Drugmakers should test any Covid-19 vaccines that have been retooled to combat new variants of the coronavirus in clinical trials designed to track the immune response of hundreds of subjects, which could take months, Reuters reported quoting US regulators.

Vaccine developers may need to modify their shots to provide protection against new variants of the coronavirus that turn up in the United States should they fail to elicit an immune response in their current form, the US Food and Drug Administration said in a statement.

The FDA said it believes currently authorised vaccines from Pfizer Inc with partner BioNTech SE and Moderna Inc are effective against variants currently circulating in the United States.

The emergence of new, more contagious variants has prompted the US government to step up efforts to track coronavirus mutations and try to keep vaccines and treatments effective against any new variants.