However, European Medicines Agency (EMA) executive director Emer Cooke told the European Parliament that, even if the new variant becomes more widespread, existing vaccines will continue to provide protection.
Andrea Ammon, chair of the European Centre for Disease prevention and Control (ECDC), said the 42 cases of the variant so far confirmed in 10 European Union countries were mild or without symptoms, although in younger age groups.
Major European stock markets, spooked by fears that vaccine resistance may prolong the two-year-old pandemic, were down about 1 percent at around 1300 GMT.
US stock index futures were down more than 1 percent.
Crude oil futures shed just under 3 percent, while Tokyo's Nikkei index closed down 1.6 percent.
"There is no world, I think, where (the effectiveness) is the same level . . . we had with Delta," Moderna 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 that it was ready to rapidly engineer an updated version of its shot, developed with AstraZeneca, if necessary.
British Prime Minister Boris Johnson said "we think it's overwhelmingly likely" that booster shots would continue to protect against severe disease.
Moderna did not reply to a Reuters request for comment, or say when it expects to have data on the effectiveness of its vaccine on omicron, which the World Health Organisation (WHO) says carries a very high risk of infection surges.
Regeneron Pharmaceuticals said its COVID-19 antibody cocktail and other similar antiviral treatments could be less effective against the latest variant.
News of omicron's emergence had wiped roughly $2 trillion off global stocks on Friday, after it was identified in southern Africa and announced on Nov 25.
And yet Dutch authorities said the variant had been detected in the Netherlands as early as Nov 19, before two flights arrived from South Africa that were known to have carried the virus.
Cooke said lab tests for "cross neutralisation" would take about two weeks. If there were a need to change COVID-19 vaccines, new ones could be approved within three or four months, she added.
"Vaccination will likely still keep you out of the hospital," said John Wherry, director of the Penn Institute for Immunology in Philadelphia.
Moderna and fellow drugmakers 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.
Uncertainty about the new variant has triggered border closures that have cast a shadow over a nascent economic recovery from the pandemic, just as parts of Europe see a fourth wave of infections as winter sets in.
Many of the travel restrictions have focused, to South Africa's fury, on banning flights to and from southern Africa.
Japan, the world's third largest economy, confirmed its first case of the new variant on Tuesday, in a traveller from Namibia. Australia found that a person with Omicron had visited a busy shopping centre in Sydney while probably infectious.
Britain and the United States have both pushed their booster programmes in response to the new variant. England made face masks compulsory once again in shops and on public transport, and London said international arrivals would have to self-isolate until they get a negative result in a PCR test.
Greece said vaccination would be compulsory for the over-60s, the group seen as most vulnerable to COVID-19.
Australia on Monday delayed the reopening of its international borders by two weeks, less than 36 hours before foreign students and skilled migrants were to be allowed back.
But in Germany, a current hotspot of the previous significant variant, delta, the seven-day average infection rate fell slightly for the first time in three weeks after new restrictions to slow transmission.
Neighbouring Austria, which imposed its fourth full lockdown last week in response to a surge in infections, also registered a drop.
The curbs on travellers from southern Africa have highlighted the inequality of vaccine distribution, which may have given the virus more opportunities to mutate.
The passenger liner Europa was docking in Cape Town on Tuesday in what was meant to be the official start of the first cruise ship season in South Africa's top tourist hub since the pandemic.
After omicron was discovered while they were at sea, many passengers were expected to fly straight home.