The United States has sent 2.5 million doses of the Moderna vaccine for the coronavirus disease 2019 (Covid-19) to Taiwan on Sunday, tripling an earlier pledge in a donation with both public health and geopolitical meaning.

The shipment arrived on a China Airlines cargo plane that left Memphis the previous day. Taiwan’s health minister, Chen Shih-chung, and Brent Christensen, the top U.S. official in Taiwan, were among those who welcomed the plane on the tarmac at the airport outside of the capital, Taipei.

“The donation reflects our commitment to Taiwan as a trusted friend, and a member of the international family of democracies,” the American Institute in Taiwan, the de facto U.S. Embassy, wrote on its Facebook page.

Taiwan, which had been relatively unscathed by the virus, has been caught off guard by a surge in new cases since May and is now scrambling to get vaccines. It has ordered 5.05 million doses directly from Moderna but so far received only 390,000, including a second shipment that arrived Friday.

The U.S. donation also signals its support for Taiwan in the face of growing pressure from China, which claims the self-governing island off its east coast as its territory. The U.S. does not have formal diplomatic ties with Taiwan under what is known as the one-China policy, but is legally bound by its own laws to ensure that Taiwan can defend itself.

The U.S. promised 750,000 vaccine doses for Taiwan earlier this month, sending Sen. Tammy Duckworth and two of her Senate colleagues to the island aboard a military transport plane to make the announcement. Taiwan’s President Tsai Ing-wen said the U.S. had decided to increase the donation through efforts on both sides over the past two weeks.

In a Facebook post, Tsai joined the U.S. in drawing attention to their shared democratic systems. China, which has been ruled single-handedly by the Communist Party since 1949, says Taiwan must eventually come under its control and reserves the right to use force if necessary.

“Whether it is for regional peace and stability or the virus that is a common human adversary, we will continue to uphold common ideas and work together,” Tsai wrote in Chinese.

She has accused China of blocking Taiwan from getting the Pfizer vaccine through BioNTech, the German co-developer. Chinese officials have repeatedly denied the charge, and say China is willing to provide vaccines to Taiwan. Taiwanese law, however, bans the import of Chinese-made medicine.

The U.S. donation follows Japan’s shipment of 1.24 million doses of the AstraZeneca vaccine in early June. Taiwan has ordered 10 million doses from AstraZeneca but has yet to receive most of them.