Congress, staffers and reporters are wearing masks again as Capitol Hill faces a new wave of the coronavirus despite widespread vaccinations, Axios' Alayna Treene reports.

Why it matters: The Delta variant is surging, and Congress is a potential petri dish for the new variant. House and Senate leaders are weighing whether to reintroduce coronavirus protocols to the Capitol. While most lawmakers have been vaccinated, they fit high-risk profiles and work in close quarters.

• Senators and representatives also have been flying around the country visiting their home districts, fundraising and campaigning.

Driving the news: Shortly after our Hans Nichols scooped that a White House official and a staff member for House Speaker Nancy Pelosi (D-Calif.) both tested positive for the virus, the phones for Axios reporters blew up with congressional aides saying the problem is even more widespread on the Hill.

• The Office of the Attending Physician indicated as much, sending a letter to members this afternoon confirming "several vaccinated congressional staff members and one member of Congress have acquired infection in this circumstance."
• "The Delta variant virus has been detected in Washington, D.C., and in the Capitol buildings," attending physician Brian P. Monahan wrote.
• The letter did not instruct lawmakers to implement new protocols but urged them to stay tuned.
• Democratic leadership aides in the House and Senate told Axios that, as of now, they have no plans to make any changes.

The latest: Many people on the Hill — particularly those with family members and children too young to get vaccinated — have taken matters into their own hands.

• After weeks of not seeing a mask, multiple people in the halls of Congress have masked up again.
• Axios' Sarah Mucha also saw the longest line in recent memory at the Capitol COVID testing site.

The good news: Those who are fully vaccinated and testing positive for the virus are overwhelmingly experiencing only mild symptoms, if any at all.

• Roughly 66% of eligible Americans have gotten at least one dose of a COVID-19 vaccine, and about 57% are fully vaccinated.

The bad news: It's still unclear how successful the vaccine is in guarding against transmission, particularly the Delta variant.

Keep reading.

Senate Democrats are weighing a Plan B if their infrastructure negotiations fail: adding the nearly $600 billion in spending Republicans have already accepted to the $3.5-trillion plan they want to enact alone — a $4.1 trillion overall price tag, Alayna also writes.

Why it matters: The combination gets the roads and bridges both parties favor; the reconciliation package covers the "soft" climate and child care items wanted by progressives, and Republicans would have to answer why if they oppose a measure that includes all of what they want.

• The bipartisan talks, which pivotal Democrats like Sens. Joe Manchin (D-W.Va) and Kyrsten Sinema (D-Ariz.) drove and said were critical to securing their votes, are in danger of falling apart in a dispute over how to pay for spending in the $579 billion framework already negotiated.
• Some Senate Democrats and Republicans also have process concerns: meeting the demand of Senate Majority Leader Chuck Schumer (D-N.Y.) to move the process along tomorrow before a bipartisan bill is even drafted.
• The vote is designed to put pressure on negotiators to wrap up talks, but it could also be what ultimately kills them. Republican senators have asked Schumer to push the vote to Monday.

What they're saying: Sen. Tim Kaine (D-Va.), a member of the Senate Budget Committee, told Axios he and his fellow members always had a hard target of $4.1 trillion in spending — similar to what President Biden initially proposed.

• "So we did have the conversation, ‘Well, what if, for some reason, the bipartisan thing doesn't come together?'" Kaine added. "And we decided we could take care of that as we write the reconciliation bill by moving it from $3.5 [trillion] to $4.1 [trillion] and just adding in all the things that the Republicans agreed to."
• "The reconciliation instruction is ‘spend up to this number,’" Kaine said. "We do look at the two bills as a combined investment, totaling about $4.1 trillion."
• Sen. Ron Wyden (D-Ore.), another member of the Budget Committee, told Axios: “The numbers are still very fluid, to give you my judgment."
• A Wyden aide said they don’t expect the total number to increase significantly but cautioned certain items could be moved around.

Between the lines: Kaine is one of the first Democrats involved in the reconciliation talks to acknowledge there’s a contingency plan in place.

• Schumer also set tomorrow as the deadline for getting all 50 Democrats on board with the framework for the budget reconciliation bill.
• This is more of an informal deadline, though, with the goal of giving the budget committee a green light to begin writing the reconciliation bill.

Keep reading.

Border officials are encountering migrants from more distant countries —rather than just Mexico or the Northern Triangle, according to the latest public figures from the U.S. Customs and Border Protection agency reviewed by Axios' Stef Kight.

Why it matters: These longer journeys to the U.S.-Mexico border underscore the desperate situation many migrants face in their home countries, as well as the multi-dimensional diplomatic, economic and moral challenge the United States faces trying to control their flow north.

• There tends to be an uptick in migrants from further-flung nations during border surges.
• But the number of encounters with migrants from countries other than Mexico or the Northern Triangle countries of Guatemala, Honduras and El Salvador have surpassed the peak of the 2019 crisis now for four months in a row.
• The far-flung journeys increase the dangers faced by the migrants as well as their risk of exploitation by smugglers.

By the numbers: At the start of the year, migrants from Nicaragua and a handful of South American nations made up just 6% of people encountered by border officials. Last month, they accounted for 18% — three times more.

• There's been a rise in migrants traveling from South America through the treacherous Darién Gap into Panama, as Axios previously reported.
• While the percentage of encounters with migrants who are Haitian or Cuban has hovered around 4% recently, the number is 2.5 times the figure from January.
• Border officials had more than 9,000 encounters with Haitians and Cubans last month.

At least a third of migrant families separated at the border during the Trump administration and reunited in the U.S. so far under President Biden were homeless initially, three people familiar with estimates discussed by advocates and government officials told Stef.

Why it matters: As the number of reunions grows, such homelessness rates have the potential to significantly strain non-governmental organizations already plagued by limited resources.

• Only about three dozen family reunifications are believed to have been completed on Biden's watch. With an estimated 2,100 families still separated, that's too small a sample to gauge with certainty the extent of the housing needs moving forward.
• But sources said it could put the rate of those in need at anywhere from 30% to 50%.

Behind the scenes: NGOs assisting the administration with these reunions have, at times, had little heads up about the housing needs of the families, and have had to scramble last minute to find a place for them to stay.

• Relatives in the U.S. or foster families who've been taking care of separated kids have not necessarily had room for whole families.
• In one case, there was uncertainty about where a family would live because the reunited mother was no longer in a relationship with the father, said Christie Turner-Herbas, director of special programs for advocacy group Kids In Need of Defense (KIND).

What they're saying: "Parents are coming back with very little to no resources and coming back to very precarious situations," Turner-Herbas told Axios. "They're just kind of focused on, 'Am I going to be able to get back to being reunited with my child?'"

• "It should be in the Biden administration’s interest not to see these families ... end up in homeless shelters," Lee Gelern, the ACLU's lead attorney in its family separation lawsuit, told Axios.
• DHS said in a statement: “The Task Force is committed to reunifying families safely and providing them with the stability and support they need and deserve. ... The Task Force is grateful for the support of the NGOs and private funders who have been supporting this mission, as we work through the challenges presented by legal and fiscal authorities.”

What we're watching: On Tuesday, the government published a request for information for a potential contract that would help the government manage and care for separated families.

• Advocates are pressing for more government assistance — financial and in the form of social services case managers — even as they pull together private resources to fund emergency housing needs for families.
• They say there's a need for more help with food, mental health services, school registration and medical checkups, as well as housing.

The bottom line: Reuniting these migrant families has been one of Biden's core pledges.

• The early housing woes illustrate just how difficult it's been to address the challenges for thousands of families more than three years after President Trump's "zero-tolerance" policy ended.

The NBC News policy editor has seen enough.