WASHINGTON (BLOOMBERG) - The Covid-19 variant that has become the dominant strain in the United States is not as deadly as earlier research indicated, although it is confirmed to be faster-spreading than other versions, according to a study.

Among 339 patients with the coronavirus, 36 per cent of those infected with the B117 strain that arose in Britain became severely ill or died, according to research published on Monday in the Lancet Infectious Diseases journal, compared with 38 per cent of those who had non-B117 infections.

"We're not saying it's nothing, but it's not worse in terms of outcome in our study, in our setting," said Dr Eleni Nastouli, a co-author of the study and an associate professor at University College London. She noted that the study differed from some earlier research, looking at patients in hospitals, rather than in the community, and making precise identifications of variants with whole-genome sequencing.

Earlier data released by a British advisory group and cited by the US Centres for Disease Control and Prevention (CDC) indicated that B117 might be as much as a third more deadly than other variants of the virus. Those findings added to concerns about mutants that have shown some ability to spread more quickly. Other variants appear to be able to avoid the protection conferred by vaccines.

Rapid circulation of the B117 variant has driven up cases in Michigan, a national hot spot, adding to concerns about a possible pandemic resurgence. While Governor Gretchen Whitmer has asked for more vaccine doses to help stem that outbreak, CDC head Rochelle Walensky said supplies cannot be diverted from one state to another. She urged wider shutdowns of public places and events.

The Lancet study looked at patients admitted to London hospitals in November and December last year. A separate study of the fast-spreading variant in the Lancet Public Health journal found no evidence that it altered symptoms or the likelihood of experiencing long Covid-19, which was defined as persistent symptoms for more than 28 days.

The spread of variants around the world threatens countries' ability to stem the pandemic even as they roll out vaccination campaigns. Drugmakers that produce vaccines that have been found to be effective are now racing to bolster their vaccines against new variants.