The first real-world study of how vaccines hold up against the Omicron variant showed a significant drop in protection against cases caused by the new and fast-spreading form of the coronaviruses.
The British government scientists who published the study said that third vaccine doses provided considerable defense against Omicron.
Government scientists on Friday offered the most complete look yet at how quickly Omicron was spreading in England, warning that the variant could overtake Delta by mid-December and cause Covid-19 cases to soar.
A computer modeling study of England released on Saturday suggested that Omicron could disrupt life and overwhelm hospitals even in populations with high levels of immunity. As they learned more about the severity of Omicron infections, scientists warned that those projections could change.
The vaccine study showed that the protection was reduced. The second dose of the Pfizer-BioNTech vaccine was 35 percent effective in preventing infections caused by Omicron, a significant drop-off from their performance against the Delta variant, the scientists found.
A third dose of the vaccine lifted the figure to 75 percent.
The two doses of the vaccine offered no protection against Omicron after several months. The Pfizer-BioNTech dose boosted effectiveness against the variant to 71 percent.
The study's authors said they expected the vaccines to remain a bulwark against hospitalizations and deaths caused by Omicron. It was too early to know how well the vaccines would work in Britain, the researchers cautioned.
New findings about how easy Omicron is to spread were released with that study. The Omicron variant is more likely to be passed on to other members of the household by a person with it than the Delta variant.
A close contact of an Omicron case is more likely to catch the virus than a close contact of someone with Delta.
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. His research team's modeling work showed that Omicron was more contagious than Delta.
Dr. Ferguson said that there is a significant amount of immune escape. It is more transmissible than Delta.
He and other scientists warned that evidence was still coming in, and that better surveilling in places where the Omicron wave is most advanced could affect their findings.
The World Health Organization said this week that it was too early to say if Omicron was causing milder illness than Delta. If the variant keeps spreading as quickly as it is in England, health systems around the world may be overwhelmed with patients.
Even if Omicron causes less severe illness than the Delta variant, 5,000 people could be admitted to hospitals daily in Britain at the peak of the Omicron wave, according to Dr. Ferguson's computer modeling.
The scientists said that widespread vaccination in countries like Britain and the United States would keep people alive. If hospitals became too full, patients with Covid and other illnesses would suffer.
It only takes a small drop in protection against severe disease for it to translate into levels of hospitalization.
It will take several weeks to understand how the current surge in Omicron infections will translate into people needing hospital care. By the time we know about severity, it may be too late to act.
The potential for a surge in hospitalizations was thrown into sharp relief by a modeling study created by a group of experts at the London School of Hygiene & Tropical Medicine. The scientists assumed that Omicron would cause disease just as severe as Delta, but also that the Omicron wave would be less severe because of the high levels of vaccinations.
The scientists said that England could be hit hard if Omicron evaded people's immune defenses in the scenario that some experts said was most likely. In April, they predicted roughly 300,000 hospitalizations and 47,000 deaths.
At the peak of the Omicron wave, English hospitals could see a bigger daily burden than at any other time in the Pandemic.
The scientists said that re-introduction of certain restrictions could save thousands of lives and spare tens of thousands of people hospital stays.
Outside experts said that Omicron remained poorly understood, that people may be able to fight off severe infections more effectively than models predicted, and that the arrival of new antiviral pills in the coming months could make a difference.
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.
Michael Head is a senior research fellow in global health at the University ofSouthampton in England.
A tactic of turning the lights off and pretending we are not in is a failed policy.
