CAMBRIDGE, Mass. – Moderna says its COVID-19 vaccine protects against worrisome emerging variants of the coronavirus but it’s taking the precaution of testing a possible booster dose against the strain discovered in South Africa.

In Monday’s announcement, Moderna CEO Stephane Bancel said the move was out of “an abundance of caution” after preliminary lab tests suggested its shot produced a weaker immune response to that variant.

Vaccine manufacturers have been testing their shots against the mutated strains including two that first emerged in Britain and South Africa. In a study conducted with the National Institutes of Health, Moderna used blood samples from eight vaccine recipients, and some immunized monkeys, in laboratory tests against the mutated viruses.

The vaccine was effective against both variants but researchers found a six-fold drop in levels of “neutralizing antibodies” against the strain from South Africa. Moderna said while the levels still were protective, it has begun developing a booster vaccine targeted to that new strain, called the B.1.351 variant. In addition, Moderna will test if simply giving an extra dose of the original vaccine could be helpful.

Pfizer, which makes a similar COVID-19 vaccine, has previously reported that its shot also appears effective against the strain from Britain. But other research has raised questions about the variant from South Africa.

Meanwhile, Dr. Anthony Fauci says scientists are already preparing to upgrade COVID-19 vaccines to address the variants of the coronavirus now circulating in the United Kingdom and South Africa.

Fauci, the top U.S. infectious disease expert, says those variants are not only more infectious but they do not respond as well to the monoclonal antibodies that have been used in treating patients. He said he was especially concerned about the South African variant, which he described as “different and more ominous than the one in the UK.”

”The data has not come out officially, but taking a look at the preliminary data that the UK scientists have analyzed, I’m pretty convinced that there is a degree of increase in seriousness of the actual infection, which we really have to keep an eye on,” Fauci told NBC’s “Today.”

Fauci said there is also “a very slight, modest diminution” of the effectiveness of COVID-19 vaccines against those variants but “there’s enough cushion with the vaccines that we have that we still consider them to be effective against both the UK strain and the South Africa strain.”