The World Health Organisation is warning countries not to impose blanket travel bans over the new Omicron coronavirus variant as governments and scientists try to determine how much protection current vaccines will offer against the strain.
Financial markets fell sharply after the head of drugmaker Moderna said existing COVID-19 vaccines would be less effective against the Omicron variant.
BioNTech's chief executive struck a cautiously positive note, saying the vaccine it makes in a partnership with Pfizer would likely offer strong protection against severe disease from Omicron, which was first reported in southern Africa a week ago.
The questions over vaccine effectiveness came as cases of the new variant spread, with the first reported case in Latin America emerging in Brazil.
The country's health regulator, Anvisa, said a traveller arriving in Sao Paulo from South Africa and his wife were both apparently infected.
Canada will extend its ban on travellers from southern Africa to also cover those from Nigeria, Malawi and Egypt, health officials said on Tuesday, for a total of 10 countries.
Canada will require people arriving by air from all nations except the United States to take a COVID-19 test, Health Minister Jean-Yves Duclos said.
The WHO asked countries to apply "an evidence-informed and risk-based approach" with any travel measures, including possible screening or quarantine of international passengers.
Blanket travel bans would not stop the spread of Omicron, it said.
However, the WHO said people over age 60 who were not fully vaccinated or did not have proof of previous infection and those with underlying health conditions should be advised to postpone travel.
WHO Director-General Tedros Adhanom Ghebreyesus said he understood the concerns about Omicron, 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".
European Medicines Agency (EMA) executive director Emer Cooke told the European Parliament that existing vaccines would continue to provide protection.
Andrea Ammon, chair of the European Centre for Disease Prevention and Control, said the cases of Omicron so far confirmed in 10 European Union countries were mild or without symptoms, although in younger age groups.
News of Omicron's emergence wiped roughly $US2 trillion ($A2.8 trillion) off global stocks on Friday, after it was first identified in southern Africa and announced on November 25.
"There is no world, I think, where (the effectiveness) is the same level ... we had with Delta," Moderna's CEO, Stephane Bancel, told the Financial Times
"I think it's going to be a material drop. I just don't know how much because we need to wait for the data. But all the scientists I've talked to ... are like, 'This is not going to be good'."
The University of Oxford said there was no evidence that current vaccines would not prevent severe disease from Omicron, but it was ready to rapidly update its shot, developed with AstraZeneca, if necessary.
The administration of US President Joe Biden suffered two more legal setbacks on vaccine policy on Tuesday, with federal judges blocking vaccination mandates for healthcare workers and for government contractors in three states.
EMA's Cooke said "cross neutralisation", a lab test for whether the approved vaccines are effective against Omicron, would take about two weeks.
Changed COVID-19 vaccines could be approved within three or four months, she said.
Moderna and fellow drug makers BioNTech and Johnson & Johnson are already working on vaccines that specifically target Omicron. Moderna has also been testing a higher dose of its existing booster.
BioNTech chief executive Ugur Sahin said BioNTech and Pfizer's vaccine was likely to offer strong protection against severe disease from the new variant.
Japan confirmed its first Omicron case on Tuesday, in a traveller from Namibia.
Britain and the United States have pushed their booster programs in response to the new variant.
Australian Associated Press