(CNN) -- At this point in the Covid-19 pandemic, much of the United States has slowly moved away from following or enforcing mitigation measures. Most public schools plan to keep masks optional this fall. The US Centers for Disease Control and Prevention no longer recommends staying at least 6 feet away from other people to reduce the risk of exposure.

Among public health leaders, there has been growing division around this pivot, and that stems from frustration around how the pandemic has been managed in the United States, said Art Caplan, founding head of the division of medical ethics at NYU Grossman School of Medicine in New York.

"This is the most powerful division I can think of among mainstream public health people," Caplan said.

"By the way, it reinforces something else: It's a division based on ethics. It's not about science," he added. "It's a fight about, what do you do at this point about Covid? Do you double down and mandate masking and mandate vaccination and insist that for schools to open, they have to improve ventilation and insist on testing? Or do you just sort of say, 'people are tired. They don't want to do it anymore. We can't make them. We'll have to live with it.' "

Since Covid-19 arrived, there has been a constant division among public health experts on what mitigation measures should be in place and how to communicate those measures -- "it's just that the proportion has changed," said Dr. Peter Hotez, co-director of the Center for Vaccine Development at Texas Children's Hospital and dean of the National School of Tropical Medicine at Baylor College of Medicine.

That division arose around the same time health officials and experts received a growing number of online attacks and even death threats from people in the public who disagreed with them.

At the height of the pandemic, "when we were losing 2,000 to 3,000 Americans a day, I think the mainstream public health and scientific communities were all on the same page. This was a devastating pandemic. This was a leading cause of death on a daily basis in the United States. We had to do everything we could to protect lives, encourage vaccinations and max out in NPI -- non-pharmaceutical interventions," Hotez said.

"There was always a group of contrarian intellectuals or pseudointellectuals who were very aggressive and tended to downplay the severity of the pandemic and say it was a hoax or nothing more than the flu," he said. "Now, things have blurred a bit more. There's many different voices now as compared to that kind of duality or dichotomy. I think what you're hearing now is a spectrum."

That spectrum has included people -- even some experts -- who still downplay the severity of Covid-19, as well as those who argue that strict measures should remain in place today. A middle ground of public health voices has also emerged over time.

"It gets a little dicey when people kind of take intermediate positions between the two," Hotez said. "And they tend to get heavily criticized."

One recent example can be found in the criticism, threats and backlash targeting Dr. Leana Wen, a CNN medical analyst and a public health professor at George Washington University.

Tensions over one doctor's work

Wen, a former Baltimore City health commissioner, came into the public eye as one of the more moderate voices in public health during the pandemic, supporting some states lifting mask mandates in schools in February and deciding not to send her own children back to school with masks this fall.

"That's what my family has decided. Our views have changed a lot since the beginning of the pandemic, when there was much unknown about the impact of Covid-19 on children. At that time, we followed extremely strict precautions, including masking at all times indoors and only associating with others outdoors," Wen told CNN in August.

"For us, the turning point was after Omicron came to dominate, because it became even more difficult to avoid Covid-19 despite precautions. Getting our children vaccinated also gave us even more reassurance that we could replace masking with the protection that vaccination provides. We know our children could still get Covid-19, but the risk of severe illness is very low."

In August, more than 600 public health professionals and allies signed a letter "demanding" that Wen be reconsidered and replaced as a speaker at the annual meeting of the American Public Health Association in Boston in November. The letter criticizes some of Wen's messaging around Covid-19 and other public health topics.

"Through her platform on news outlets and social media, Dr. Wen has promoted unscientific, unsafe, ableist, fatphobic, and unethical practices during the COVID-19 pandemic," the letter states. "For instance, in a recent article, Dr. Wen suggested that infection should be accepted as a 'new normal.' "

The American Public Health Association responded, saying in a statement that Wen will participate in the upcoming meeting as a panelist in a moderated discussion about backlash against public health.

In effect, Wen is receiving backlash for being on a panel about backlash.

"This is something she and many other public health officials across the country have encountered at unprecedented levels in recent years, and that she is well-qualified to address. Dr. Wen is not a keynote speaker and is not being paid to speak at the meeting," the statement said. It added that more than 5,000 people are scheduled to speak during sessions at the meeting, "none of whom we plan to disinvite from the program."

Wen said in an email to CNN that disagreements around Covid-19 measures among public health experts are "important policy conversations to have" and that the guidance around Covid-19 has changed since the start of the pandemic as the nation now has access to vaccines and treatments.

"In addition, as it's become clear that this coronavirus is going to be with us for many years to come, the majority of Americans have decided that they cannot limit their activities in perpetuity. Guidance from the CDC has evolved to take into account this new reality, and I am among the public health experts who support this evolution by federal health officials. Some others disagree; these are important policy conversations to have," Wen wrote.

"Ultimately, we need to be honest that all mitigation measures come with tradeoffs that must be continually evaluated as circumstances change," she added. "We also need to be concerned about the growing backlash to public health that's, in part, caused by what some perceive to be extreme covid-19 mitigation efforts."

That backlash differs from the divide emerging within the medical and public health fields; such threats, made directly against some public health experts, mostly come from people in the general public.

For instance, a Texas man was sentenced to federal prison for sending a threatening message to Wen. Another man, from Massachusetts, has been charged with making direct threats against Wen.

"Scientists and medical professionals should never have to fear for our lives. It is really tragic that the intimidation and bullying of public health officials has intensified due to polarization around COVID-19," Wen said in email. "People who have disagreements over policy issues need to air them in a respectful and civil manner, not through harassment and threats."

Medicine vs. public health

Overall, medicine and public health are two different things, Alex John London, author of "For the Common Good: Philosophical Foundations of Research Ethics" and director of the Center for Ethics and Policy at Carnegie Mellon University, suggested in an email to CNN.

"It has been disappointing to see public health largely equated with medicine since medicine tends to focus on the health of the individual while public health focuses on factors that impact many people. Vaccines are important and it is remarkable that we have such safe and effective tools at [our] disposal. But polarization around false dichotomies has resulted in the neglect of practices such as improving ventilation, encouraging sick leave rather than asking workers to come to work sick, and encouraging the use of masks whenever someone is sick and must go out in public," London wrote.

"There has been some confusion about when people are arguing about evidence and when they are arguing about values. My guess is that the backlash around Dr. Wen stems, at least in part, from what appears to be her willingness to embrace a status quo that still poses very high risks for large numbers of people," he added. "I view disability rights activists, in particular, as pressing, among other things, for an approach to public health that improves the baseline that we regard as socially acceptable rather than aiming to get back to what we had before."

There's a major divide in Covid-19 messaging between the individual, medicalized model of medicine and a public health model, said Steven Thrasher, author of the book "The Viral Underclass" and a professor at Northwestern University's Medill School of Journalism, Media, Integrated Marketing Communications.

He thinks Wen is "pushing the medical model where, basically, you assess your own risk, and you talk to your doctor about what you should do. And the [Biden] administration is leaning into that in a lot of ways. They're planning to 'commercialize' -- that's their word -- Covid treatment and medication," Thrasher said.

Congress has failed to pass the additional Covid-19 relief funding that the White House says is needed to pay for vaccines, testing and treatments. Officials have said the coming effort to move the response to the commercial market is one of the consequences.

"But the way that you get collective health around infectious disease is that you have to make medication free at the point of service to as many people as possible," Thrasher said. "The second it starts getting paid for, the people who can pay for it get more protection, and for those who can't, the virus pulls amongst them, and that's kind of a major dynamic happening right now between the two camps, of which I think Dr. Wen is a prime example."

Thrasher added that although he did not sign it, as he is not a member of the association, he is "in support" of the letter calling for Wen not to speak at the American Public Health Association meeting. He has been critical of Wen's views on Twitter and told CNN that she has blocked him on the social media platform.

"I think any time people are active members of an organization, we should be able to have critical voice in who's allowed to speak and who's not," he said. "If I were part of that organization, I would sign the letter myself."

Caplan does not agree with all of Wen's views on the Covid-19 pandemic.

"She's in the camp that says we didn't battle this well, and we're going to have to live with it now. ... We're going to treat Covid at this point like we do the flu and live with the consequences, including deaths," he said. "I think that's wrong. I think public health should still try hard to fight for people who are at high risk, to do what it can to cut back on deaths, long-term Covid disability and loss from work."

Yet he also does not agree with the backlash against Wen or the letter protesting her appearance at the meeting.

"I think trying to turn Wen into a target to cancel is ridiculous," Caplan said.

"I consider public health response to Covid to be a mess, and I think part of the reason Wen has become a lightning rod is, she's kind of being put out there as an example by people in public health who are disappointed or frustrated of that failure," he said. "It's public health divided because of frustration about poor management of Covid."

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