A previous Covid-19 recovery provides little shield against infection with the omicron variant, a research team from Imperial College London showed in a large study that underlines the importance of booster shots.
Having had Covid probably only offers 19% protection against omicron, the study showed on Friday. That was roughly in line with two doses of vaccine, which the team estimated were as much as 20% effective against omicron. Adding a booster dose helped dramatically, blocking an estimated 55% to 80% of symptomatic cases.
The Imperial College London team analyzed all the PCR test-confirmed Covid cases in England between Nov. 29 and Dec. 11, making it one of the most expansive examinations yet at omicron’s potential to evade the body’s defenses. The results were in line with the picture emerging of the variant’s capacity to elude protection from previous infection or inoculation and spread faster than earlier iterations of the virus.
There was no evidence of omicron cases being less severe than delta, based on the proportion of people testing positive who had symptoms or went to the hospital, the team said.
“It’s very early days,” said Neil Ferguson, a professor at Imperial, who helped lead the study. Rising hospitalizations in places like Manchester and London will provide clearer evidence, he said. “We should have data within a week to make a more definitive judgment on relative severity.”
In South Africa, which announced the discovery of the variant on Nov. 25, authorities said on Friday the rate of hospitalizations seems to be lower than during the country’s earlier wave of delta infections.
Europe is bracing for an omicron-driven fifth wave even as intensive-care units in many areas remain filled with patients sick with the delta variant. Some governments are already imposing new measures to slow the omicron wave and buy time for booster campaigns to gear up.
Another open question is how long the shield provided by a booster shot will hold against the new variant. In a worst-case scenario, with efficacy dwindling as quickly as it does after the first two doses, boosters might prevent only four-fifths of hospitalizations by two months after they’re given, a second Imperial study showed on Friday. That makes it important to think about a long-term strategy, ideally with shots that could work against multiple variants, said Azra Ghani, a professor at Imperial who led that research.
The proportion of omicron among all Covid cases was probably doubling every two days up to Dec. 11, the U.K. team said, estimating that every person infected with the variant passed it on to more than three other people.