Omicron may be intrinsically as severe as previous coronavirus variants, according to a US study which contradicts assumptions that the strain was more transmissible but less severe.

The yet-to-be-published study, posted as preprint on Research Square on May 2, adjusted for factors such as demographics, vaccination status, and the Charlson comorbidity index that predicts the risk of death within a year of hospitalisation for patients with specific comorbid conditions.

The B.1.1.529 (Omicron) variant has previously been reported as more transmissible, but less severe than other SARS-CoV-2 variants.

To test this assumption, the researchers linked state-level vaccination data with quality-controlled electronic health records from a large healthcare system, including 13 hospitals, in Massachusetts, US.

The team, including researchers from Massachusetts General Hospital, Minerva University and Harvard Medical School, US then compared risks of hospital admission and mortality across the SARS-CoV-2 waves in over 130,000 COVID-19 patients.

Although the unadjusted rates of hospital admission and mortality appeared to be higher in previous waves compared to the Omicron period, the risks of hospitalisation and mortality were nearly identical.

"After adjusting for confounders we found that the risks of hospitalisation and mortality were nearly identical between periods. Our analysis suggests that the intrinsic severity of the Omicron variant may be as severe as previous variants, the authors of the study noted.

The researchers noted that the Omicron variant has been reported as more transmissible, but less severe, than previous variants in a variety of locations including South Africa, Scotland, England, and Canada.

However, understanding the intrinsic severity of Omicron is challenging, they said.

"A number of confounding factors affecting severity in COVID-19 have changed since the start of the pandemic and may continue to change," the authors of the study said.

"Any comparison between SARS-CoV-2 variants without adequately adjusting and controlling for important confounders that may change over time such as vaccination status and healthcare utilisation, can mislead both the public and medical experts of the true danger of the variant," they added.