Drugmaker Moderna's CEO set off fresh alarm bells in financial markets on Tuesday after he warned that Covid-19 vaccines were 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 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.