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Two studies from Britain published Wednesday showed COVID-19 infections with omicron are less likely to result in hospitalization compared to the delta variant, the latest research confirming a trend first identified in South Africa.

The preliminary studies — one paper from Scotland and the other from England — were cautiously welcomed by experts, who nonetheless stressed that any advantage in milder outcomes could still be negated by the new strain's heightened infectiousness, which may still lead to more severe cases overall.

"We're saying that this is qualified good news — qualified because these are early observations, they are statistically significant, and we are showing a reduced risk of hospitalizations," said Jim McMenamin, a co-author of the Scottish study and the national COVID-19 incident director for Public Health Scotland, which conducted the study with the University of Edinburgh and the University of Strathclyde.

The Scottish paper examined COVID-19 cases recorded in November and December, and grouped them by cases caused by delta against those caused by omicron.

It found that "Omicron is associated with a two-thirds reduction in the risk of Covid-19 hospitalization when compared to Delta," while also showing that a booster vaccine offered substantial additional protection against symptomatic infection.

The study was small and there were no people under 60 hospitalized at the time, but the authors said they had adjusted for these limitations using statistical methods.

The second paper, from England, found there was a 20% to 25% reduction in any attendance at hospital for omicron compared to delta, and a 40% to 45% reduction in hospitalizations lasting one night or longer, in other words "admissions."

The Scottish study only looked at admissions so this may account for part of the difference seen.

Azra Ghani of the Imperial College London, who coauthored the England study, said in a statement: "Whilst the reduced risk of hospitalization with the omicron variant is reassuring, the risk of infection remains extremely high.

"With the addition of the booster dose, vaccines continue to offer the best protection against infection and hospitalization."

Neither of the studies has been peer reviewed, but they add to growing evidence about disease outcomes with omicron.

Two new studies from British researchers suggest that omicron infections more often result in mild illness, compared with previous variants, offering hope that the current surge may not be quite as catastrophic as feared. |  ANDREW TESTA / THE NEW YORK TIMES
Two new studies from British researchers suggest that omicron infections more often result in mild illness, compared with previous variants, offering hope that the current surge may not be quite as catastrophic as feared. | ANDREW TESTA / THE NEW YORK TIMES

It remains unclear whether the decreased rate of severe cases seen with omicron is because of characteristics of the variant, or whether it appears milder because it is coming up against populations with greater immunity from prior infection and from vaccination.

"It’s important that we don’t get ahead of ourselves,” said McMenamin. "A smaller proportion of a greater number of cases requiring treatment might still mean a substantial number of people that may experience severe COVID.”

Anthony Fauci, who serves as the top medical adviser to U.S. President Joe Biden, echoed those comments. While the Scotland study "appears to validate and verify” the data from South Africa, he warned that U.S. demographics may lead to different outcomes, and that the total caseload might eliminate any benefits from lower severity.

"Even if you have a diminution in severity, if you have a much larger number of individual cases, the fact that you have so many more cases might actually obviate the effect of it being less severe,” Fauci said at a briefing.

Public health leaders have cautioned that other factors, such as higher numbers of people who are vaccinated or have previously had COVID-19, may also complicate any comparison with previous points in the pandemic.

"When we measure severity of omicron, we are measuring it as South Africa did, in a very immune population,” said Neil Ferguson, a professor at Imperial, who helped lead the English study. After two doses of the Pfizer Inc.-BioNTech SE vaccine, the risk of hospitalization from omicron is probably about the same as the risk from delta, he said — perhaps reflecting the fact that the new variant, though potentially somewhat less severe, is better at eluding the vaccine in people who haven’t had a booster.

The English and Scottish studies had different follow-up times, meaning that the results may change as people’s illnesses progress, said Penny Ward, a visiting professor in pharmaceutical medicine at King’s College London. "It remains important for all of us to take reasonable care, test, test, test and get our boosters as soon as possible.”

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