PRESIDENT TRUMP touted his “America First” philosophy in his inaugural address as making the United States “unstoppable.” But the reality is that under Mr. Trump, the United States is in retreat. His refusal to join a global vaccine effort organized in part by the World Health Organization is yet another example of America isolated and weak.

The United States has decided not to participate in a global drive to develop, manufacture and equitably distribute a coronavirus vaccine known as the Covid-19 Vaccines Global Access (Covax) Facility. The project is led by the WHO; Gavi, the Vaccine Alliance; and the Coalition for Epidemic Preparedness Innovations, a public-private global vaccine development project. The Covax effort now has 172 nations signed up — Japan, Germany and the European Commission have joined — with nine candidate vaccines in its portfolio, four more under discussion and nine others being evaluated for the longer term. The project is essentially a joint effort to procure vaccines and pool the risk so that when a safe and effective vaccine is found, it can be made available to all nations in the project as soon as possible. Those countries that join the program will be promised enough vaccine doses to cover 20 percent of their population at the outset. The goal is to acquire 2 billion doses by the end of next year.

There are many advantages to such a global effort, especially for smaller nations that are being stricken with the virus but lack resources to develop or purchase vaccine supplies on their own. If the poorer populations of the globe are left unprotected — and if the virus continues to ravage them — then the richer nations will not be free of the scourge, nor will the global economy rebound to full health. An important facet of this is the Covax Advance Market Commitment, launched by Gavi, a financing instrument aimed at helping 92 lower- and middle-income economies in the Covax project. It has raised more than $600 million of an initial goal of $2 billion in seed money from sovereign donors as well as philanthropy and the private sector.

Nothing in the Covax project would prevent the United States from separate contracts to acquire vaccines directly. But by joining the global project, the United States would have a backstop, in case vaccines stumble in development, and it would help a huge swath of the world’s population get an equitable shot at fighting the pandemic.

Once upon a time, the United States would gladly embrace such a program as an expression of leadership and lofty ideals — why generations looked to America as a beacon of hope. Mr. Trump’s decision to walk away is based in part on his irrational grievances against the WHO, from which he is withdrawing the United States on wholly misguided conclusions about how it operates. Mr. Trump’s “America First” is a prescription for retreat from the world, an approach that leaves the United States more isolated and vulnerable than ever before.

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