New concerns over the variant pummelled financial markets, especially stocks of airlines and others in the travel sector, and oil, which tumbled by about $10 a barrel.
The discovery of a new coronavirus variant named Omicron has triggered global alarm as countries rushed to suspend travel from southern Africa and stock markets on both sides of the Atlantic suffered their biggest falls in more than a year.
The World Health Organisation (WHO) said on Friday Omicron may spread more quickly than other forms, and preliminary evidence suggested there is an increased risk of reinfection.
Those worries pummelled financial markets, especially stocks of airlines and others in the travel sector, and oil, which tumbled by about $10 a barrel.
The Dow Jones Industrial Average closed down 2.5 percent, its worst day since late October 2020, and European stocks had their worst day in 17 months.
Cruise operators Carnival Corp, Royal Caribbean Cruises and Norwegian Cruise Line plunged more than 10 percent each, while shares in United Airlines, Delta Air Lines and American Airlines slumped almost as much.
Too late for travel curbs?
Epidemiologists warned travel curbs may be too late to stop Omicron from circulating globally.
The new mutations were first discovered in South Africa and have since been detected in Belgium, Botswana, Israel and Hong Kong.
The United States will restrict travel from South Africa and neighbouring countries effective Monday, a senior Biden administration official said.
Going further, Canada said it was closing its borders to those countries, following bans on flights announced by Britain, the European Union and others.
But it could take weeks for scientists to fully understand the variant's mutations and whether existing vaccines and treatments are effective against it. Omicron is the fifth variant of concern designated by the WHO.
The variant has a spike protein that is dramatically different than the one in the original coronavirus that vaccines are based on, the UK Health Security Agency said, raising fears about how current vaccines will fare.
Scientists issued similar warnings.
"This new variant of the Covid-19 virus is very worrying. It is the most heavily mutated version of the virus we have seen to date," said Lawrence Young, a virologist at Britain's University of Warwick.
"Some of the mutations that are similar to changes we've seen in other variants of concern are associated with enhanced transmissibility and with partial resistance to immunity induced by vaccination or natural infection."
Several other countries including India, Japan, Israel, Turkey, Switzerland and the United Arab Emirates also toughened travel curbs.