The World Health Organization yesterday said preliminary evidence indicated that Covid-19 vaccines might be less effective against infection and transmission linked to the Omicron coronavirus variant, a day after a major US study said all three US-authorised vaccines appear to be significantly less protective against the variant.

The warning comes as the WHO on Tuesday warned Omicron was spreading at an unprecedented rate and urged countries to act. The WHO said the variant also carries a higher risk of reinfection.

The US study from researchers at Massachusetts General Hospital (MGH), Harvard and MIT that has not yet been peer reviewed tested blood from people who received the Moderna, Johnson & Johnson and Pfizer/BioNTech vaccines against a pseudovirus engineered to resemble the Omicron variant.

It showed fully vaccinated people do not carry enough antibodies to prevent the new variant, but a booster dose restores most of the protection.

The WHO, in its weekly epidemiological update, said that more data was needed to better understand the extent to which Omicron may evade immunity derived from either vaccines or previous infection.

Omicron, first detected by South A9.frica and reported to the WHO on November 24, has a large number of mutations, setting alarm bells ringing since its discovery.

WHO chief Tedros Adhanom Ghebreyesus told reporters the strain had been reported in 77 countries and "probably" spread to most nations undetected "at a rate we have not seen with any previous variant".

Omicron now accounts for around three percent of cases in the United States, a figure that is expected to rise rapidly as has been seen in other countries.

The United States is the nation hit hardest by the pandemic, and it crossed 800,000 known Covid-19 deaths on Tuesday, according to a Johns Hopkins University tracker.

Although Britain on Monday confirmed what is thought to be the world's first Omicron death, there is no proof yet that the variant causes more severe illness.

WHO expert Bruce Aylward strenuously warned against "jumping to a conclusion that this is a mild disease".

"We could be setting ourselves up for a very dangerous situation."

Europe is the global coronavirus hotspot, recording 62 percent of the world's total cases in the past seven days, while the five countries with the world's highest infection rates are all European.

EU chief Ursula von der Leyen yesterday warned the Omicron variant of Covid-19 could become dominant in Europe next month, but said her 27-nation bloc had ample vaccines to fight the pandemic.

The Netherlands followed other European nations in reintroducing restrictions on Tuesday as Prime Minister Mark Rutte announced primary schools will shut next week and a night-time lockdown will be extended over Omicron fears.

In neighbouring Britain, the ruling Conservative government on Tuesday suffered a major parliamentary rebellion as almost 100 of its MPs rejected new restrictions as the country responds to Omicron.

Boris Johnson's administration will introduce new rules on mask-wearing, daily testing to avoid isolation and vaccine passes for certain settings in England.

France yesterday began implementing a new tightening of rules in the fight against Covid-19, meaning people aged 65 and over will lose the validity of their health pass unless they have had a third booster shot.

Italy yesterday said it will tighten restrictions for arrivals from the rest of the EU from Thursday, requiring coronavirus tests of everyone and a five-day quarantine for those who are not vaccinated.