A new move by the US has sparked fears the country will hog a Covid-19 vaccine when one is developed.

The US bought up almost all of the world's supply of Remdesivir - when the drug showed promising signs in early attempts to treat people with Covid-19.

Remdesivir, which is currently administered through infusions, is the only drug that has an emergency use authorisation from the US Food and Drug Administration to treat coronavirus infections. 

However, The Guardian reports that the move by the US means that no other country will be able to access the drug for at least three months.

Former Prime Minister Helen Clark has signed an international letter - calling for an eventual vaccine to be shared.

She told Heather du Plessis-Allan it's a real concern.

"We need to be clubbing together with all those countries saying it should have an equitable rollout.

"If it's just monopolised by just upper income countries, that would be desperately unfair."