The first realistic study of how vaccines are resistant to the Omicron variant showed a significant reduction in protection against symptomatic cases caused by the new, rapidly spreading form of the coronavirus.
But the study, published by scientists from the British government on Friday, also indicated that the third doses of the vaccine provided a significant defense against Omicron.
Government scientists on Friday also provided the most complete look yet at how quickly Omicron is spreading among England’s heavily vaccinated population, warning that the variant could cross Delta by mid-December and, without any precautionary measures, cause Covid-19 cases to spike.
The scientists found that four months after subjects received a second dose of the Pfizer-BioNTech vaccine, the vaccines were approximately 35% effective in preventing accidental infections caused by Omicron, a significant decrease in their performance against the delta variant.
However, a third dose of the Pfizer-BioNTech vaccine raised the number to nearly 75%.
Two doses of the AstraZeneca vaccine appear to provide almost no protection against symptomatic infection caused by Omicron several months after vaccination. But for those who benefit, an extra dose of Pfizer-BioNTech paid off big, boosting efficacy against the alternative to 71 percent.
However, the study authors said they expected vaccines to remain a bulwark against hospitalizations and deaths, if not infections, caused by Omicron. The researchers cautioned that even in a country that tracks the alternative as closely as Britain, it is too soon to know exactly how well the vaccines are performing.
This study is released along with new findings on how easily Omicron can spread. Britain’s Health Security Agency reports that someone with the Omicron variant, for example, is roughly three times more likely than someone with the delta variant to transmit the virus to other individuals in their home.
Close contact of an Omicron case is twice as likely to contract the virus when close contact with a person infected with delta.
Neil Ferguson, an epidemiologist at Imperial College London, said Omicron’s ability to evade the body’s immune defenses accounts for most of its advantage over previous variants. But modeling work by his research team and other groups in Britain also suggested that Omicron was simply more contagious than Delta, by about 25 to 50 percent.
“I think there is a great deal of immune escape,” Dr. Ferguson said, referring to the virus’s ability to evade the body’s defences. “But it’s also substantially more transmissible than Delta.”
He and other scientists have cautioned that evidence is still coming in, and that better monitoring in places where the Omicron wave is more advanced may affect their results.
The World Health Organization said this week that some evidence has emerged that Omicron causes milder disease than Delta, but it is too early to be certain. However, scientists have warned that if the variant continues to spread at the same speed as in England, where cases are doubling every 2.5 days, health systems around the world could be overwhelmed with patients.
Dr Ferguson said that even if Omicron caused serious illness at only half a variable delta rate, computer modeling suggested 5,000 people could be admitted to hospitals a day in Britain at the peak of the Omicron wave – a number higher than anyone seen at any other point in the pandemic. .
Scientists said that widespread vaccination in countries such as Britain and the United States would protect as many people from death as happened in previous waves. But experts have also warned that patients with Covid and other diseases will suffer if hospitals become too full.
“It takes a small reduction in protection from severe illness for those very high numbers of infections to translate to levels of hospital treatment that we can’t handle,” Dr. Ferguson said.
It will take several weeks to understand how the current increase in omicron infections could translate to people needing hospital care. “I worry that by the time we know the risk, it may be too late to act,” Dr. Ferguson said.