British studies warn of Omicron’s speed, and one notes the need for boosters.

The first real-world study of how vaccines hold up against the Omicron variant showed a significant drop in protection against symptomatic cases caused by the new and fast-spreading form of the coronavirus.

But the study, published by British government scientists on Friday, also indicated that third vaccine doses provided considerable defense against Omicron.

Government scientists on Friday also offered the most complete look yet at how quickly Omicron was spreading in England’s highly vaccinated population, warning that the variant could overtake Delta by mid-December and, without any precautionary measures, cause Covid-19 cases to soar.

Those warnings were reinforced by a computer modeling study of England released on Saturday suggesting that even in populations with high levels of immunity, Omicron could significantly disrupt life and overwhelm hospitals. Scientists cautioned that those projections could change as they learned more about the severity of Omicron infections.

The vaccine study published Friday indicated reduced levels of protection. Four months after people received a second dose of the Pfizer-BioNTech vaccine, the shots were roughly 35 percent effective in preventing symptomatic infections caused by Omicron, a significant drop-off from their performance against the Delta variant, the scientists found.

A third dose of the Pfizer-BioNTech vaccine, though, lifted the figure to roughly 75 percent.

Two doses of the AstraZeneca vaccine appeared to offer virtually no protection against symptomatic infection caused by Omicron several months after vaccination. But for those recipients, an additional Pfizer-BioNTech dose paid big dividends, boosting effectiveness against the variant to 71 percent.

Still, the study’s authors said they expected that the vaccines would remain a bulwark against hospitalizations and deaths, if not infections, caused by Omicron. And the researchers cautioned that even in a country tracking the variant as closely as Britain is, it was too early to know precisely how well the vaccines would perform.

That study was released alongside new findings about how easily Omicron is managing to spread. Someone infected with the Omicron variant, for example, is roughly three times as likely as a person infected by the Delta variant to pass the virus to other members of his or her household, Britain’s Health Security Agency reported.

And a close contact of an Omicron case is roughly twice as likely as a close contact of someone infected with Delta to catch the virus.

Neil Ferguson, an epidemiologist at Imperial College London, said that Omicron’s ability to evade the body’s immune defenses accounted for most of its advantage over previous variants. But modeling work by his research team also suggested that Omicron was simply more contagious than Delta, by roughly 25 to 50 percent.

“I think that there’s a significant amount of immune escape,” Dr. Ferguson said, referring to the virus’s ability to dodge the body’s defenses. “But it’s also more intrinsically transmissible than Delta.”

He and other scientists have cautioned that evidence was still coming in, and that better surveillance in places where the Omicron wave is most advanced could affect their findings.

The World Health Organization said this week that some evidence had emerged that Omicron was causing milder illness than Delta, but that it was too early to be certain. Still, scientists have warned that if the variant keeps spreading as quickly as it is in England, where cases are doubling every 2.5 days, health systems around the world may be deluged with patients.

Even if Omicron causes severe illness at only half the rate of the Delta variant, Dr. Ferguson said, his computer modeling suggested that 5,000 people could be admitted to hospitals daily in Britain at the peak of its Omicron wave — a figure higher than any seen at any other point in the pandemic.

Scientists said that widespread vaccination in countries like Britain and the United States would keep as many people from dying as have in earlier waves. But the experts also warned that patients with Covid and with other illnesses would suffer if hospitals became too full.

“It only requires a small drop in protection against severe disease for those very large numbers of infections to translate into levels of hospitalization we can’t cope with,” Dr. Ferguson said.

It will take several weeks to understand how the current surge in Omicron infections may translate into people needing hospital care. “I’m concerned that by the time we know about severity,” Dr. Ferguson said, “it may be too late to act.”

The potential for a surge in hospitalizations was thrown into sharp relief by the modeling study released on Saturday, created by a separate group of experts at the London School of Hygiene & Tropical Medicine. For now, those scientists assumed that Omicron would cause disease just as severe as Delta does in unvaccinated people, but also that mounting levels of immunity from vaccinations and previous infections would temper the Omicron wave, as has happened in South Africa.

In the scenario that some outside experts said was most likely — in which Omicron evaded people’s immune defenses to a large degree, but booster doses also proved highly effective — the scientists said that England could be hit hard. Through April, they predicted roughly 300,000 hospitalizations and 47,000 deaths.

That could place a bigger daily burden on English hospitals at the peak of the Omicron wave than was seen at any time earlier in the pandemic.

Crucially, the scientists said that reintroducing certain restrictions could save thousands of lives and spare tens of thousands of people hospital stays.

Outside experts emphasized that Omicron remained poorly understood, that people may be able to fight off severe infections more effectively than the models predicted, and that the arrival of new antiviral pills in the coming months could soften the blow of infections.

Still, scientists urged governments to speed up inoculation campaigns, share doses with less-vaccinated nations and consider measures like more self-testing, if not new restrictions.

“The coronavirus has not finished with us,” said Michael Head, a senior research fellow in global health at the University of Southampton in England.

“A tactic of ‘turning the lights off and pretending we are not in’ is a failed policy.”