Chinese biopharmaceutical firm Sinovac said Friday a third production line for its Covid-19 vaccine has been put into use, doubling its annual capacity of the jabs to two billion doses.

Its CoronaVac is one of four domestic vaccines given conditional approval by Chinese authorities, which helps rush emergency drugs to market.

On Wednesday, experts from the World Health Organization said an interim analysis of clinical trial data from two Chinese vaccines, including Sinovac's product, showed they demonstrated "safety and good efficacy", although more data is still needed.

"Over 200 million doses of CoronaVac have been delivered to over 20 countries, including China," said Sinovac in its latest statement.

Although Sinovac's jab has been approved by domestic regulators, it has yet to receive authorisation by what the WHO considers "a stringent regulatory authority", noted the UN health agency's Strategic Advisory Group of Experts on Immunization (SAGE).

Sinovac is among Chinese firms to have submitted data in applications for the WHO's emergency use listing, which opens the door for the jabs to join the global Covax programme, which aims to ensure equitable access to Covid-19 vaccinations.