Up to 100 million COVID-19 vaccine doses could be made available for poorer countries by 2021 under a deal struck Friday for production at the Serum Institute of India.

The deal was announced by Gavi, the Vaccine Alliance, for production of two different potential vaccines at SII, the world's largest vaccine manufacturer by volume. The vaccines will be priced at a maximum $3 per dose and made available to up to 92 countries as part of Gavi's coronavirus vaccine advance market commitment programme.

The deal provides money up front to SII to help them expand capacity. Once the vaccines gain regulatory approval, the doses could be produced as early as the first half of 2021 for distribution to low- and middle-income countries.

"Too many times we've seen the most vulnerable countries left at the back of the queue when it comes to new treatments, new diagnostics and new vaccines," Gavi chief executive Seth Berkley said. "With COVID-19 vaccines we want things to be different. If only the wealthiest countries in the world are protected, then international trade, commerce and society as a whole will continue to be hit hard as the pandemic continues to rage across the globe."

Under the arrangement, SII would manufacture candidate vaccines from British pharmaceuticals group AstraZeneca and US biotech company Novavax, which will be available for procurement if they are successful in attaining full licences.

A total of 165 candidate vaccines are being worked on around the world, according to a World Health Organization overview. Of those, 139 are still in pre-clinical evaluation, while the other 26 are already in the various phases of being tested on humans, with six the furthest ahead, having reached Phase 3 of clinical evaluation.

According to the World Health Organization's overview of candidate vaccines, the AstraZeneca one is in Phase 3, while Novavax is one of six in Phase 1/2. Under the new collaboration, AstraZeneca's candidate vaccine, if successful, will be available to 57 countries through Gavi, while Novavax's would be available to all 92 countries.

Geneva-based Gavi helps vaccinate half the world's children against some of the world's deadliest diseases.