PRESIDENT BIDEN’S announcement that the United States will purchase and share with lower-income nations 500 million doses of the Pfizer-BioNTech vaccine this year and next is a sign of much-needed empathy for millions of people whose lives are threatened by the global pandemic. Mr. Biden also makes a large down payment on restoring American leadership in a world that has been doubting it.

Just last year, then-President Donald Trump announced that he had pulled the United States out of the World Health Organization, frozen U.S. funding for the WHO and refused to participate in the Covax facility to help vaccinate the poorest countries. He advocated an “America first” strategy that pleased his political base but left many allies feeling bruised and wondering whether the United States could reassert the leadership role it had held for more than half a century.

Mr. Biden is showing that it can. He reversed the pullout from the WHO, made the single largest donation of any country to Covax, and now, with the vaccine donation, demonstrated that the United States is ready and able to compete with Russia and China, both of which have boasted that their authoritarian systems are better than democracy and have exported millions of doses of their own vaccines in a bid for favor around the world.

The mRNA vaccine invented by Pfizer-BioNTech and Moderna is the gold standard, delivering extraordinary results against the original virus and different variants. The $3.5 billion purchase of 500 million doses, on top of 80 million surplus U.S. doses pledged earlier, serves the interests of the United States, since an outbreak anywhere is a threat to everyone, everywhere. But it also shows the United States is sensitive to the yawning gap in vaccine access. Thus far, rich nations have grabbed most doses, leaving poor countries empty-handed. The donation will ease the crushing economic pain of the pandemic in the less-developed world, and it will give Mr. Biden important leverage at the Group of Seven summit to persuade other leaders of major democracies to join him.

The doses will go to Covax and the nations of the African Union. The Covax initiative was originally designed to reach the neediest 20 percent of populations in 92 lower-income nations, and it has so far shipped 81.9 million doses of 163 million ordered. But the effort fell seriously behind its original schedule because of the setback in India, where the Serum Institute of India, which had committed to providing a large portion of Covax’s supply, halted exports of vaccine to cope with India’s pandemic surge. The U.S. donation will boost Covax’s ambitions.

The world also needs a vastly improved outbreak surveillance system. The Rockefeller Foundation has just pledged more than $20 million for programs working toward the goal of improved genomic surveillance and data analytics, vital tools for tracking and understanding the spread of pathogens. The foundation is setting up an institute that, working with others, has a goal of identifying disease outbreaks early and stopping them in the first 100 days. When it comes to preparing for the next catastrophe, it is never too early.

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