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 rapidly spreading form of the coronavirus.
But the study, published Friday by UK government scientists, also indicated that the third dose of vaccine offered a significant defense against Omicron.
Government scientists 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 catch up with Delta by mid-December and, without any precautions, could increase the number of Covid-19 cases. increase.
Those warnings were reinforced by a computer modeling study from England released Saturday that suggested that Omicron, even in high-immune populations, could significantly disrupt life and overwhelm hospitals. Scientists warned those projections could change as they learn 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 about 35 percent effective at preventing symptomatic infections caused by Omicron, a significant drop in their performance over the Delta variant, the researchers found. scientists.
However, a third dose of the Pfizer-BioNTech vaccine brought the figure up to about 75 percent.
Two doses of the AstraZeneca vaccine were found to provide 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, boosting effectiveness against the variant to 71 percent.
Still, the study authors said they expected the vaccines to continue to be a bulwark against hospitalizations and deaths, if not infections, caused by Omicron. And the researchers warned that even in a country that tracks the variant as closely as Britain, it was too early to know exactly how well the vaccines would perform.
That study was released alongside new findings about how easily Omicron spreads. Someone infected with the Omicron variant, for example, is about three times as likely as a person infected with the Delta variant to pass the virus to other members of his or her household, the UK’s Health Security Agency reported. .
And close contact with an Omicron case is about twice as likely as close contact from someone infected with Delta to contract the virus.
Neil Ferguson, an epidemiologist at Imperial College London, said Omicron’s ability to evade the body’s immune defenses explains most of its advantage over previous variants. But his research team’s modeling work also suggested that Omicron was simply more contagious than Delta, at about 25 to 50 percent.
“I think there’s a significant amount of immune escape,” said Dr. Ferguson, referring to the virus’s ability to evade the body’s defenses. “But it’s also inherently more transferable than Delta.”
He and other scientists have warned that evidence is still coming in and that better monitoring of places where the Omicron wave is most advanced could influence their findings.
The World Health Organization said this week there was evidence that Omicron causes 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 the number of cases doubles every 2.5 days, health systems around the world could be overrun with patients.
Even if Omicron causes severe disease at only half the rate of the Delta variant, said Dr. Ferguson, his computer modeling suggested that 5,000 people could be admitted to hospitals in Britain every day at the height of the Omicron wave — a higher figure than ever seen at any other time in 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 the experts also warned that patients with Covid and with other illnesses would suffer if hospitals became too full.
“It only takes a small drop in protection against serious disease for those very high numbers of infections to translate into hospitalizations that we can’t handle,” said Dr. Ferguson.
It will take several weeks to understand how the current rise in Omicron infections might translate into people needing hospital care. “I’m afraid by the time we know about the severity,” said Dr. Ferguson, “it may be too late to act.”
The potential for an increase in hospital admissions was sharply lit by the modeling study released Saturday, created by a separate group of experts from the London School of Hygiene & Tropical Medicine. For now, those scientists assumed that Omicron would cause disease as serious as Delta in unvaccinated people, but also that increasing immunity to vaccinations and previous infections would dampen the Omicron wave, as has happened in South Africa.
In the scenario most likely, according to some outside experts – in which Omicron largely evaded people’s immune defenses, but booster doses also proved highly effective – the scientists said England could be hit hard. As of April, they predicted about 300,000 hospitalizations and 47,000 deaths.
That could be a greater daily burden on English hospitals at the height of the Omicron wave than was ever seen before during the pandemic.
Crucially, the scientists said reintroducing certain restrictions could save thousands of lives and tens of thousands of people save hospital stays.
Outside experts stressed that Omicron was still poorly understood, that humans may be able to fight serious infections more effectively than the models had 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 vaccination campaigns, share doses with less vaccinated countries and consider measures such as more self-testing, if not new restrictions.
“The coronavirus isn’t done with us yet,” said Michael Head, a senior research fellow in global health at the University of Southampton in England.
“A tactic of ‘turning off the lights and pretending we’re not there’ is a failed policy.”