The head of drugmaker Moderna said Covid-19 vaccines are unlikely to be as effective against the Omicron variant of the coronavirus as they have been previously, sparking fresh worry in financial markets about the trajectory of the pandemic.

"There is no world, I think, where (the effectiveness) is the same level . . . we had with Delta," Moderna Chief Executive Stéphane 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.'"

Vaccine resistance could lead to more sickness and hospitalisations and prolong the pandemic, and his comments triggered selling in growth-exposed assets like oil, stocks and the Australian dollar.

Bancel added that the high number of mutations on the protein spike the virus uses to infect human cells meant it was likely the current crop of vaccines would need to be modified.

He had earlier said on CNBC that it could take months to begin shipping a vaccine that does work against Omicron.

Fear of the new variant, despite a lack of information about its severity, has already triggered delays to some economic reopening plans and the reimposition of some travel and movement restrictions.