The drug maker Moderna is set to begin testing a new version of its Covid-19 vaccine that was specifically designed to target an emerging variant of the coronavirus that was first reported in South Africa.

The company announced that it has shipped doses of its variant-specific booster shot to the U.S. National Institutes of Health for clinical trials. This development is part of Moderna’s tactics to counter strains of the coronavirus that are circulating and to get ahead of the constantly mutating virus.

“Moderna is committed to making as many updates to our vaccine as necessary until the pandemic is under control,” Moderna CEO Stéphane Bancel said in a statement.

There is no evidence yet to suggest that the coronavirus has mutated in a way that makes it able to evade the existing vaccines, but that prospect remains a serious concern for scientists around the world.

Early studies found that Moderna’s vaccine was less protective against the South African variant, but the company said the observed level of neutralizing antibodies — which can bind to viruses and block them from getting into cells — stayed above protective levels.

Still, the preliminary findings prompted Moderna to begin tweaking its vaccine to make it more effective against the South African variant and other known strains of the virus, including one that was first reported in the United Kingdom and another that is thought to have emerged in Brazil.

An early analysis found that Moderna’s vaccine is effective against the U.K. variant, which is already spreading rapidly around the world. A separate vaccine developed by Pfizer-BioNTech was also found to protect against the U.K. variant, with that vaccine only slightly less effective against the South African strain, according to a study that has yet to be peer-reviewed.

Cases of the U.K. variant have been reported in more than 80 countries, and in more than half of the states in the U.S. The South African strain has been detected in at least 40 countries, including the U.S.

Moderna has already begun tests using a third dose of its vaccine as a booster shot in people who already received two doses, but will now start clinical trials to evaluate the safety and effectiveness of its variant-specific booster. The company will also test a “multivalent booster candidate” that combines its original vaccine with the variant-specific version into a single dose.

As part of the new trials, Moderna will evaluate the immune responses in people who are already vaccinated as well as participants who are receiving a vaccine for the first time.