Omicron cases appear less severe and less likely to result in hospitalization, new UK studies suggest

• The Omicron variant may result in milder illness than Delta, two new studies from the UK suggest.
• Omicron patients in the United Kingdom had a 15%-20% lower risk of hospitalization compared to Delta patients.
• Many Omicron patients are vaccinated or previously infected. More research is needed to tell whether the variant is milder than Delta.

The Omicron coronavirus variant may result in milder illness than its predecessors, two studies conducted in the United Kingdom and released Wednesday suggest.

Preliminary estimates suggest that patients infected with Omicron in the UK had a 15%-20% lower risk of hospitalization and 40%-45% lower risk of being admitted to the hospital overnight than those infected with Delta, according to researchers at the Imperial College of London. To come to that conclusion, scientists analyzed coronavirus specimens in England between December 1 and December 14 and linked those COVID-19 cases to hospital data.

The researchers did not, however, find that Omicron is significantly less severe than Delta at face value. Instead, their data suggest Omicron is more skilled than Delta at reinfecting people with prior immunity from vaccination or natural infection, meaning they are less likely to be admitted to the hospital with COVID-19 in the first place. 

A separate preliminary analysis, from Scotland, examined hospital data between November 23 and December 19, and found that hospitalizations in Scotland were 70% lower for Omicron infections than Delta ones.

“Although two-thirds reduction is significant, Omicron can cause severe illness in the doubly vaccinated,” James Naismith, director of the Rosalind Franklin Institute and professor of structural biology at the University of Oxford, told reporters.

“If Omicron continues to double every few days, it could generate many more hospitalizations than Delta from the double vaccinated population,” Naismith, who was not affiliated with the Scottish research, added. 

“In my view, the best news in the study is the observation that the booster is highly effective at reducing serious illness from Omicron.”

Both sets of scientists stressed the results were preliminary. More research is needed to know whether Omicron is intrinsically milder than Delta.

The UK data join a growing body of early Omicron research 

Early lab data, which has not yet been published in a peer-reviewed journal, offer clues about Omicron’s behavior compared to past variants.

In a petri dish, Omicron spread 70 times faster than Delta in tissue taken from human airways, which could explain why the variant spreads so quickly and overwhelms early immune defenses from vaccines.

The same study found that Omicron spread more slowly in human lung tissue than the original strain of the virus or the Delta variant, which may be why Omicron doesn’t appear as severe as past strains. Still, questions remain, and scientists are rushing to explain what happens inside the human body when Omicron invades.

Until then, the real world may offer additional hints.

Tim Spector, an epidemiologist at King’s College London, told Insider he is seeing similar patterns in his own data-gathering since the Omicron variant emerged.

Spector runs the UK’s ZOE COVID Symptom Study, an app that logs more than 1 million people’s COVID symptoms each day, and predicts COVID-19 trends.

His latest findings, which have not yet been published, suggest that among 2,500 people who had suspected Omicron cases, “self-reported hospitalization was also less common than with Delta,” he told Insider. 

Given the preliminary and self-reported nature of the data, putting it in larger context is key, Spector said. His cohort includes data from populations that tend to be younger and vaccinated, making it hard to tell how Omicron infections compare to Delta infections among people who are unvaccinated or immunocompromised.

Spector cautioned that, although hospitalization rates may be lower for Omicron than Delta, given the large number of infections, the absolute number of people hospitalized may be high.

Still, Spector said, he’s “cautiously optimistic,” adding, “I’d say so far the data aren’t living up to the rather pessimistic predictions.”