The Global Alliance for Vaccines and Immunization (GAVI) said on Monday it had signed two advance purchase agreements with Chinese drugmakers Sinopharm and Sinovac to provide up to 550 million Covid-19 vaccines to the COVAX programme.

The new deals include up to 170 million doses of the Sinopharm shot and up to 380 million shots of the Sinovac vaccine, through to the middle of 2022, the statement said. Sinovac confirmed the agreement in a statement.

"The agreements, which come at a time when the Delta variant is posing a rising risk to health systems, will begin to make 110 million doses immediately available to participants of the COVAX Facility, with options for additional doses," GAVI said.

GAVI, which runs the global vaccine sharing scheme COVAX with the World Health Organization (WHO) did not immediately provide details of which countries would receive the doses.

Deliveries can start quickly because both vaccines have already been granted emergency use listing by the WHO, GAVI Chief Executive Officer Seth Berkley said.

"This is yet another example of Gavi's active portfolio management strategy, ensuring the Facility has options in the face of constraints such as supply delays," he added.

COVAX, which distributes vaccines to poorer countries, has struggled to meet its early commitments amid Indian export disruptions, forcing many countries to freeze their inoculation programmes in their early phases.

However, its latest supply forecast shows that the programme is on track to deliver more than 2 billion doses by early 2022. Sinovac and Sinopharm join nine other vaccines and vaccine candidates already in the programme including those by AstraZeneca and Moderna.

Some countries including Thailand and Indonesia which used Sinovac as a first dose are switching to other shots for the second dose to increase protection against Covid-19.

GAVI did not immediately respond to a request for comment about whether it had concerns about the vaccine's efficacy.