An advisory panel of the World Health Organisation (WHO) classified on Friday a new Covid-19 variant first detected in South Africa as a "highly transmissible" virus of concern and named it Omicron under its Greek-letter system.
The panel said early evidence suggests an increased risk of reinfection.
The discovery of a new coronavirus variant sent a chill through much of the world on Friday as nations raced to halt air travel, markets fell sharply and scientists held emergency meetings to weigh the exact risks, which were largely unknown.
Medical experts and bodies, including the WHO, warned against any overreaction before the variant that originated in southern Africa was better understood. But a jittery world feared the worst nearly two years after Covid-19 emerged and triggered a pandemic that has killed more than five million people around the globe.
There was no immediate indication whether the variant causes more severe disease. As with other variants, some infected people display no symptoms, South African experts said.
Even though some of the genetic changes appear worrisome, it was unclear if the new variant would pose a significant public health threat. Some previous variants, like the Beta variant, initially concerned scientists but did not spread very far.
The 27-nation European Union (EU) imposed a temporary ban on air travel from southern Africa, and stocks tumbled in Asia, Europe and the US.
The Dow Jones Industrial Average dropped more than 1,000 points. The S&P 500 index was down 2.3 per cent, on pace for its worst day since February. The price of oil plunged nearly 12pc.
Meanwhile, EU Commission President Ursula von der Leyen said, "flights will have to be suspended until we have a clear understanding about the danger posed by this new variant, and travellers returning from this region ( southern Africa) should respect strict quarantine rules.
She insisted on extreme caution, warning that mutations could lead to the emergence and spread of even more concerning variants of the virus that could spread worldwide within a few months.
The United Kingdom also banned flights from South Africa and five other southern African countries at noon on Friday and announced that anyone who had recently arrived from those countries would be asked to take a coronavirus test.
The Japanese government announced that Japanese nationals travelling from Eswatini, Zimbabwe, Namibia, Botswana, South Africa and Lesotho will have to quarantine at government-dedicated accommodations for 10 days and take Covid-19 tests on the third, sixth and tenth days. Japan has not yet opened up to foreign nationals.
Belgium became the first EU country to announce a case of the variant.
"It's a suspicious variant,” Health Minister Frank Vandenbroucke said. "We don't know if it's a very dangerous variant."
Showing how complicated the spread of a variant can be, the Belgian case involved a traveller who returned to Belgium from Egypt on November 11 but did not become sick with mild symptoms until Monday, according to professor Marc Van Ranst, who works for the scientific group overseeing the Belgian government's Covid-19 response.
Israel, one of the world's most vaccinated countries, announced on Friday that it also detected its first case of the new variant in a traveller who returned from Malawi. The traveller and two other suspected cases were placed in isolation. Israel said all three were vaccinated, but officials were looking into the travellers' exact vaccination status.
After a 10-hour overnight trip, passengers aboard KLM Flight 598 from Capetown, South Africa, to Amsterdam were held on the edge of the runway on Friday morning at Schiphol airport for four hours pending special testing. Passengers aboard a flight from Johannesburg were also being isolated and tested.
"It's ridiculous. If we didn't catch the dreaded bugger before, we're catching it now," said passenger Francesca de Medici, a Rome-based art consultant who was on the flight.
Meanwhile, Dr Anthony Fauci, the US government's top infectious disease expert, said, "It has yet to be detected in the United States."
Abroad, the variant seems to be spreading at a reasonably rapid rate, he told CNN. And although it may be more transmissible and resistant to vaccines than other variants, “we don't know that for sure right now.
Some experts said the variant's emergence illustrated how rich countries' hoarding of vaccines threatens to prolong the pandemic.
Fewer than six per cent of people in Africa have been fully immunised against Covid-19, and millions of health workers and vulnerable populations have yet to receive a single dose. Those conditions can speed up the spread of the virus, offering more opportunities for it to evolve into a dangerous variant.
This is one of the consequences of the inequity in vaccine rollouts and why the grabbing of surplus vaccines by richer countries will inevitably rebound on us all at some point, said Michael Head, a senior research fellow in global health at Britain's University of Southampton. He urged Group of 20 leaders “to go beyond vague promises and actually deliver on their commitments to share doses".
The new variant has also added to investor anxiety that months of progress containing Covid-19 could be reversed.
Investors are likely to shoot first and ask questions later until more is known, said Jeffrey Halley of foreign exchange broker Oanda.
In a sign of how concerned Wall Street has become, the market's so-called fear gauge, known as the VIX, jumped 48pc to a reading of 26.91. That's the highest reading for the volatility index since January, before vaccines were widely distributed.
Speaking before the EU announcement, Dr Michael Ryan, head of emergencies at the WHO, warned against “knee-jerk responses.”
"We've seen in the past, the minute there's any kind of mention of any kind of variation and everyone is closing borders and restricting travel,” Ryan said. "It's really important that we remain open and stay focused."
The Africa Centres for Disease Control and Prevention agreed and strongly discouraged any travel bans on countries that reported the new variant. It said past experience shows that such travel bans have not yielded a meaningful outcome.