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3 studies now suggest Omicron is milder than the Delta COVID variant

December 23, 2021, 10:42 AM UTC

Three studies have now indicated that the Omicron variant of SARS-CoV-2 is much less likely than the Delta variant to put people in the hospital, and also potentially less likely to lead to severe disease. However, this is preliminary data—all three papers are yet to be peer-reviewed—and Omicron’s more transmissible nature still heralds terrible strain on health-care systems in the near future.

Markets appear to be focusing more on the positives than the caveats. The STOXX Europe 600 Travel & Leisure Index rose more than 2.5% on Thursday morning, the biggest sector-gainer, while the broader STOXX 600 was up a far more modest 0.4%. U.S. futures, meanwhile, point to gains at the open.

The studies were conducted in South Africa—where Omicron was first identified, and where its stark wave of infections appears to have already peaked—and in England and Scotland.

The South African study, published Tuesday, showed Omicron was about 80% less likely than Delta to lead to hospitalization. It also suggested the new variant was also 30% less likely to lead to severe disease in those who have been hospitalized. “Some of this reduction is likely a result of high population immunity,” the paper’s authors noted.

Comparison difficulties

Experts warned of a couple of shortcomings in the study. Paul Hunter, professor in medicine at the University of East Anglia in England, pointed out that the results may have been skewed by the use of different time periods in the comparison—April to November for Delta, and October to November for Omicron—making it difficult to predict the new strain’s effect on health services.

“Even though cases of Omicron were less likely to end up in hospital than cases of Delta, it is not possible to say whether this is due to inherent differences in virulence or whether this is due to higher population immunity in November compared to earlier in the year,” Hunter said in a statement from the Science Media Centre.

Hunter also warned that the South African data about disease severity came with such wide confidence intervals (a standard measure of uncertainty in clinical studies) that “we cannot say that severity was indeed less in hospital patients with Omicron.”

The English study (from Imperial College London) and Scottish study (from the University of Edinburgh) were both published on Wednesday.

The Scottish conclusions were more upbeat, suggested a two-thirds reduction in Omicron’s hospitalization risk, compared with Delta. In the English data, there was only a 40-45% reduction in the likelihood of an infected person needing to stay in hospital for a day or longer, and an overall 20-25% reduction when considering all attendances at hospital, no matter how brief.

“That they report different outcomes is part of science. Both studies are early and we should expect different results as the science evolves,” commented the University of Oxford’s Professor James Naismith.

The papers both underscored the need for boosters in warding off Omicron’s worst effects—though again, the fact that Omicron appeared less than two months ago makes it hard to compare the severity of outcomes for patients requiring a long hospital stay.

Lessons for lockdowns

Both of the British studies stressed the fact that Omicron’s extremely high infectiousness will still send a lot of people to hospital, even if it is apparently milder for those who already have some immunity through full primary vaccination or from previous infection.

When it comes to lessons for the policymakers who set lockdown rules, different experts drew slightly different conclusions.

“Decreasing the spread of the virus to give time to improve population coverage with the booster is the best strategy,” Naismith said in reference to the Imperial study. However, Professor Penny Ward of King’s College London said of the same paper that, “based on this preliminary information, the decision to delay imposition of greater restrictions on social mixing than are currently advised under [the U.K.’s] Plan B might be more reasonable than some have suggested.”

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