LONDON—The U.K. is emerging as a testing ground in the battle for dominance between the new Omicron variant of the coronavirus and Delta, the earlier strain that is currently driving most infections in the U.S. and Europe.

How Britain fares against Omicron will offer clues to the U.S. and the rest of the industrialized world about how the variant behaves in a highly vaccinated population, how sick those who are infected get and if its dozens of mutations have given Omicron enough of an advantage on the evolutionary ladder...

LONDON—The U.K. is emerging as a testing ground in the battle for dominance between the new Omicron variant of the coronavirus and Delta, the earlier strain that is currently driving most infections in the U.S. and Europe.

How Britain fares against Omicron will offer clues to the U.S. and the rest of the industrialized world about how the variant behaves in a highly vaccinated population, how sick those who are infected get and if its dozens of mutations have given Omicron enough of an advantage on the evolutionary ladder to starve Delta of the hosts it needs to stay on top.

South Africa alerted the world to Omicron in late November and is experiencing a rapid increase in infections with the new variant.

But scientists say it may not be a reliable template for what might happen with Omicron in the U.S. and Europe because vaccination coverage is lower, its population is younger, and the variant isn’t competing with a large number of Delta cases. It is also summer in the Southern Hemisphere, and the virus tends to spread more easily when people are huddled together indoors in the winter. Many South African cases have been mild.

Fresh hospitalization data released from South Africa indicated that Omicron provokes a milder infection than past variants even though it spreads more quickly.

Michelle Groome, who heads the public-health division at South Africa’s National Institute for Communicable Diseases, told a press conference Friday that researchers see a disconnect between infections and hospitalizations compared with previous strains, with fewer hospitalizations among infected people and a small uptick in the number of deaths. She cautioned that the results could be due to the limited sample size at the wave’s early stage.

The U.K. looks more like the U.S. and will likely prove a better guide to how Omicron will play out—and not for the first time, having been hit by the Alpha variant sooner than North America. Its population is older than South Africa’s, vaccination coverage is widespread and it has, since an almost-complete reopening in the summer, experienced a sustained but manageable spell of Delta cases, hospital admissions and deaths. It also has a sophisticated surveillance system for monitoring variants and its doctors and scientists publish reams of data and analysis on Covid-19.

Scientists and vaccine makers are investigating Omicron, a Covid-19 variant with around 50 mutations, that has been detected in many countries after spreading in southern Africa. Here’s what we know as the U.S. and others implement travel restrictions. Photo: Fazry Ismail/EPA-EFE/Shutterstock The Wall Street Journal Interactive Edition

Now, with more confirmed cases than the U.S. or anywhere in Europe, the U.K. is facing Omicron. Prime Minister Boris Johnson reimposed some public-health restrictions in England this week in an effort to stem the spread of the new variant, as public-health officials warned Omicron was on course to cause one million infections in Britain by the end of the month. Omicron cases are doubling every two to three days, public-health officials estimate, highlighting the variant’s potential to displace Delta as the most common variant within weeks.

Still, there are unanswered questions over the extent of Omicron’s transmission advantage over Delta, and how far it can evade immunity conferred by prior infection or vaccination. Another urgent question is whether vaccination will keep people safe from severe illness. Delta may prove hard to dislodge. How the Omicron wave plays out in the U.K. should yield some answers, scientists say. Omicron has been detected in 23 U.S. states, according to the Centers for Disease Control and Prevention.

Omicron samples in Elandsdoorn, South Africa.

Photo: Jerome Delay/Associated Press

“There’s no way one can understand what’s going on over here without knowing what’s going on in the U.K. and South Africa and other places where the variant has cropped up,” said Philip Landrigan, director of the Program for Global Public Health and the Common Good at Boston College.

Confirmed Omicron cases in the U.K. totaled 817 Thursday, up 249 on the day before. Health Secretary Sajid Javid said this week the true number of infections could be closer to 10,000 and the variant is rapidly gaining ground.

Scientists on a panel that advises the government said Wednesday that without some restrictions, hospital admissions from Omicron could reach 1,000 a day by the end of the year if the variant continues to spread at its current rate. That would be on top of however many admissions will have been caused by Delta. The seven-day average of new Covid-19 cases overall in the U.K. was 48,000 Thursday—almost 500 per 100,000 people—and hospital admissions are at around 760 a day.

The panel sketched out six scenarios of how the Omicron phase of the pandemic could play out.

The first three, which modelers advising the government say are the most likely, are that Omicron does show a transmission advantage over Delta and that it can partially evade the immunity conferred by vaccination or prior infection, broadening the pool of people who can be infected. Those advantages mean Omicron would ultimately outcompete Delta, just as Delta beat back earlier variants such as Alpha.

A Covid-19 testing clinic in London.

Photo: Stephen Chung/Zuma Press

“The only way that one infection drives another to extinction is when they compete for the same resources—susceptible people,” said Matt Keeling, director of the Zeeman Institute for Systems Biology and Infectious Disease Epidemiology Research at the University of Warwick.

The difference between the three scenarios is in how severe it is in terms of how many infected people need to be hospitalized. Though scientists are optimistic that vaccines will continue to shield most people from severe illness with Omicron, the downbeat prognosis is that even if the number of admissions tied to each Omicron case is lower than with Delta, the variant’s other advantages mean the scale of the outbreak could still lead to a surge in serious illness, stretching the capacity of the state-run National Health Service to cope. Greater severity of illness would mean a deadlier wave.

In an effort to slow Omicron down, Mr. Johnson on Wednesday asked people in England to work from home where possible, wear masks in indoor public spaces, and be prepared to show proof of vaccination or a negative Covid-19 test to gain entry to nightclubs and other venues.

The remaining scenarios the panel said are less likely. In one, Omicron doesn’t turn out to possess a greater ability than Delta to sidestep immunity, but does have a transmission advantage. That would mean it eventually displaces Delta but the bulwark of immunity in Britain keeps the epidemic under control.

In another, Omicron turns out to be less dangerous than feared because exposure to Delta, which wasn’t widespread in South Africa when the new variant emerged, bolsters protection from Omicron. The final possibility is Delta stays on top and Omicron flares sporadically in small outbreaks. That happened with the Beta variant, which also spread rapidly in South Africa but never got established in Britain or the U.S.

Evidence from lab studies suggests that booster shots should enhance people’s protection against Omicron. The U.K. has administered boosters to almost 22 million people, a third of its population, and the government has broadened those eligible to everyone over 18 and shortened the interval between second doses and boosters to three months.

Graham Medley, professor of infectious-disease modeling at the London School of Hygiene and Tropical Medicine and co-chair of the panel’s modeling group, likened the latest phase of the pandemic to driving a car. Vaccines, booster shots and precautions such as mask wearing act like seat belts and safety checks, reducing the risk to an individual of getting hurt in an accident.

“Omicron has made the road a bit icier,” he said.

—Joanna Sugden in London and Aaisha Dadi Patel in Johannesburg contributed to this article.

Write to Jason Douglas at jason.douglas@wsj.com