The US company that produces the Moderna Covid-19 vaccine says it's ready to test a booster shot to target a variant of the virus first discovered in South Africa .

Early studies showed that its original vaccine - which is approved in the US and EU - is less protective against this strain, which has now been detected in at least 40 countries.

The company says it's ready to do human trials with several different strategies to see what gives more protection - an additional booster to the original vaccine, combining the booster with the original, or giving two separate variant-specific vaccines.

Moderna says the new trials will also monitor the immune responses of people already vaccinated and those receiving their first dose of the vaccine.