TAOYUAN, Taiwan—Taiwan welcomed 2.5 million COVID-19 vaccine doses from the United States on Sunday as help from a true friend, more than doubling the major semiconductor-producing island’s arsenal of shots as it deals with a cluster of domestic infections. Washington, competing with Beijing to deepen geopolitical clout through “vaccine diplomacy,” initially had promised to donate…

The UK government has announced the world’s first clinical trial of switching between different COVID-19 vaccines for the first and second doses. Scientists are hoping that the trial—which won’t yield initial results until the summer—will reveal how vaccines could be used more flexibly, potentially resolving logistical and supply challenges. “It is also even possible that…