The United States plans to send millions of doses of the AstraZeneca vaccine to Mexico and Canada, the White House said today, a notable step into vaccine diplomacy just as the Biden administration is quietly pressing Mexico to curb the stream of migrants coming to the border.

Jen Psaki, the White House press secretary, said the United States was planning to share 2.5 million doses of the vaccine with Mexico and 1.5 million with Canada, adding that it was “not finalized yet, but that is our aim.”

Tens of millions of doses of the vaccine have been sitting in U.S. manufacturing sites. But while their use has already been authorized in dozens of countries, the vaccine has not yet been approved by U.S. regulators.

Several European countries suspended use of the AstraZeneca vaccine this week, a precaution because some people who had received the shot later developed blood clots and severe bleeding. But today, Europe’s drug regulator declared the vaccine safe. AstraZeneca has also said that a review of 17 million people who received the vaccine found they were less likely than others to develop dangerous clots.

The announcement of the vaccine distribution came at a critical time in negotiations with Mexico. President Joe Biden has moved quickly to dismantle some of former President Donald Trump’s signature immigration policies, halting construction of a border wall, stopping the swift expulsion of children at the border and proposing a pathway to citizenship for millions of immigrants in the United States.

But he is clinging to a central element of Trump’s agenda: relying on Mexico to restrain a wave of people making their way to the United States.

Anticipating a surge of migrants and the most apprehensions by U.S. agents at the border in two decades, Biden asked President Andrés Manuel López Obrador of Mexico in a video call this month whether more could be done to help solve the problem, according to Mexican officials and another person briefed on the conversation.

The two presidents also discussed the possibility of the United States sending Mexico some of its surplus vaccine supply, a senior Mexican official said. Mexican officials say the efforts to secure vaccines are separate from the negotiations over migration. But they acknowledge that relations between the United States and Mexico, which has suffered one of the world’s deadliest coronavirus epidemics, would be buoyed by a shipment of doses south.

“Both governments cooperate on the basis of an orderly, safe and regular migration system,” Roberto Velasco, director general for the North America region at Mexico’s foreign ministry, said in a statement, referring to the engagement between the two countries on migration and vaccines.

But he said there was no quid pro quo for vaccines: “These are two separate issues, as we look for a more humane migratory system and enhanced cooperation against COVID-19, for the benefit of our two countries and the region.”

A Biden administration official declined to comment on discussions with Mexico but noted that both countries shared a common goal of reducing migration by addressing its root causes and said they were working closely to stem the flow of people streaming to the border.

Mexico has agreed to increase its presence on its southern border with Guatemala to deter migration from Central America, one of the government officials said, and local Mexican officials say their country has recently stepped up efforts to stop migrants on the northern border with the United States as well.

But there are also signs that Mexico’s commitment to policing migration — a central demand of Trump, who wielded the threat of tariffs against all Mexican goods unless migration was curbed — may have flagged in the waning months of the Trump administration.

From October through December 2020, the number of Central Americans apprehended by Mexico declined, while detentions by U.S. agents increased, according to Mexican government numbers and data compiled by The Washington Office on Latin America, a research organization that advocates for human rights.

“The likelihood of the outgoing Trump administration threatening tariffs again was low, so there was an incentive for Mexico to go back to its default state of low apprehensions,” said Adam Isacson, an expert on border security at The Washington Office on Latin America.

The Biden administration’s appeal to do more against migration has put Mexico in a difficult position. While Trump strong-armed Mexico into militarizing the border, some Mexican officials argue that his harsh policies may have at times helped lessen their load by deterring migrants from attempting to make the journey north.

Biden is less likely to resort to threats of tariffs to get his way, officials and analyst say. But now Mexico is being asked to hold the line against a surge of migrants — while the Biden administration is signaling that the United States is more welcoming to migrants.

“They get to look like the good guys, and the Mexicans look like the bad guys,” said Cris Ramón, an immigration consultant based in Washington, D.C.

“All the positive humanitarian policies are being done by the Biden administration.” Ramón added, “and then the Mexicans are left with the dirty work.”

As for Canada, several of Prime Minister Justin Trudeau’s political opponents repeatedly pressed him to lobby the new Biden administration for the release of vaccines. Many Canadians have expressed dismay that the United States had not shared any supplies with Canada, where no coronavirus vaccines are manufactured.

Until today, all of Canada’s vaccine supply had come from Europe or India, and Canada’s rollout has proceeded at a slow pace compared with the United States and many other countries.

With Mexico, the Biden administration has been urging the country to take in more families being expelled by U.S. authorities and to step up enforcement at Mexico’s southern border with Guatemala, according to two Mexican officials and two others briefed on the discussions.

López Obrador is also trying to find a way of increasing capacity to house migrants in shelters, which are bursting at the seams. In a statement Tuesday, the secretary for homeland security, Alejandro Mayorkas, said he was “working with Mexico to increase its capacity to receive expelled families.”

Local officials in Chihuahua and shelter operators say that coordination has broken down between Mexican and U.S. authorities. During the last years of the Trump administration, U.S. officials would notify their Mexican counterparts before expelling migrants across the border and would orchestrate the crossings at a handful of well-staffed border checkpoints, they say.

Under the Biden administration, they say, Customs and Border Protection agents now deposit migrants at some of the most obscure, understaffed checkpoints, leaving their Mexican counterparts scrambling when they discover dozens of migrants walking in from the United States.

Local government officials in Ciudad Juárez and shelter operators say Mexico is dialing up operations to capture and deport migrants along the northern border. On a nearly daily basis, two of them said, Mexican authorities are stopping vans stuffed with families and pickup trucks carrying livestock — along with migrants crouching on the floor to avoid detection.

Part of the reason Mexico is willing to continue cracking down is that, despite being a country that has long sent people north, there is a lot of resentment toward Central American migrants.

“The level of negative attitudes that we have toward migrant flows has gone up, so there won’t be a political cost” for López Obrador, said Tonatiuh Guillén, who ran Mexico’s National Migration Institute in the first half of 2019. “But with Trump, we negotiated nothing; we gave them a lot, and they didn’t give us anything back,” he added, arguing that the strategy should be different with Biden.

Despite the very public tensions with Mexico under Trump, López Obrador has been wary of the Biden administration, concerned that it might be more willing to interfere on domestic issues like labor rights or the environment.

Instead, several Mexican officials say, his government has pushed the United States to deter Central Americans from migrating by sending humanitarian aid to Honduras and Guatemala in the wake of two hurricanes that devastated those countries and, many experts believe, pushed even more people to migrate.

Mexican officials have also asked the United States to send more Hondurans and Guatemalans apprehended in the United States directly to their home countries rather than releasing them to Mexico, making it even harder for them to try to cross the border again.

© 2021 The New York Times Company