In July, Congress saw a months-long streak with no COVID-19 cases among its member ranks during the pandemic come to a halt, and a rise in the new Delta variant threatens progress made this year.
This month, Republican Rep. Vern Buchanan said he had tested positive for the illness, despite being fully vaccinated. Buchanan, who lost a staffer to the illness last year, said he was experiencing "very mild flu-like symptoms."
A day later, on July 20, a senior communications staffer for House Speaker Nancy Pelosi tested positive for the illness, the office said. And it apparently was one of multiple recent cases.
Dr. Brian Monahan, the Capitol's attending physician, issued new guidance noting that "several" staff members had been infected in addition to the one member of Congress. Monahan warned that the Delta variant had indeed been detected in the Washington, D.C. area and the Capitol buildings.
"Future developments in the coronavirus Delta variant local threat may require the resumption of mask wear for all as now seen in several counties in the United States," Monahan wrote in a guidance notes to the Capitol community. The Centers for Disease Control does not generally require vaccinated individuals to wear a mask indoors at this time.
The positive result for the Pelosi staffer follows the aide's meeting with members of a group of Texas state Democrat lawmakers who have seen at least five cases in their ranks. The lawmakers, who are in Washington, D.C. to evade a special Texas legislative session to approve new voting restrictions in the state, have met without masks with Vice President Kamala Harris, members of Congress and staffers. The group issued a statement stating that they were all vaccinated.
Before these latest cases Congress' nearly six-month pause for lawmakers dealing with the virus began after a majority of more than 500 lawmakers were fully vaccinated by January 2021, and the Capitol's attending physician lifted much of the mask mandates that dominated the past year. However, a minority of House Republican members have refused to seek vaccination.
Lawmakers impact by the coronavirus
This threat arrives after the coronavirus already exacted a large toll on the Capitol.
Last year, the pandemic upended daily work for months, sickening dozens of members and hundreds of workers. A sitting congressman, a member-elect and an aide died.
By February 2021, more than 60 lawmakers and 360 Capitol Hill workers had tested positive, or were presumed so, for the coronavirus, according to NPR's tracker and congressional aides.
See a list of all members who tested positive or quarantined.
That same month, Rep. Ron Wright, R-Texas, became the first sitting member of Congress to die after a more than two-week battle with COVID-19. In December, Rep.-elect Luke Letlow of Louisiana died days before he was due to be sworn in. And last summer, a Florida House member's aide died from COVID-19.
The rash of infections among members had continued at a consistent pace since the outbreak began, with the longest stretch without any such cases lasting about three weeks during recess periods away from Washington, D.C.
But this past March, Capitol Hill saw the first month-long stretch without any reported cases or quarantines among members. The most recent positive case before Buchanan was reported by Rep. Stephen Lynch, D-Mass., on Jan. 29, despite his having received his second dose of the vaccine days earlier.
Another member, House Oversight Committee Chair Carolyn Maloney, D-N.Y., reported that she quarantined on Feb. 1. It ended Feb. 8 without a report of a positive test.
In June, Monahan issued a new 18-page guidance document lifting the mask mandate for vaccinated members and workers.
January 6 insurrection caused spike in cases on Capitol Hill
Up until this summer the most recent outbreak of cases on Capitol Hill for members was triggered by the Jan. 6 insurrection, which forced members to cluster together in rooms for several hours. Some House Republicans were seen refusing to wear masks during the ordeal.
Later, Monahan told Congress that at least one of these holding rooms for House members included a lawmaker who was already positive.
More than a half-dozen members quarantined as a result, and several tested positive, including Democratic Reps. Bonnie Watson Coleman of New Jersey and Brad Schneider of Illinois.
In all, January proved to be one of members' worst month for the coronavirus, with at least 16 lawmakers testing positive. Before the insurrection, the House met to launch a new session before reinstating rules allowing proxy voting.
This, as congressional leaders and lawmakers received the Pfizer-BioNTech COVID-19 vaccine, which became available to members in December. The doses were provided to meet long-standing requirements for continuity of government operations, Monahan told members in a Dec. 17 note.
Pandemic impacted congressional operations
Ahead of the vaccinations, both chambers of Congress recessed multiple times last year as the Capitol went largely without a widespread testing program.
The Democratic-controlled House installed emergency proxy voting and remote hearings in May 2020, and Pelosi extended the covered period several times due to the pandemic. It remains in place until at least until mid-August.
In November, Pelosi triggered a broader coronavirus testing program for Congress following new requirement for travelers to the Washington, D.C., area.
Pelosi has previously said with about 75% of House members vaccinated, and several Republicans who refuse to do so, she's unclear when Congress will fully get back to normal.
"One of the most substantial steps that can be taken is that everybody should be vaccinated," Pelosi told reporters in March, later adding, "We need 100% of the members vaccinated, because it just takes one to endanger others."
The pandemic delayed other rituals, such as President Biden's first address before a joint session of Congress, which was held with only 200 socially distanced guests on April 28.
Last year remains a reminder of how quickly outbreaks spiraled out of control among lawmakers.
In December, at least nine House members, including six Republicans, tested positive for the coronavirus. And November proved to be another one of the worst months of the pandemic for Congress, when at least 15 lawmakers said they were infected, including two of the oldest, Republicans Rep. Don Young of Alaska and Sen. Chuck Grassley of Iowa.
A previous outbreak in September was tied to a White House Rose Garden ceremony to announce Amy Coney Barrett as then-President Donald Trump's Supreme Court justice nominee. Trump, then-first lady Melania Trump and dozens of others in attendance tested positive, including Republican Sens. Mike Lee of Utah and Thom Tillis of North Carolina.
It was one of several instances forcing leaders to change the congressional schedule.
Starting in the first month of the pandemic, in March 2020, House and Senate leaders delayed bringing back members in light of public health guidelines recommending social distancing.
The Senate returned in May 2020, but the much larger House remained mostly away under the advice of Monahan, the Capitol's attending physician. That same month, the House approved its historic rule changes to allow remote voting and hearings.
The first spate of cases began March 8, 2020, when two Republican lawmakers, Sen. Ted Cruz of Texas and Rep. Paul Gosar of Arizona, were the first members of Congress to announce self-quarantines. Both attended the Conservative Political Action Conference in National Harbor, Md., where another guest fell ill.
The following week, the first two members of Congress said they tested positive for the coronavirus. Republican Rep. Mario Diaz-Balart of Florida and then-Democratic Rep. Ben McAdams of Utah both said they developed symptoms after a March 14 floor vote on a coronavirus relief package.
By last summer, dozens of Capitol workers reported a positive test or were presumed so, and Gary Tibbetts, a longtime staffer for Buchanan died from COVID-19 on July 24.
Some lawmakers also took antibody tests to see if they were previously infected. They include Democratic Sens. Tim Kaine of Virginia and Bob Casey of Pennsylvania, who both said they tested positive months after experiencing symptoms in the spring.
To stem the flow of new cases, both chambers early on issued new social distancing guidance. Also, the U.S. Capitol closed to public tours and remained open only to members, staff, the press and official business visitors, and aside from the Jan. 6 breach by pro-Trump extremists, it has remained so.
In late July 2020, Pelosi also issued a temporary, new mask mandate after Texas GOP Rep. Louie Gohmert, who often rejected such protocols, tested positive. Gohmert had attended several hearings a day earlier and returned to the Capitol following a White House screening that caught his infection.
Gohmert's case triggered quarantines for five House members, including Arizona Democratic Rep. Raúl Grijalva. Days later, Grijalva tested positive but fully recovered symptom free. Until recent changes lifting the mask mandates, members could be fined or be forcibly removed by Capitol Police if they are not wearing masks.
A previous version of this story incorrectly identified Rep. Joe Cunningham of South Carolina as a Republican. He is a Democrat.