mistake to underestimate the risks posed by the omicron variant, despite some evidence that it causes milder symptoms more akin to a cold than flu.

Experts have judged omicron as being far more transmissible than the delta variant and believe it will soon become the dominant variant internationally. Omicron's rise to prominence is remarkable given the fact it was only designated as a "variant of concern" by the World Health Organization on Nov. 26, two days after South Africa reported that it had detected it.

Early, small studies showed that while it was more virulent than the delta variant, it might cause less severe infections but that remains to be seen at a wider, real-world level with an infected person's age (younger people tend to experience milder Covid infections), general state of health and vaccination status (including when they were fully vaccinated as we know immunity wanes after six months) being factors in how an illness is experienced.

The South African doctor who first spotted the variant among her patients has said that the initial symptoms she saw in her own surgery were "extremely mild"but this was observational evidence on a small group of people.

Vaccine makers have said that the variant undermines the efficacy of a full course of Covid vaccination but that a booster shot helps to restore much of the shots' protection against severe infection, hospitalization and death.

Rising hospitalization

Experts are warning that a rise in hospitalizations is now inevitable given the increased transmissibility of omicron.

South Africa has seen a rise in hospitalizations (although the majority of admissions have been unvaccinated people) and the U.K. is seeing an increase too, with U.K. Prime Minister Boris Johnson noting on Wednesday that the country was now seeing what he called "the inevitable increase in hospitalizations, up by 10% nationally week on week and up by almost a third in London."

To date, there have been just over 10,000 cases of omicron in the U.K., with case numbers doubling every two days or less, and experts predict this is a vast understatement of the true number of omicron infections.

The first two confirmed U.K. cases of the variant were announced on Nov. 27 and they had links to travel to South Africa. Although soon after, cases of community transmission were confirmed, meaning the variant was likely circulating earlier.

Omicron is making its presence felt in the U.K. with Covid cases surging prompting the government to race to get booster shots into people's arms. On Wednesday, the U.K. reported its highest number of Covid cases since the pandemic began with 78,610 new infections.

England's chief medical officer, Professor Chris Whitty, called on Wednesday for "serious caution" over hospitalization data, particularly in South Africa, that suggested omicron might cause a milder disease. There, he said, immunity levels were different among the population due to a recent wave of delta infections.

He noted that more data was still needed on hospitalizations, severe disease and deaths but that "all the things that we do know [about omicron] are bad."

David Nabarro, the World Health Organization's special envoy on Covid-19, later told Sky News Wednesday that there was a "very serious situation indeed" in the U.K.

"The rise that you're seeing in the U.K. today is just the beginning of an extraordinary acceleration," he said.

"There are two epidemics going on; delta and omicron. And it is an emergency situation for the British health service. It will get extremely serious within the next two weeks, perhaps quicker."

Nabarro added the spread of the omicron variant was "serious in the U.K., it's serious in Europe, and it's serious for the world."

"We're concerned that people are dismissing omicron as mild," he said. "Even if omicron does cause less severe disease the sheer number of cases will once again overwhelm health systems.

CNBC