The WHO said Tuesday that those not fully vaccinated against Covid-19 who are also vulnerable to the disease, including over-60s, should put off travel to areas with community transmission.

The World Health Organization also said blanket travel bans would not stop the spread of the Omicron variant.

The new Covid-19 variant of concern, which the WHO says poses a "very high" risk globally, has prompted many countries to shut their borders.

"Blanket travel bans will not prevent the international spread, and they place a heavy burden on lives and livelihoods," the WHO said in a travel advice statement on Omicron.

"In addition, they can adversely impact global health efforts during a pandemic by disincentivising countries to report and share epidemiological and sequencing data."

First reported to the WHO less than a week ago after being detected in southern Africa earlier this month, Omicron has already appeared in several countries.

The WHO noted the increasing number of governments introducing travel measures, including temporarily banning arrivals from countries where the variant has been found.

The WHO said that as of Sunday, 56 countries were reportedly implementing travel measures aimed at potentially delaying the importation of the new variant.

"It is expected that the Omicron variant will be detected in an increasing number of countries as national authorities step up their surveillance and sequencing activities," it said.

The WHO later issued a correction to the final part of that travel advice, relating who should be advised to postpone travel, and to where.

"Persons who have not been fully vaccinated or do not have proof of previous SARS-CoV-2 infection and are at increased risk of developing severe disease and dying, including people 60 years of age or older or those with comorbidities that present increased risk of severe Covid-19 (e.g. heart disease, cancer and diabetes) should be advised to postpone travel to areas with community transmission," the WHO's corrected line said.

Elsewhere, the WHO advised countries to apply an "evidence-informed and risk-based approach" when implementing travel measures.

The UN health agency said national authorities in countries of departure, transit and arrival could apply mitigation measures that might delay or reduce the exportation and importation of the variant.

They could include screening passengers, testing and quarantine.

"All measures should be commensurate with the risk, time-limited and applied with respect to travellers' dignity, human rights and fundamental freedoms."

The WHO said that "essential international travel", including for humanitarian missions, repatriations and transport of vital supplies, should always be prioritised during the pandemic.

- 'Calm, coordinated, coherent' -

Earlier Tuesday, WHO chief Tedros Adhanom Ghebreyesus told member states to keep calm and take "rational" steps in response to Omicron.

"We call on all member states to take rational, proportional risk-reduction measures," he said.

"The global response must be calm, coordinated and coherent."

Tedros stressed that it remains unclear how dangerous the variant is.

"We still have more questions than answers about the effect of Omicron on transmission, severity of disease, and the effectiveness of tests, therapeutics and vaccines," he said.

The WHO chief said it was understandable that countries wanted to protect their citizens "against a variant that we don't yet fully understand".

"But I am equally concerned that several member states are introducing blunt, blanket measures that are not evidence-based or effective on their own, and which will only worsen inequities."

Omicron in Europe before South Africa reported first cases

The Omicron coronavirus variant was present in Europe before the first cases were reported in South Africa, new data from the Netherlands showed Tuesday, as Latin America reported its first two cases in Brazil.

In the week since the new virus strain was reported to the World Health Organization by South Africa, dozens of countries around the world have responded with travel restrictions -- most targeting southern African nations.

But the World Health Organization warned Tuesday -- as Canada expanded its restrictions to also include Egypt and Nigeria -- that "blanket" travel bans risked doing more harm than good.

And the likely futility of broad travel restrictions was underscored as Dutch authorities reported that Omicron was present in the country before South Africa officially reported its first cases, on November 25.

The new variant -- whose high number of mutations the WHO believes may make it more transmissible or resistant to vaccines -- was found in two Dutch test samples from November 19 and 23, with one having no travel history.

With countries now on alert for the Omicron variant, a clearer picture is emerging by the day of where it has been circulating, and for how long.

So far, well over a dozen countries and territories have detected cases, including Australia, Britain, Canada, Hong Kong, Israel, Italy and Portugal.

Among European countries, Belgium and Germany have both reported cases of the new strain prior to November 25, but both linked to foreign travel.

Latin America reported its first two cases Tuesday -- in people who traveled from South Africa to Brazil -- and a first case was confirmed in Japan, one day after it barred all foreign arrivals.

- Vaccine-resistant? -

While much is still unknown about the Omicron variant -- it could take weeks to determine whether and to what extent it is vaccine-resistant -- it has highlighted that the global fight against Covid-19 is far from over.

Asian, European and US markets all fell on Tuesday, while the benchmark US oil price tumbled more than five percent after the boss of vaccine manufacturer Moderna warned existing jabs might be less effective against the new variant.

"All the scientists I've talked to ... are like 'this is not going to be good'," Moderna boss Stephane Bancel told the Financial Times, warning against a "material drop" in the effectiveness of current jabs against Omicron.

Moderna, US drugmaker Pfizer and the backers of Russian vaccine Sputnik V are already working on an Omicron-specific vaccine.

On the treatment front, meanwhile, a panel of US health experts voted Tuesday to endorse Merck's Covid pill for high-risk adult patients, which is already authorised in Britain.

- 'Heavy burden' -

In a briefing to WHO member states, the body's chief Tedros Adhanom Ghebreyesus said it was understandable for countries to seek to protect their citizens "against a variant we don't yet fully understand".

But he called for the global response to be "calm, coordinated and coherent", urging nations to "take rational, proportional risk-reduction measures".

The UN agency cautioned in a travel advisory that "blanket" travel bans risked placing a "heavy burden on lives and livelihoods" and could ultimately dissuade countries from sharing data about the evolving virus -- as South Africa did in reporting the latest variant.

But it did advise that people not fully vaccinated and considered vulnerable to Covid-19, including over-60s, should put off travel to areas with community transmission of the virus -- after correcting a previous statement that indicated all over-60s should defer travel.

- Tougher measures -

Omicron has emerged as much of the world was already bracing for a new winter wave of the pandemic -- leaving even nations with high vaccination rates scrambling to stem infections and prevent health services from being overwhelmed. 

Governments, particularly in western Europe, have already reintroduced mandatory mask-wearing, social-distancing measures, curfews or lockdowns -- leaving businesses fearing another grim Christmas.

In Germany, incoming Chancellor Olaf Scholz said parliament would vote on making Covid vaccines compulsory by the end of the year -- and a source from Scholz's party told AFP he had "signalled his sympathy" for such a rule.

Greece went ahead Tuesday in making vaccines compulsory for over-60s, while Norway will offer booster shots to all adults before Easter, as preferable to a lockdown.

Britain meanwhile set a target of delivering third jabs to all adults within two months, with Prime Minister Boris Johnson saying vaccination centres would be "popping up like Christmas trees."