US biotechnology firm Moderna on Monday said lab studies showed its Covid-19 vaccine would remain protective against variants of the coronavirus first identified in the United Kingdom and South Africa.

But out of caution, the company will test adding a second booster of its vaccine -- to make three shots in total -- and has begun preclinical studies on a booster specifically for the South African variant.

"We are encouraged by these new data, which reinforce our confidence that the Moderna COVID-19 Vaccine should be protective against these newly detected variants," said Stephane Bancel, Moderna's CEO.

"Out of an abundance of caution and leveraging the flexibility of our mRNA platform, we are advancing an emerging variant booster candidate against the variant first identified in the Republic of South Africa into the clinic."

To study the impact of the existing vaccine, called mRNA-1273, Moderna took blood samples from eight people who had received two doses of the vaccine, and two primates that had also been immunized.

For the B.1.1.7 variant, first identified in the UK, there was no impact on the level of neutralizing antibodies -- which bind to the virus and prevent it from invading human cells -- that were produced by the shots.

But for the South African variant, B.1.351, there was a sixfold reduction in the neutralizing antibody level.

Even so, it remained above the quantity that was shown to be protective in earlier tests on primates that were infected on purpose.

The company, which carried out the studies with the National Institutes of Health, has submitted the study to a preprint server so it can be analyzed by the wider scientific community.