The world is fast becoming ever more reliant on China for vaccines, with India’s raging virus outbreak stifling its ability to deliver on supply deals, even as the U.S. tries to position itself as a champion of wider access.

Over the past few weeks, leaders of some of the globe’s most populous nations have sought more shots from China despite concerns about their effectiveness. Demand is expected to rise even further if the World Health Organization, as expected, authorizes vaccines from China’s Sinovac Biotech Ltd. and Sinopharm Group Co. Ltd., allowing developing countries in Asia, Africa and Latin America to access them through COVAX, the global vaccination effort.

"China has become not just the largest exporter,” said Yanzhong Huang, a China specialist and senior fellow for global health at the Council on Foreign Relations. "In many countries it has become the only option.”