ap photo A sign at Twitter headquarters is shown in San Francisco on Nov. 18.

Twitter will no longer enforce its policy against COVID-19 misinformation, raising concerns among public health experts and social media researchers that the change could have serious consequences if it discourages vaccination and other efforts to combat the still-spreading virus.

Eagle-eyed users spotted the change Monday night, noting that a one-sentence update had been made to Twitter’s online rules: “Effective November 23, 2022, Twitter is no longer enforcing the COVID-19 misleading information policy.”

By Tuesday, some Twitter accounts were testing the new boundaries and celebrating the platform’s hands-off approach, which comes after Twitter was purchased by Elon Musk.

“This policy was used to silence people across the world who questioned the media narrative surrounding the virus and treatment options,” tweeted Dr. Simone Gold, a physician and leading purveyor of COVID-19 misinformation. “A win for free speech and medical freedom!”

Twitter’s decision to no longer remove false claims about the safety of COVID-19 vaccines disappointed public health officials, however, who said it could lead to more false claims about the virus, or the safety and effectiveness of vaccines.

“Bad news,” tweeted epidemiologist Eric Feigl-Ding, who urged people not to flee Twitter but to keep up the fight against bad information about the virus. “Stay folks — do NOT cede the town square to them!”

While Twitter’s efforts to stop false claims about COVID weren’t perfect, the company’s decision to reverse course is an abdication of its duty to its users, said Paul Russo, a social media researcher and dean of the Katz School of Science and Health at Yeshiva University in New York.

Russo added that it’s the latest of several recent moves by Twitter that could ultimately scare away some users and even advertisers. Some big names in business have already paused their ads on Twitter over questions about its direction under Musk.

“It is 100 percent the responsibility of the platform to protect its users from harmful content,” Russo said. “This is absolutely unacceptable.”

The virus, meanwhile, continues to spread. Nationally, new COVID cases averaged nearly 38,800 a day as of Monday, according to data from Johns Hopkins University — far lower than last winter but a vast undercount because of reduced testing and reporting. About 28,100 people with COVID were hospitalized daily and about 313 died, according to the most recent federal daily averages.

Cases and deaths were up from two weeks earlier. Yet a fifth of the U.S. population hasn’t been vaccinated, most Americans haven’t gotten the latest boosters, and many have stopped wearing masks.

Musk, who has himself spread COVID misinformation on Twitter, has signaled an interest in rolling back many of the platform’s previous rules meant to combat misinformation.

Last week, Musk said he would grant “amnesty” to account holders who had been kicked off Twitter. He’s also reinstated the accounts for several people who spread COVID misinformation, including that of Rep. Marjorie Taylor Greene, whose personal account was suspended this year for repeatedly violating Twitter’s COVID rules.