The first real-world study of how vaccines resist the Omicron variant showed a significant drop in protection against symptomatic cases caused by the new, rapidly spreading form of the coronavirus.
But the study, published by scientists in the British government on Friday, also indicated that the third doses of the vaccine offered considerable defense against Omicron.
Government scientists also offered the most comprehensive review yet of the rate at which Omicron was spreading through England’s heavily vaccinated population on Friday, warning that the variant could overtake Delta by mid-December and , without any precautionary measures, to skyrocket the cases of Covid-19.
Four months after people received a second dose of the Pfizer-BioNTech vaccine, the injections were about 35% effective in preventing symptomatic infections caused by Omicron, a significant drop in their performance against the Delta variant, the scientists found. .
A third dose of the Pfizer-BioNTech vaccine, however, brought the figure to around 75 percent.
Two doses of AstraZeneca vaccine appeared to offer virtually no protection against symptomatic infection caused by Omicron several months after vaccination. But for those recipients, an extra dose of Pfizer-BioNTech paid big dividends, increasing the efficacy against the variant to 71%.
Still, the study’s authors said they expected vaccines to remain a bulwark against hospitalizations and deaths, and even infections, caused by Omicron. And the researchers warned that even in a country following the variant as closely as Britain, it was too early to know precisely how well the vaccines would work.
This study was published with new findings on how easily Omicron manages to spread. A person infected with the Omicron variant, for example, is about three times more likely than a person infected with the Delta variant to pass the virus on to other family members, the UK Health Security Agency reported.
And close contact from an Omicron case is about twice as likely as close contact from a person infected with Delta to catch the virus.
Neil Ferguson, an epidemiologist at Imperial College London, said Omicron’s ability to evade the body’s immune defenses was 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 around 25 to 50 percent.
“I think there’s a significant amount of immune evasion,” Dr Ferguson said, referring to the virus’s ability to dodge the body’s defenses. “But it’s also more inherently transmissible than Delta.”
He and other scientists have warned that evidence is still coming in and better monitoring 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 caused milder illness than Delta, but it was too early to be sure. Still, scientists have warned that if the variant continues to spread as quickly as it does in England, where cases double every 2.5 days, healthcare systems around the world could be inundated with patients.
Even though Omicron causes serious illness at only half the rate of the Delta variant, Dr Ferguson said, computer modeling has suggested that 5,000 people could be admitted to UK hospitals daily at the height of its Omicron wave – a figure higher than that observed at any other time of the pandemic.
Scientists said widespread vaccination in countries like Britain and the United States would prevent as many people from dying as in previous waves. But experts have also warned that patients with Covid and other illnesses will suffer if hospitals become too full.
“It only takes a small dip in protection against critical illness for these very large numbers of infections to translate into levels of hospitalization that we cannot cope with,” said Dr Ferguson.
It will take several weeks to understand how the current outbreak of Omicron infections may translate into people requiring hospital care. “I’m afraid by the time we know gravity,” said Dr Ferguson, “it may be too late to act.”