President Biden faces growing pressure to distribute coronavirus vaccines to other countries in response to aggressive “vaccine diplomacy” campaigns by China and Russia, a step he has until now ruled out while millions of Americans are still urgently seeking immunizations.

China’s efforts in particular will be a focus of a meeting Friday between Biden and the leaders of Australia, India and Japan. Aides say the four leaders may discuss an agreement in principle to pass out vaccine surpluses after their home populations’ needs have essentially been met.

The United States’ authoritarian adversaries — who worry far less about domestic opinion — have the field mostly free for the moment to send vaccines to nations from Mexico to Lebanon to Uzbekistan.

That has sparked a debate within the administration about how to balance national security, humanitarian needs and political concerns. Activists and experts warn that the United States could be missing a unique opportunity to regain worldwide influence after the isolationism of the Trump years.

J. Stephen Morrison, director of the Global Health Policy Center at the Center for Strategic and International Studies, said the Biden administration should not underestimate the risk of losing out in a soft-power contest that authoritarian nations could exploit for years.

“The Chinese and the Russians are advancing their vaccine diplomacy and are winning friends and influencing people and expanding their sphere of influence,” he said.

Beijing, for example, has promised to deliver Chinese-made vaccines to more than 50 nations, including nuclear-armed Pakistan, whose cooperation is key to a successful U.S. military withdrawal from Afghanistan, and the Philippines, an anchor for U.S. operations in Southeast Asia and traditionally a bulwark against Chinese military expansion.

Russia is pushing hard to raise the profile of its homegrown Sputnik V vaccine by pursuing numerous licensing deals. Russian vaccine backers signed an agreement Tuesday that could pave the way for production in Italy, a potentially big step in expanding Moscow’s efforts into the West.

Krishna Udayakumar, director of the Duke Global Health Innovation Center, said that the United States is “missing the opportunity to more strongly assert U.S. leadership on the global stage.”

Udayakumar said the United States could donate more vaccines to other countries without significantly affecting their availability to Americans. “That’s especially striking in our own backyard in Latin America, where the burden of covid is enormous and countries are struggling,” he said.

Biden disappointed Mexican President Andrés Manuel López Obrador last week by taking U.S. vaccine donation off the table for now.

Still, the reaction among Americans could be explosive if Biden supplies vaccines to other countries while many in the United States are still struggling to get the shots. Biden signaled this week that he feels pressure to balance domestic and global needs.

“If we have a surplus, we’re going to share it with the rest of the world,” he said Wednesday, when pressed on the issue. “We’re going to start off making sure Americans are taken care of first, but we’re then going to try to help the rest of the world.”

His remark followed an announcement that the United States has secured 100 million more doses of the single-dose vaccine developed by Johnson & Johnson, bringing the projected U.S. supply far past the number of Americans in line to receive shots this year.

Demand still outpaces supply in most of the country, but that is beginning to change.

The administration has said it has secured enough supply to offer all American adults a vaccine by the end of May, although officials caution that having vaccine on hand is not the same as getting doses administered, given challenges such as access to remote areas and the wariness of some communities to get the vaccine.

Biden administration officials also say they want to build in redundancy, in case of problems with delivery schedules. “I’m doing this because, in this wartime effort, we need maximum flexibility,” Biden said about the additional doses.

Still, if all goes well, the United States would have an oversupply of vaccine later this year. The administration has not said how much vaccine it considers sufficient, or what threshold it would set before considering exporting vaccines.

Biden has joined Covax, an international vaccine consortium that aims to make vaccine distribution more equitable, but the United States’ promised $4 billion contribution is not a direct handout of U.S.-made vials on the model that China and other nations are pursuing. It is also a departure from the muscular U.S. efforts directed at Ebola, HIV, smallpox and other scourges.

Even so, Biden appeared this week to be laying the groundwork for an argument that by protecting other nations, the United States also would be protecting Americans. “This is not something that can be stopped by a fence, no matter how high you build a fence or a wall. So we’re not going be ultimately safe until the world is safe,” he said.

Biden has staked the success of his presidency on an all-out effort to turn the corner on the coronavirus pandemic, which has killed more than 528,000 Americans and hampered the country for a year. Rapid vaccination is crucial to reopening businesses and schools.

Biden’s advisers say that his practical approach to the pandemic was a major factor in defeating former president Donald Trump and that he must deliver for Americans before all else.

Biden is navigating a complex political landscape when it comes to the coronavirus. Every Republican in Congress voted against his $1.9 trillion economic stimulus package this week, but Republicans are welcoming the national vaccine rollout efforts. Atop that is a general suspicion among many Americans of foreign aid and engagement that has taken on a more acute, nationalist tone during the pandemic.

Trump greeted news of the additional vaccine purchase Wednesday by taking credit for vaccine development and repeating his racially charged characterization of the respiratory illness that originated in China.

“I hope everyone remembers when they’re getting the COVID-19 (often referred to as the China Virus) Vaccine, that if I wasn’t President, you wouldn’t be getting that beautiful ‘shot’ for 5 years, at best, and probably wouldn’t be getting it at all,” Trump wrote in a statement. “I hope everyone remembers!”

More than 30 million people in the United States — about 10 percent of the population — have been fully vaccinated. The nation is averaging about 2.1 million doses administered per day, up from about 1.5 million a month ago.

Britain, Canada and many European nations also are among the wealthy parts of the world that have secured vaccine supplies more than equal to their populations. Despite occasional problems with delivering doses, the overall global picture is one of haves vs. have-nots, as the countries receiving handouts generally are poorer and have less domestic manufacturing capability.

Some activists and scholars warned that for both moral and strategic reasons, the Biden administration should not wait too long to help other countries in need.

The ONE Campaign is one of several advocacy groups urging the administration to share the U.S. stockpile with countries unable to procure the vaccines, as well as to lay out a distribution plan for excess doses.

Jenny Ottenhoff, the senior policy director for the nonpartisan anti-poverty organization, cautioned against matching U.S. adversaries with one-off deliveries to friends or would-be friends of the United States.

“I think it’s a really slippery slope for countries to start using vaccines as a political bargaining chip,” Ottenhoff said. “We need to ensure that the right vaccines are going to the right places and getting into the arms of those who are most vulnerable.”

Morrison, of the Global Health Policy Center, said the Biden administration is taking a “let’s just wait a bit” approach to vaccine diplomacy.

“There is considerable caution about leaning heavily in on the international agenda under current circumstances, where there is this hypersensitivity and great uncertainty” around vaccine doses in the United States, Morrison said. “The outstanding question is: How do you sell this to Americans under these circumstances?”

Secretary of State Antony Blinken has called China the world’s “biggest geopolitical test.” He and other administration officials have pledged to counter Chinese efforts to win influence and financial dependency among nations in Africa and elsewhere, and to head off Chinese attempts to attain clout within international organizations such as the World Health Organization.

Blinken and national security adviser Jake Sullivan will meet face-to-face with their Chinese counterparts next week.

Thomas Bollyky, the director of the Global Health Program at the Council on Foreign Relations, said that the countries donating vaccines have either largely gotten their coronavirus outbreaks under control, or do not have the domestic capacity to distribute many of the doses they procured or produced.

A few, including India, are U.S. partners. Israel, a close ally of the United States, plans to distribute excess supply to about 20 friendly nations in the coming weeks. Prime Minister Benjamin Netanyahu drew criticism for saying that some far-flung recipients were being rewarded for actions benefiting Israel, while Palestinians in the West Bank are struggling to get vaccines.

“The donations that we’ve seen so far have been useful in that they’ve started vaccination campaigns in some countries,” Bollyky said, citing India’s donations to neighboring countries as an example. “But they are no answer for meeting global demand for vaccines.”