Drugmaker Moderna set off fresh alarm bells in financial markets on Tuesday as the firm's chief warned that COVID-19 vaccines are unlikely to be as effective against the Omicron variant as they have been against the Delta version.

Crude oil futures, shed more than a dollar, the Australian currency hit a year low, and Nikkei gave up its gains as Stéphane Bancel's comments spurred fears that vaccine resistance could lead to more sickness and hospitalisations, prolonging the pandemic.

"There is no world, I think, where (the effectiveness) is the same level...we had with Delta," Moderna CEO Bancel told the Financial Times in an interview.

"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'," Bancel said.

Omicron - which the World Health Organization (WHO) said carries a "very high" risk of infection surges - has triggered global alarm, with border closures casting a shadow over a nascent economic recovery from a two-year pandemic.

News of its emergence wiped roughly $2 trillion off the value of global stocks on Friday, although some calm was restored this week as investors waited for more data on the characteristics of Omicron.

Remarks by President Joe Biden that the United States would not reinstate lockdowns had also helped soothe markets before comments from the Moderna chief spooked investors.

Biden has called for wider vaccination, while the U.S. Centers for Disease Control and Prevention has urged everyone aged 18 years and older to get a booster shot. Britain too has expanded its COVID-19 booster programme amid Omicron fears.

Hong Kong expands curbs

Fear of the new variant has prompted countries around the world to move quickly to tighten border controls to prevent a recurrence of last year's strict lockdowns and steep economic downturns. 

Hong Kong authorities have expanded a ban on entry for non-residents from several countries. It said non-residents from Angola, Ethiopia, Nigeria and Zambia would not be allowed to enter as of Nov. 30.

Additionally, non-residents who have been to Austria, Australia, Belgium, Canada, the Czech Republic, Denmark, Germany, Israel and Italy in the past 21 days would not be allowed to enter the city from Dec. 2, it added.

The global financial hub, among the last places pursuing a zero-COVID strategy, has already banned non-residents arriving from South Africa, Botswana, Eswatini, Lesotho, Malawi, Mozambique, Namibia and Zimbabwe.

In Australia, five travellers tested positive for Omicron.

They are vaccinated and in quarantine, officials said, adding they are asymptomatic or display very mild symptoms.

Singapore's health ministry said two travellers from Johannesburg who tested positive for the variant in Sydney had transited through its Changi airport.

Australian authorities have also identified a sixth traveller who was most likely infected with the variant and had spent time in the community. 

Canberra delayed on Monday the reopening of the nation's borders for international students and skilled migrants, less than 36 hours before they were due to be allowed back in.

"We're doing this out of an abundance of caution but our overwhelming view is that whilst (Omicron) is an emerging variant, it is a manageable variant," Federal Health Minister Greg Hunt told a media conference. 

Curbs and concerns

First reported on Nov. 24 from South Africa, Omicron has since spread to over a dozen countries.

The WHO has urged countries to use a "risk-based approach to adjust international travel measures".

The global curbs have, however, triggered concerns about vaccine inequality.

"The people of Africa cannot be blamed for the immorally low level of vaccinations available in Africa – and they should not be penalized for identifying and sharing crucial science and health information with the world," U.N. Secretary-General Antonio Guterres said in a statement.

India, home to the world's largest vaccine maker, has approved supplies of COVID-19 vaccines to many African countries and said it stands ready to "expeditiously" send more. China too has pledged 1 billion doses to the continent.