LONDON — With cases of the Omicron variant doubling every three days and the government doing an about-face on restrictions it had long resisted, Britain is bracing for a new coronavirus surge, unsure if it will be a relatively minor event or a return to the dark days of earlier pandemic waves.
So far, the number of Omicron cases — 817 confirmed by Thursday, though officials say the real figure is likely much higher — is small compared with the daily average of 48,000 new coronavirus cases overall. But the government’s Health Security Agency warned that if the recent growth rate continues, “we expect to see at least 50 percent of Covid-19 cases to be caused by the Omicron variant in the next two to four weeks.”
Early evidence in Britain backs up tentative findings elsewhere, notably in South Africa, where the heavily mutated new variant is already widespread: It appears to be the most contagious form of the virus yet, a previous case of Covid-19 provides little immunity to it, and vaccines seem less effective against it. But it also seems to cause less severe illness than earlier variants.
Britain’s experience with Omicron may be a harbinger of what others can expect. Until now, it has been looser about social restrictions than many other nations in Western Europe, and Britain ordinarily has extensive travel to and from South Africa, so it could be the first wealthy country to be hit hard by Omicron. It also has one of the world’s most robust systems for sequencing viral genomes, so it can identify and track new variants earlier and more thoroughly than countries.
“I think we are looking at a horrible winter,” said Peter English, a retired consultant in communicable disease control, noting the exponential spread of Omicron.
Much remains to be learned about the variant, but experts say that what is known so far is worrying. Jeffrey Barrett, the director of the Covid-19 genomics initiative at the Wellcome Sanger Institute in Britain, said the new data made one thing clear: “It will spread very fast, even in countries that have a very high vaccination rate like the U.K.”
The current estimate that Omicron is doubling roughly every three days, in a country where 70 percent of people are fully vaccinated and 32 percent have had a booster dose, is “really striking,” he added.
“We haven’t seen that kind of rate of growth since I think the March 2020 time when the original virus was spreading in a totally naïve population, when none of us really knew anything about it,” he said.
Even if Omicron infection is less severe on the whole, experts warn that if it leads to an enormous surge in cases, even a small percentage of them resulting in seriously ill patients could once again overwhelm hospitals and cause a spike in deaths.
Dr. Barrett said he was more worried than he was about the previous variants. The possibility that Omicron cases are less severe and that vaccines could still offer some protection mean that the picture could be less pessimistic, he said, but he added, “I don’t think any country should be gambling on that chance right now.”
Michael Ryan, the head of the World Health Organization’s health emergencies program, warned during a news conference that as the world has seen before with other variants, “if they are allowed to spread unchecked even though they are not individually more virulent or more lethal, they just generate more cases, they put pressure on the health system and more people die. That’s what we can avoid.”
The W.H.O. chief, Tedros Adhanom Ghebreyesus, acknowledged that there was some evidence that Omicron caused milder illness then Delta, though it was too early to be definitive.
On Wednesday, Britain adopted a new strategy in response to Omicron, urging people to work from home where possible, introducing new mask mandates and requiring people to show vaccine passports for entry to some venues. It was a striking reversal for Prime Minister Boris Johnson, who had opposed stricter controls that have been adopted around Europe, which was suffering through its biggest coronavirus wave so far before Omicron appeared.
Britain’s Health Security Agency released new data on Wednesday that it said “suggests that Omicron is displaying a significant growth advantage over Delta,” which had previously been the fastest-spreading variant and has become the dominant one worldwide.
Analysis of the data collected in Britain showed increased household transmission risk, a key indicator of how fast the variant can spread. The health agency cautioned that the data was still sparse and the conclusions tentative, with deeper studies underway. But Britain’s genomic sequencing system offers some of the strongest evidence yet on the variant.
All positive coronavirus tests from people arriving in Britain are sent for genomic sequencing, and as part of the country’s routine surveillance, around 15 to 20 percent of all positive P.C.R. tests of people already in the country are also sent for sequencing.
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“It’s not going to take long before it becomes obvious in other places, but it’s clearer earlier here,” Dr. Barrett said. “I think other countries should basically assume the same thing is happening.”
The genomic surveillance could also give Britain a head start in determining how severe Omicron cases are, though there will be a lag because it takes days or weeks for a person who gets infected to become seriously ill.
“It is increasingly evident that Omicron is highly infectious and there is emerging laboratory and early clinical evidence to suggest that both vaccine-acquired and naturally acquired immunity against infection is reduced for this variant,” Susan Hopkins, the chief medical adviser to the Health Security Agency, said in a statement.
Experts fear what that could mean for Britain’s already struggling National Health Service.
“A lot of staff have left or are burnt out,” Dr. English said, after months of dealing with the strains of the pandemic. “Now we’ve going to have another big hit — very likely — from Omicron. I am really, really sympathetic toward my poor colleagues working in clinical practice at the moment.”
Chaand Nagpaul, the chair of the British Medical Association, a trade union for doctors and medical students, said the government decision was the right one and had come at a crucial moment.
He said in a statement that the country had been having “increasingly high incidences of Covid-19 for some time,” adding that “health care workers are rightly worried about the impact the Omicron variant could have” on the health system’s ability to function if caseloads rise fast.
Some hospitals have already canceled elective care again, a strategy seen at the start of the pandemic to free up resources for treating coronavirus patients. Patients are already experiencing hourslong waits for ambulances as a result of the existing pressures on the system, Dr. Nagpaul added.
“While the number of Covid hospitalizations today is much lower than last winter, we must not risk complacency by ignoring the rapid doubling of omicron cases every two to three days,” he said.