The Omicron variant of the Sars-CoV2 virus is intrinsically as severe as previous variants, according to a preprint version of a large US study that counters assumptions in other studies that it was more transmissible but less severe.

The findings, which estimated Omicron's severity after accounting for the impact of vaccines, should reinforce the importance of inoculations and booster shots, experts said. Vaccines helped keep hospital admissions and deaths relatively low during the Omicron surge compared with previous variants.

The study, which is undergoing peer review at Nature Portfolio, was posted on Research Square on May 2. The authors, from Massachusetts General Hospital, Minerva University and Harvard Medical School, declined to comment until peer review is completed.

"We found that the risks of hospitalisation and mortality were nearly identical" between the Omicron era and times in the past two years when different variants were dominant, the researchers said in their report.

The new study, based on records of 130,000 Covid patients in Massachusetts, is unique and "pretty strong," said Dr Arjun Venkatesh of Yale School of Medicine and the Yale Centre for Outcomes Research and Evaluation, who was not involved in the research.

Rather than just looking at numbers of deaths and hospital admissions, as earlier studies have done, it accounted for patients' vaccination status and medical risk factors and compared similar groups of people, Venkatesh said.

The authors cited potential limitations in their report, including the possibility that the analysis underestimated the number of vaccinated patients in more recent Covid waves, and the total number of infections, because it excluded patients who performed at-home rapid tests.

The study did not account for treatments patients may have received, such as monoclonal antibodies or antiviral drugs "that are known to reduce hospitalisations," Venkatesh noted. "It's possible that if we didn't have these treatments available today, Omicron would be even worse." (Reuters)