Congress crossed a critical threshold Wednesday that is a testament to the power of vaccines: 75 straight days without a single member announcing a positive test for the coronavirus.

As cases surged inside the Capitol in the fall and early winter, congressional health officials began a vaccination program just before Christmas. The pandemic continued to wreak havoc among lawmakers throughout January, especially after the Jan. 6 attack on the Capitol forced hundreds of lawmakers to shelter together in cramped quarters for several hours as some Republicans refused to wear masks.

But, as vaccine shots became more widely available and the number of infections plummeted nationally, the coronavirus has all but disappeared among members of Congress.

Not since Jan. 29, when Rep. Stephen F. Lynch (D-Mass.) announced that he was infected, has a lawmaker revealed testing positive for the coronavirus, according to a database maintained by NPR. That is easily the longest stretch without a lawmaker contracting the deadly virus.

Epidemiologists caution that it takes a lot more medical research to declare that herd immunity has been achieved for this unique bubble, but a coronavirus-free Congress would be a significant trend as cases have risen again nationally for the past month and appear poised to continue rising for at least the next few weeks.

Perhaps no one is as big a vaccination evangelist as Rep. Bonnie Watson Coleman.

“I am really blessed,” the New Jersey Democrat said in a recent telephone interview. “I tell my story.”

Watson Coleman is presumed to have contracted the virus during the Jan. 6 security lockdown. She understandably feared for her life as a diabetic who battled lung cancer in 2018 and 2019, and a Black woman “of age” — she turned 76 in February.

But Watson Coleman received an immediate infusion of monoclonal antibodies, which help the immune system fight viruses. And she had received her first vaccination on Dec. 29, in keeping with the advice of Brian Monahan, the head of Congress’s Office of Attending Physician.

The lone shot could not prevent her developing the disease, but it is very likely, along with the antibody infusion, to have protected her against deadly illness, she said Monahan told her.

“That probably saved your life,” Watson Coleman recalled that he said.

Ron Wright’s covid-19 case went the other direction.

Rep. Wright (R-Tex.) also battled lung cancer and, last September, was briefly hospitalized. He announced on Jan. 21 that he had tested positive for the virus after he and staffers came in contact a few days earlier with someone who had it.

Aides to Wright told Texas media that his doctors had cleared him to be vaccinated — but that he did not follow through.

Wright, 67, died 18 days later.

Former aides to Wright declined to discuss why the late congressman put off getting his vaccinations.

Medical experts declined to discuss the specific cases of these two lawmakers, given that they were not their patients, but in general urged that nearly everyone get vaccinated.

“This vaccine is really safe for lots of people with lots of different chronic conditions,” said Lindsey Leininger, a public health educator and researcher of data-driven health policy at Dartmouth College’s Tuck School of Business.

Leininger said members of Congress can serve as North Stars for the vaccination movement because they are generally respected in their individual districts.

“Nobody trusts Congress, but everybody loves their member of Congress,” she said, expressing concern that nearly 50 percent of Republican voters do not want to be vaccinated. “Oh my gosh. With the fact that they got vaccinated, these congresspeople could be great ambassadors.”

But Republican lawmakers are reluctant to talk about being vaccinated.

According to estimates from GOP leaders, at least half of the 212 House Republicans received shots.

Yet just 53 Republicans would publicly acknowledge being vaccinated when CNN did an expansive survey of the House; 13 Republicans declined to get the shots, and the remaining GOP lawmakers would not comment.

Democrats are largely practicing what they preach: 189 told CNN they had been vaccinated, one declined and 29 did not respond.

The irony here is that Republicans are demanding that Congress return to normal on Capitol Hill while they refuse to publicly take the steps that health officials say would get them there. For a year, House and Senate committees have held virtual hearings, while the House also allows remote voting via proxy.

“I look forward to the day that when Congress changes, that you have to show up to work to be paid,” House Minority Leader Kevin McCarthy (R-Calif.) told reporters Thursday, predicting that the GOP would win back the majority next year. “That will be one of the first things I will do.”

In recent weeks some Republicans have flouted mask requirements or have simply ripped the coverings off their faces the moment they walk off the House floor.

Watson Coleman’s experience tells her that the best tactics to defeat the virus are a mixture of vaccines, continued social distancing and testing the public for the coronavirus.

When the House voted last May to allow proxy voting, she left Washington and would not be back for more than seven months.

“I stayed in Jersey and worked virtually,” she said. “I am telling you, I didn’t even leave the house for months.”

She returned to the Capitol on Dec. 29 to receive her first shot in the attending physician’s office, then came back for the Jan. 3 start of the 117th Congress.

She was sworn in for a fourth term and received a negative coronavirus test. Rather than heading straight home, she stuck around for the certification of Joe Biden’s electoral college victory.

“It was an important statement. This is part of what our democracy is all about,” she said.

Watson Coleman and her husband spent the morning of Jan. 6 in her Capitol Hill apartment — until their building was evacuated because it shares an alley with the Republican National Committee building, where a pipe bomb was placed.

When they got into the House office buildings, she didn’t like how crowded the area was, with all the staff clustered together, and told her husband to go with her to the Capitol because it would be easier there to avoid crowding.

“We’ll be safe,” she told him.

Instead, the attack started, and after hiding in a small room close to where rioters clashed with police, Watson Coleman and her husband were taken to a secure location — along with hundreds of lawmakers, staffers and journalists. She sheltered in a packed room, and she and several others who were there contracted the virus.

She dealt with a persistent cough and was very tired for a few weeks but never spent a night in the hospital.

She received her second vaccine shot Wednesday and is planning a full return to work in the Capitol, possibly next month. Watson Coleman often recalls Monahan’s voice telling her that the first shot helped keep her alive.

“And I’ll never forget that,” she said. “You don’t hear that too often.”