The Cambridge Institute of Therapeutic Immunology and Infectious Disease study concluded mutations on the virus's spike protein, which makes it able to avoid antibodies, may also reduce how it replicates in the lungs and causes severe disease.

"These observations highlight that Omicron has gained immune evasion properties whilst compromising on properties associated with replication and pathogenicity," the study's abstract stated.

Study leader Ravi Gupta, a professor of clinical microbiology at Cambridge, said there were still challenges ahead despite the seemingly positive findings.

He tweeted: "What does this all mean? Efficient infection of lung cells could correlate with severity of lung disease. Syncitia or fused cells are often seen in respiratory tissues taken following severe disease. Delta was very good at both, in contrast to Omicron. Further work is needed.

"In summary this work suggests that Omicron does appear to have become more immune evasive, but that properties associated with disease progression *may* be attenuated to some extent. The significant growth of Omicron nevertheless represents a major public health challenge."

It comes after advisory group Sage's "situation update" said it is "almost certain that there are now hundreds of thousands of new Omicron infections per day" in England, prompting reports that tighter pandemic restrictions could be introduced after Christmas.

Australian Associated Press