ROME — From the opening moments of the Group of 20 summit on Saturday, the leaders of the world’s largest economies wanted to send a strong message about ending the coronavirus pandemic: During an unconventional group photograph, they were joined on the dais by doctors in white coats and first responders from the Italian Red Cross.

In his remarks opening the meeting — the first gathering in person for the group since the pandemic struck — Prime Minister Mario Draghi of Italy pointed to the stark disparity in access to vaccines between richer and poorer countries.

“Going it alone is simply not an option,” said Mr. Draghi, whose country is hosting the summit. Now, he added, the world could “finally look at the future with great — or with some — optimism.”

But as the leaders gathered to discuss plans to protect against future pandemics, health experts and activists expressed concerns that the world’s richest nations were still not doing enough to help people in poor nations survive the current one.

Advisers said President Biden, who has promised to make the United States an “arsenal of vaccines,” would not announce concrete plans related to closing the gap between rich and poor nations on vaccination rates. He would instead be pushing other leaders to explore debt relief and emergency financing for poor countries whose economies have been battered by the pandemic.

The president came into the summit focused on other problems, including fixing global supply chains, urging investments to curb climate change, and meeting with the leaders of France, Britain and Germany to discuss ways to return to a 2015 nuclear accord with Iran that the Trump administration scuttled.

On Saturday, Mr. Biden and other world leaders endorsed a landmark global agreement that seeks to block large corporations from shifting profits and jobs across borders to avoid taxes — a win for the president, whose administration pushed hard to carry the deal over the finish line.

The leaders were set to formally back the accord in a communiqué to be released on Sunday, an administration official said.

But health experts and influential advocates, including Pope Francis, have urged Mr. Biden during his trip to stay focused on closing the vaccine gap for poor nations, who are particularly vulnerable to the virus and its variants.

While wealthy nations are offering people third vaccine doses and increasingly inoculating children, poor countries have administered an estimated four doses per 100 people, according to the World Health Organization.

Jake Sullivan, the president’s national security adviser, told reporters on Air Force One en route to Rome that “the main thrust of the effort on Covid-19 is not actually traveling through the G20.” He said that a virtual summit that Mr. Biden convened in September had set “more ambitious targets” for countries to pledge to share doses of the vaccine.

Although Secretary of State Antony J. Blinken is scheduled to host a meeting of dozens of countries and nongovernmental organizations this year to secure commitments on vaccine sharing, Mr. Sullivan said the focus for the Group of 20 was on the future.

Mr. Biden said in June that the United States would buy 500 million Pfizer-BioNTech coronavirus vaccine doses for poorer nations. He followed up in September by announcing an additional 500 million Pfizer doses, along with the promise of an additional $750 million for vaccine distribution, roughly half of it through a nonprofit involved in global vaccinations.

Only about 300 million of those doses are expected to be shipped this year, a number that experts say falls short of the amount needed for meaningful protection against the virus.

“You really have a failure of developed countries’ leadership post-Covid,” said Célia Belin, a visiting foreign policy fellow in the Center on the United States and Europe at the Brookings Institution. “This is going to have consequences.”

Indeed, offering vaccine doses to developing countries is more than an altruistic gesture on the part of wealthy nations. The more the virus continues to circulate globally, the more likely it is to continue producing lethal variants, making it harder to end the pandemic and rendering vulnerable rich and poor alike.

Since arriving in Rome, Mr. Biden has already heard a personal appeal to do more: During a meeting at the Vatican on Friday, Pope Francis pushed the president on the issue, a senior official said afterward.

And in an open letter to the Group of 20, the head of the World Health Organization, Dr. Tedros Adhanom Ghebreyesus, urged the leaders of the world’s largest economies to “to help stem the pandemic by expanding access to vaccines and other tools for the people and places where these are in shortest supply.”

As the summit got underway, it is also drew a mélange of protesters who raised security concerns — laid-off factory workers, climate activists, anti-globalization campaigners, unions, feminist groups, Communists and some vaccine skeptics.

“There will be many of us,” said Gino Orsini, a representative for the Si Cobas union, one of the organizers of a demonstration planned for Saturday to coincide with the gathering.

This year is the 20th anniversary of the Group of 8 summit that Italy hosted in the northern city of Genoa that was marred by rioting. It is also a moment of tension between the authorities and opponents of the Italian government’s coronavirus vaccination requirements, which have resulted in violent clashes.

“The level of attention is maximum,” said Giovanni Borrelli, a local government official, adding that 5,500 extra law enforcement officers were being deployed this weekend.

Emma Bubola contributed reporting.