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.
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. Modelling work done by his research team and other groups in Britain suggested 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.
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.
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.
