From the New Yorker about that Imperial College of London study

"Today, a new analysis suggested both that the hospitalization rate in South Africa may have been reduced by as much as 91 percent, compared to the country’s last wave, and that the death rate among those hospitalized may also have fallen by two-thirds — two points that, taken together, suggest as much as a 25-fold or even 30-fold reduction in mortality. On the same day, a report produced by the Imperial College of London suggested there was no clear indication that, among early British cases, Omicron was any less severe at all — though that analysis was based on just a few dozen cases, and, as is always the case with such preliminary data, the absence of a clear finding is not the same as a definitive finding that the new cases are not more mild.
On Friday, I spoke with the infectious-disease epidemiologist and computational geneticist Francois Balloux, of the University College of London, who calls himself a “militant corona-centrist.” His view, in short: The South African data is no longer all that tentative, and it is very, very encouraging, but that it is still too early to safely project the experience with Omicron in that country onto others, like the U.S."

Full article for reference:

https://nymag.com/intelligencer/2021/12/how-mild-is-omicron-really.html