Half a year in, however, that campaign has lost some of its luster. Officials in several countries have raised doubts about the efficacy of Chinese vaccines, especially against the more transmissible Delta variant. Indonesia, which was early to accept Chinese shots, was recently the epicenter of the virus. Others have complained about the conditions that accompanied Chinese donations or sales.

The setback to China’s vaccine campaign has created a diplomatic opening for the United States when relations between the two countries are increasingly fraught, in part because of the coronavirus. China has criticized the American handling of the crisis at home and even claimed, with no evidence, that the pandemic originated in a military lab at Fort Detrick, Md., not in Wuhan, where the first cases emerged in late 2019.

As more countries turn away from Chinese shots, vaccine aid from the United States offers an opportunity to restore relations in a region that American officials have mostly ignored for years while China extended its influence. The Biden administration has dispatched a crowd of senior officials, including Vice President Kamala Harris, who is scheduled to arrive on Sunday to visit Singapore and Vietnam. It has also, at last, made its own vaccine pledges to Southeast Asia, emphasizing that the American contribution of roughly 23 million shots as of this week comes with “no strings attached,” an implicit reference to China.