Moderna's vaccine should be effective against all COVID-19 mutations but may require a booster to protect against the variant first detected in South Africa, the US biotechnology company said.
The firm said its early laboratory tests showed the vaccine did trigger sufficient quantities of antibodies to fight all key emerging variants of the virus.
But in the South Africa case, it is developing a booster “out of an abundance of caution”.
A Moderna announcement on Monday said this was because its tests had observed a six-fold reduction in neutralising antibodies compared to other variants.
The vaccine is effective against a separate variant identified in Britain, it added.
“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 Moderna's chief executive Stéphane Bancel.
The additional clinical studies would help “determine if it will be more effective to boost titers (antibody concentrations) against this and potentially future variants”, he added.
In addition to the United States, the original vaccine has been approved for use by Canada, Britain and the European Union.
Moderna said its studies covered "all key emerging variants" of COVID-19, but Monday's announcement only contained the results from its testing against the variants first identified in South Africa and the UK.
Earlier this month South Africa's government announced urgent testing to see if vaccines for coronavirus were effective against the variant spreading in the country.