The coronavirus vaccine developed by Moderna triggers an immune response that protected in laboratory tests against two variants of the virus first detected in Britain and South Africa, the company said Monday. 

The finding was largely encouraging but contained a clear warning sign: Even though the vaccine generated disease-fighting antibodies that worked against the South African variant, that efficacy was diminished. Moderna said there was a reduction in response, prompting the company to design a new potential vaccine that could be added to the current two-dose regimen.

As a precaution, Moderna announced that it will begin two new studies. The company will test adding a third shot of its current vaccine to boost its two-dose regimen. The biotechnology company also plans early human tests of the all-new vaccine specific to the South African variant, using it as a booster to prime the immune system to the new strain.

"The virus is changing its stripes, and we will change to make sure we can beat the virus where it's going," Moderna President Stephen Hoge said in an interview. "The unknown is: Would we feel it's necessary to do that, would public health officials want this at that point or would they still be comfortable? What we're trying to do is create an option."

The success of two remarkably effective coronavirus vaccines in record time last year provided the world some breathing room and hope, even as the pandemic surged. But the detection of both variants late last year caused immediate concern, first because of evidence that they were spreading far more easily. Many of the mutations in each variant - eight of those found in the British variant and 10 of those in the South African variant - drew special concern because they sit in the spiky proteins that dot the outside of the coronavirus and have been the key target for vaccines and therapeutics. That raised the specter that the current generation of vaccines might be rendered obsolete before they have even been fully rolled out.

The company's announcement puts some of those fears to rest, suggesting that the vaccines will still work against both variants. But the science on vaccines is far from resolved, as was illustrated by the surprising announcement Monday that Merck, a longtime vaccine company, was abandoning its two candidates, finding that both triggered inferior immune responses compared with other vaccines and natural infection.

The Merck vaccine effort started later and significantly lagged other companies, and the nation's vaccination plan wasn't heavily reliant on a possible Merck shot. The two authorized vaccines use a novel approach that had never been used in an authorized vaccine, while Merck employed a more traditional approach.

Late last year, Sanofi and GlaxoSmithKline announced that they were redesigning their vaccine after a first version did not cause a robust immune response in older adults and didn't think that their candidate would be available until the end of the year.

In the Moderna study, antibody-containing blood serum taken from people and monkeys who received the vaccine showed that it was just as effective at blocking the British variant as the original strain of virus in the study. And it remained above the threshold for efficacy for the South African variant, despite the diminution in effectiveness. The work has been submitted to a preprint server, but has not been peer-reviewed and was not available for review before publication.

The company reported a decrease in the ability of antibodies to block the South African variant - a drop that Hoge said was concerning but not alarming - underscoring the need to remain vigilant.

The report comes after similar news from Pfizer-BioNTech, which released data last week that also was not yet peer-reviewed. Pfizer found that antibody-rich blood serum samples from 16 vaccinated people showed that the vaccine was equally as effective at blocking the British variant as it was against the original version of the virus that took hold in Wuhan, China, a year ago. That publication did not address the South African variant, which has been of most concern and shares many mutations with a concerning variant detected in Brazil.

Laboratories worldwide have been scrambling to study whether vaccines and treatments, particularly monoclonal antibodies, are likely to be as effective against the new variants. Such tests have mainly relied on one part of the multifaceted immune response and found evidence that the vaccines are likely to be effective. But those tests have underscored the need to track changes in the virus and prepare for the eventuality that, like flu shots, a coronavirus vaccine may need to be updated and administered on a regular basis.