Air travellers to the US will face tougher Covid-19 testing rules to try to slow the spread of the omicron variant and other countries tightened border controls as a European leader urged all concerned to “prepare for the worst”.

A World Health Organisation official said 24 countries may have reported cases of the variant so far but that some of the early indications were that most cases were mild, with none severe. Travel bans had consequences, he said, but there would be more mutations without other measures to contain its spread.

Staving off omicron while scientists establish how easily it can spread and whether it can evade vaccine protection is a “race against time” the president of the EU’s executive commission said, emphasising the role of vaccines.

“Prepare for the worst, hope for the best,” Ursula von der Leyen told a news conference, adding that according to scientists, full vaccination and a booster shot provide the strongest possible protection.

Ghana, Nigeria, Norway, Saudia Arabia and South Korea were among the latest countries to report cases of the variant. Britain reported 22 cases so far, a number it said would certainly go up.

Australia said at least two people visited several places in Sydney while likely infectious and Denmark said an infected person had taken part in a large concert.

Japan, which had already barred all new foreign entrants, reported its second case of the new variant.

Such curbs have become more tangled as they have spread.

Hong Kong added Japan, Portugal and Sweden to its travel restrictions while Uzbekistan said it would suspend flights with Hong Kong as well as South Africa. Malaysia temporarily barred travellers from eight African countries and said Britain and the Netherlands could join the list.

The WHO said “blanket travel bans will not prevent the international spread and they place a heavy burden on lives and livelihoods”, while advising those unwell, at
risk or 60 years and over and unvaccinated to postpone travel.

Global shares came off lows plumbed on Tuesday after remarks by the CEO of Moderna raised questions about the efficacy of Covid-19 vaccines against omicron. Health officials have since offered reassurances and said it is very likely vaccines will still prevent people getting seriously ill.