Investors remained on edge on Wednesday, even as financial markets came off lows plumbed a day earlier following remarks by the CEO of Moderna that raised questions about the efficacy of COVID-19 vaccines against Omicron.

In an attempt to stave off hasty border curbs, the World Health Organization (WHO) urged countries to apply “an evidence-informed and risk-based approach” to travel measures, saying “blanket travel bans will not prevent the international spread, and they place a heavy burden on lives and livelihoods”.

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Air travellers to the United States will face tougher COVID-19 testing rules, while more countries tightened their borders amid uncertainty around the virulence of the Omicron variant and its ability to evade vaccine protection.

Japan and Hong Kong said they would expand travel curbs, Malaysia temporarily banned travellers from countries deemed at risk, while Australia braced for more cases of the coronavirus variant after at least two people visited several locations in its biggest city while likely infectious.

In an attempt to stave off hasty border curbs, the World Health Organization (WHO) urged countries to apply “an evidence-informed and risk-based approach” to travel measures, saying “blanket travel bans will not prevent the international spread, and they place a heavy burden on lives and livelihoods”.

Investors remained on edge on Wednesday, even as financial markets came off lows plumbed a day earlier following remarks by the CEO of Moderna that raised questions about the efficacy of COVID-19 vaccines against Omicron.

Global health officials have since offered reassurances and reiterated calls for people to get vaccinated.

“Our best form of defence still remains our vaccines,” British Health Secretary Sajid Javid told Sky News.

“It’s possible of course, it’s possible that it might be less effective. We just don’t know for sure yet. But it’s also very likely that it will remain effective against serious disease,” he said.

Javid said he expected to know more about the variant within two weeks, echoing remarks by other health experts.

European Medicines Agency Executive Director Emer Cooke earlier said that laboratory analyses should indicate over the next couple of weeks whether the blood of vaccinated people has sufficient antibodies to neutralise the new variant.

Forty-two cases of the COVID-19 Omicron variant have been confirmed in 10 European Union countries, the head of the EU’s public health agency said on Tuesday.

Authorities in the 27-nation EU were analysing another six “probable” cases, Andrea Ammon, who chairs the European Centre for Disease prevention and Control (ECDC), told an online conference organised by the EU’s Slovenian presidency.

She said the confirmed cases were mild or without symptoms, although in younger age groups.

BioNTech’s CEO said the vaccine it makes in a partnership with Pfizer would likely offer strong protection against severe disease from Omicron.

Britain and the United States have both pushed their booster programmes in response to the new variant.

First reported in southern Africa a week ago, Omicron has triggered global alarm, roiled markets, led to travel bans, and highlighted the disparity between massive vaccination pushes in rich nations and sparse inoculation in the developing world.

It has spread to more than a dozen countries, with Nigeria among the latest to report cases of the variant.

Some 56 countries were reportedly implementing travel measures to guard against Omicron as of Nov. 28, the WHO said. WHO head Tedros Adhanom Ghebreyesus said he’s concerned that several member states were “introducing blunt, blanket measures”, which “will only worsen inequities”.