Democratic and Republican senators are considering cutting as much as $5 billion in funding for the global vaccination effort from an emerging aid package — assistance that is central to President Biden’s strategy of reducing vaccine inequality and limiting the impact of the next coronavirus variant.

On Monday, the Senate will continue to trying to resolve a battle over a broader emergency coronavirus response package by slimming it to $10 billion from $15.6 billion. It was not clear whether the narrower bill would have the votes to move forward in either chamber of Congress.

While access to vaccines has gradually expanded around the world, administering the shots remains a challenge for a multitude of reasons. In many low-income countries, only about 15 percent of the population has received at least one vaccine dose, compared with about 80 percent of the population in many middle- and high-income countries, according to the Our World in Data project at the University of Oxford.

Hence the need for urgent additional financial assistance, said Gayle Smith, a former State Department coordinator for global Covid response and health security. Cutting the aid, she said, could “signal that the United States kind of thinks the pandemic has been managed.” And while $5 billion is a significant sum, she said, it is far less than the “double-digit trillions this pandemic has cost the world.”

The issue is not only saving lives abroad. Uncontrolled outbreaks allow more dangerous virus variants to emerge, as occurred with Omicron, prolonging the pandemic and threatening yet more damage to the global economy. “Our goal has to be — not just the U.S. but every country in the world — has to be shutting this down as quickly as possible so that we, No. 1, minimize the risk of new variants,” Ms. Smith said.

The United States has led the way in the world’s pandemic response, Ms. Smith said, allotting the effort $11 billion as part of the American Rescue Plan passed last year. The country has also donated hundreds of millions of vaccine doses through the Covax global initiative and pledged many more.

The $4 billion the United States dedicated to Covax for the last fiscal year made up 36 percent of the initiative’s budget, according to an analysis by the Kaiser Family Foundation.

When asked at a White House press briefing on Thursday whether Mr. Biden would sign or veto a bill that did not include significant international aid, Kate Bedingfield, the communications director, said the president had been clear on the importance of funding efforts globally.

“Right now, countries in fact are declining our doses because they don’t have the infrastructure in place to take our lifesaving vaccines,” Ms. Bedingfield said. “Funding will obviously help solve this issue.”

The aid would also go toward other supplies, including protective gear and Covid treatments. Republicans have said they will support the aid, but have demanded that it be paid for by repurposing money already approved by Congress earlier in the pandemic.

Public health experts worry that wealthier nations are leaving other countries behind as they turn to third and fourth vaccine doses. The World Health Organization has been pushing for wealthier nations to share access to vaccines and therapeutics with the world’s more vulnerable populations.

“We can end the acute phase of #COVID19 this year but only if we work together to close inequities in access to vaccines and other lifesaving tools,” Dr. Tedros Adhanom Ghebreyesus, the organization’s director general, said in a video he posted to Twitter on Tuesday.

Devi Sridhar, a professor and head of the global public health program at the University of Edinburgh, described the cut to the proposed package as a “major step backward.”

“I am worried that as we declare the end of the pandemic in Britain and the United States, we forget that it’s still causing a lot of devastation in other places, even if we’re not reading about it in the headlines,” Professor Sridhar said.

The uncertainty over Covid funding comes as the vaccination campaign has stalled in the United States, where total cases have now surpassed 80 million, according to a New York Times database. Daily inoculation rates have fallen to their lowest levels since vaccines became widely available to the public in early 2021.

About 32,730 people received their first shot this week, according to the Centers for Disease Control and Prevention. Booster shots are also waning, with only about 52,000 people getting their booster this week, compared with about a million people per week in early December, shortly after Omicron was detected in the United States.