President Trump has spent the pandemic pulling the United States away from global health diplomacy. President-elect Joe Biden wants to jump back in.

Biden has signaled his desire to re-engage with the World Health Organization and other global institutions but will have to do so amid a roiling health crisis — and deep skepticism among U.S. allies about American reliability. So U.S. engagement with the global coronavirus response will be a test of the country’s ability to work with traditional allies in a post-Trump world.

“I don’t think it is going to be possible to glue all the pieces back together in the short or medium term,” said Lawrence Gostin, a professor of global health law at Georgetown University. “There is a ‘we can’t depend on you, America’ attitude right now, and that will not be easily mended. But there are steps we can take.”

During the pandemic, Trump has shown disdain for cooperation on global public health. He promised to leave the WHO. He spurned collaboration on vaccines.

Biden plans a different approach. According to his campaign platform, he will immediately rescind the letter withdrawing the United States from the WHO. The letter, which the Trump administration sent in July, gave the U.N. health agency a year’s notice, though it remained unclear whether the White House could withdraw without congressional approval.

Since the United States had not technically left the organization, it will not “rejoin” but instead recommit to the U.N. health agency after months of rancor and finger-pointing.

Although Trump initially praised the WHO’s handling of the outbreak in China, he changed his tune as the virus hit the United States, accusing the organization of amplifying false Chinese data in the early days of the outbreak and allowing the virus to spread.

Although some of his critique of the WHO’s response to the outbreak in Wuhan resonated beyond the White House — Japan’s deputy prime minister at one point called the WHO the “China Health Organization,” for instance — other parts were widely dismissed as false and disruptive. The White House claim that WHO secrecy allowed the virus to spread to the United States, for instance, was undercut by the not-so-secret revelation that U.S. researchers, physicians and public health experts at the agency’s Geneva headquarters sent real-time updates to the White House.

The WHO has said it will welcome back the United States, and U.S. officials involved in the coronavirus response and other health programs will continue their work in Geneva, just as they have for months.

It remains unclear, however, whether the United States will play a central or sideline role in ongoing talks about how to overhaul the WHO and improve its response to emerging threats. So far, France and Germany have taken the lead.

In terms of international efforts to develop and distribute vaccines, Biden’s team has been less specific, alluding to re-engagement with allies but stopping short of making commitments.

In September, the Trump administration declined to join a global effort to develop, manufacture and equitably distribute a coronavirus vaccine, in part because the United States would not work with the WHO.

Under the plan, known as the Covid-19 Vaccines Global Access Facility, or Covax, rich and poor countries pool money to offer manufacturers volume guarantees for potential vaccines. The idea is to discourage hoarding and focus on vaccinating high-risk people in every participating country first. The goal is 2 billion doses of vaccine by the end of 2021.

Many traditional U.S. allies, including the European Union, Canada and Japan, joined the initiative, as has China. But many of these relatively wealthy countries are backing Covax while also cutting deals directly with manufacturers — a two-track policy the United States may well pursue.

Indeed, several public health experts said it is very likely, though not certain, that the United States will contribute money to the Covax facility while simultaneously pushing ahead on domestic efforts.

Mara Pillinger, an associate in global health policy and governance at Georgetown’s O’Neill Institute for National and Global Health Law, said the United States is also likely to maintain or increase its support for other programs affected by the pandemic, including polio, malaria, HIV/AIDS and tuberculosis.

But the United States’ inability to bring its own coronavirus outbreak under control and its ­falling-out this year with the WHO may hurt U.S. standing and influence in the months ahead.

“How do governments have confidence that the U.S. won’t revert again in four years?” said Pillinger. “There are questions about how much appetite there is for a return to U.S. leadership.”

China has tried to seize on the U.S. retreat, playing up its commitment to multilateral efforts, pledging money to the global fight and joining Covax not long after the United States opted out.

Beijing’s moves have deepened U.S. concern about China’s growing role in multilateral organizations. But European leaders remain skeptical of Chinese efforts and are willing to work with the United States, said J. Stephen Morrison, director of the Global Health Policy Center at the Center for Strategic and International Studies.

He expects European leaders and diplomats to work closely, if somewhat warily, with the Biden administration. “The fact that there is an urgent threat out there will help drive things forward,” he said. “Amid this emergency, there is not a lot of time to be licking their wounds.”