By Michael Erman

(Reuters) – 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 test subjects, which could take months, U.S. regulators said on Monday.

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 immune response in their current form, the U.S. Food and Drug Administration said in a statement.

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

Acting FDA Commissioner Janet Woodcock said drugmakers should not wait until a mutated virus is demonstrated to be able to escape the vaccines to begin developing new versions.

“We need to anticipate this and work on it, so that we have something in our back pocket before the threshold is upon us,” she said at a news conference.

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

Pfizer and Moderna have both said they plan to run clinical trials of versions of their vaccine that have been redesigned to combat the highly contagious COVID-19 variant that has become prevalent South Africa and has turned up in several U.S. States.

The FDA made the comments as part of a newly updated guidance for companies making vaccines, tests and therapeutics for COVID-19.

As part of its updated guidance, the FDA recommended that vaccine makers test any modified vaccines in both previously unvaccinated people and vaccinated people. The manufacturers should compare the immune response of a modified vaccine against both the new variant as well as the original virus.

The FDA also recommended monitoring test subjects’ safety for at least seven days, to support emergency use authorization for modified vaccines.

The agency is trying to allow drugmakers to make the change with a minimal amount of extra data needed, said Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research.

Still, the type of trial the agency wants, “might take a few months, whether it be two or three,” Marks said. “I can’t say exactly how long but again, that type of the scale of the amount of time.”

“We want to be ready so that we can move it into production, when it’s ready and when it’s needed,” he said.

(Additional reporting by Manojna Maddipatla and Mrinalika Roy in Bengaluru; Editing by Jonathan Oatis and Bill Berkrot)