WASHINGTON, Dec 16 ― Booster doses of currently available Covid-19 vaccines work against the Omicron variant of the coronavirus and there appears to be no need for variant-specific boosters, top US infectious disease expert Anthony Fauci said yesterday.

"Our booster vaccine regimens work against Omicron. At this point, there is no need for a variant-specific booster,” Fauci told reporters at a White House briefing.

He said the neutralising activity of two doses of Moderna's Covid-19 vaccine is "substantially low” against Omicron, citing data from the National Institute of Allergy and Infectious Diseases, of which he is director.

"However, if you look at two weeks post the third dose, note the substantial degree of elevation of the neutralising title; well within the range of neutralising Omicron,” he said of the study's findings, on which Moderna collaborated with the NIAID.

BioNTech and Pfizer said last week that a three-shot course of their vaccine was able to neutralise Omicron in a laboratory test, but two doses resulted in significantly lower neutralising antibodies.

J&J has yet to release any of its own data about how its vaccine performs against the new coronavirus strain.

All three US-authorised Covid-19 vaccines appear to be significantly less protective against Omicron in laboratory testing, but a booster dose likely restores most of the protection, according to a study published on Tuesday.

There are confirmed cases of the variant in at least 36 states, representing about 3 per cent of Covid-19 cases in the country, US Centres for Disease Control and Prevention Director Rochelle Walensky said at the same briefing. The Delta variant remains responsible for the vast majority of cases, she added.

"We expect to see the proportion of Omicron cases here in the United States continue to grow in the coming weeks. Early data suggests that Omicron is more transmissible than Delta, with a doubling time of about two days,” Walensky said.

She said officials were starting to see Covid-19 cases increase in fully vaccinated nursing-home residents, but residents who have had a booster shot have an infection rate that is 10 times lower. ― Reuters