Two members of Congress from Massachusetts have tested positive for the coronavirus, one after receiving both doses of the vaccine, a reminder that people can still be vulnerable to infection after being vaccinated, particularly in the two weeks after receiving the second dose.

Rep. Stephen F. Lynch (D-Mass.) tested positive for the virus on Friday afternoon after a staff member in his Boston office tested positive earlier in the week, his spokeswoman Molly Rose Tarpey said.

Lynch received a second dose of the Pfizer-BioNTech vaccine before the inauguration of President Biden on Jan. 20, but his office declined to specify the date it was administered. Lynch had tested negative for the virus before attending the inaugural ceremonies, Tarpey said.

“While Mr. Lynch remains asymptomatic and feels fine, he will self-quarantine and will vote by proxy in Congress during the coming week,” she said.

Tarpey added that Lynch “has followed CDC guidelines and continues to do so since he received the vaccine.”

Another Democrat from Massachusetts, Rep. Lori Trahan, announced Thursday that she had tested positive for the virus and was asymptomatic. Trahan, whose staff members have been working remotely, also said she planned to vote by proxy next week.

“I encourage everyone to continue taking this virus seriously and to follow the science and data-driven guidance to wear a mask, maintain a safe social distance from others, avoid large gatherings and stay home whenever possible,” Trahan said.

Trahan received her first shot of one of the vaccines last week, spokeswoman Francis Grubar told The Washington Post.

Occasional cases of people testing positive after receiving one or both doses are not unexpected, medical experts say. Clinical trial data published by Pfizer show that the vaccine is about 52 percent effective at preventing illness after the first shot, compared to 95 percent effectiveness seven days after the second dose.

A small number of patients can still become mildly sick even after they are fully vaccinated. But only one of the roughly 20,000 people who received both doses in the clinical trial developed severe covid-19, suggesting the vaccine is powerful protection against the most dangerous cases of the disease.

Members of Congress began getting vaccinated as early as Dec. 18, but Lynch at the time said he was “waiting for the vaccine to be first offered to health care personnel, first responders and vulnerable seniors” in his district, the Boston Herald reported. It is unclear when Lynch ultimately received his first dose of the vaccine; he would have received the second dose of the Pfizer-BioNTech vaccine about three to four weeks after the first.

Public health experts have emphasized that it usually takes one week after the second dose of the Pfizer-BioNTech vaccine to reach 95 percent efficacy and two weeks after the second dose of the Moderna vaccine to reach 94 percent efficacy.

“There’s no vaccine that I know that protects you the same day you get it,” Onyema Ogbuagu, the principal investigator for Pfizer’s vaccine trial at Yale University, told The Post’s Allyson Chiu. “On a population level, 95% efficacy still translates to 5/100, or 50/1,000, or 500/10,000 vaccinated persons still being vulnerable to symptomatic disease and maybe even more having asymptomatic carriage.”

At least 23.2 million people in the United States have received one or both doses of the vaccine. The Centers for Disease Control and Prevention recommends that vaccinated people continue to wear masks, socially distance, avoid poorly ventilated spaces and wash their hands frequently to prevent the spread of the virus.

“We also don’t yet know whether getting a covid-19 vaccine will prevent you from spreading the virus that causes covid-19 to other people, even if you don’t get sick yourself,” CDC guidelines state. “While experts learn more about the protection that covid-19 vaccines provide under real-life conditions, it will be important for everyone to continue using all the tools available to help stop this pandemic.”

Mask-wearing in particular has become politicized, including in the hallways of Congress. After the Jan. 6 siege at the Capitol, several Democrats said they feared they had been exposed to the virus after sheltering with Republican lawmakers who refused to wear masks. In the following, at least three lawmakers tested positive for the virus.

On Friday, Rep. Cori Bush (D-Mo.) accused Rep. Marjorie Taylor Greene (R-Ga.) of berating her in the hallways after she told Greene to put on a mask. The incident, coupled with other hostile rhetoric and Greene’s refusal to abide by rules and protocols put in place because of the pandemic, prompted Bush to decide to move her office away from Greene’s for safety reasons, the Missouri lawmaker said.