(CNN) With the highly contagious Delta variant spreading, particularly among unvaccinated Americans, it may be time for much of the country to put masks back on, experts said.
"We are at a very different point in the pandemic than we were a month ago," Dr. Leana Wen told CNN on Tuesday. "And, therefore, we should follow the example of LA County and say that if there are places where vaccinated and unvaccinated people are mixing, then indoor mask mandates should still apply."
Los Angeles County reinstated a mask mandate over the weekend, requiring masking indoors regardless of vaccination status. On Wednesday, the county reported a 20-fold increase in new cases over the previous month, according to the public health department.
Wen, a CNN medical analyst, gave two exceptions to the occasions she thinks people should be wearing masks indoors in public: when everyone is vaccinated and has provided proof or if there is a very high level of community vaccination.
Ideally, mask mandates would be in place while leaders move toward methods of proving vaccination status to boost vaccination rates, said Wen, an emergency physician and visiting professor of health policy and management at the George Washington University Milken Institute School of Public Health.
Meanwhile, the man who was US surgeon general when the pandemic began says he wants the Centers for Disease Control and Prevention to formally change its masking guidance because of the Delta variant and rising case numbers.
Vaccinated and unvaccinated people should wear masks "in places where cases are rising yet vaccinations rates remain low," former Surgeon General Jerome Adams wrote in an opinion piece published Wednesday by the Washington Post.
That would be a change from the CDC's announcement May 13, when it said fully vaccinated people don't have to wear masks indoors or outdoors.
"Instead of vax it or mask it, people might need to vax it and mask it," Adams wrote.
About 28% of the US population, or more than 91 million people, lives in a county considered to have "high" Covid-19 transmission, according to data from the CDC. Only 48.8% of the total US population is fully vaccinated against the virus, according to the CDC.
Cases are surging in the US. The country averaged 37,055 new cases a day across a week as of Tuesday -- 54% higher than the prior week and more than two and a half times the average recorded about two weeks ago (13,665), according to data from Johns Hopkins University.
On Monday, US Department of Health and Human Services Secretary Xavier Becerra signed a renewal of the public health emergency status due to Covid-19 for another 90 days, as some states are seeing particularly worrying impacts of the pandemic.
The current rise of cases may continue to put pressure on the health care system in Mississippi, state officials warned.
"We are going to have a rough few weeks, Delta is hitting us very strongly," State Health Officer Dr. Thomas Dobbs said Tuesday. "We're gonna watch people needlessly die over the next month or two, for no good reason."
Once Covid-19 vaccines receive full approval, it will be easier to mandate vaccination in the US, National Institutes of Health Director Dr. Francis Collins said Wednesday.
"If it was approved with the full approval from FDA -- which we all anticipate may be coming pretty soon, maybe in the next couple of months -- then the legal ability to mandate becomes a lot stronger," Collins told CNN's Jim Acosta.
Collins noted that some universities are already mandating Covid-19 vaccines for students, including the schools his grandchildren will be attending in the fall.
"I'm personally glad to see that," he said. "It's so straightforward here to try to keep us from having outbreaks on college campuses with vaccines that are safe and effective."
"What's not to be in favor of?" Collins added.
Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, recently said it is likely that Pfizer's Covid-19 vaccine will receive full approval by the end of August.
While the virus spreads among unvaccinated adults, children -- many of whom are not yet eligible to be vaccinated -- are feeling the impacts.
"It doesn't look like this virus is selectively targeting children," Dr. Peter Hotez, professor and dean of Tropical Medicine at Baylor College of Medicine, told CNN. "It's just that so many unvaccinated individuals are getting Delta that children are getting swept up along with it."
Last week, more than 23,000 kids caught Covid-19, which is nearly double what was reported at the end of June, the American Academy of Pediatrics reported Tuesday. Children represent nearly 16% of weekly reported cases.
And though children are at a lower risk to develop serious illness from Covid-19 than older adults, CDC Director Dr. Rochelle Walensky pushed back on claims that they are not being affected.
"One thing I just want to note with the children is, I think we fall into this flawed thinking of saying that only 400 of these 600,000 deaths from Covid-19 have been in children," Walensky said. "Children are not supposed to die. And so, 400 is a huge amount for respiratory season."
Currently, only people 12 and older are eligible for vaccinations, though studies are underway that aim to provide protection to younger children.
It is "very likely" that data about Covid-19 vaccines in children under 12 may be available by early winter, Fauci has said.
And when they do become available, Fauci said he would not be surprised if schools considered including Covid-19 vaccines as a required immunization. If this year and next there is a still a problem with coronavirus, "it very well might be required," Fauci said Tuesday on "CBS This Morning."
Many experts have suggested local vaccination mandates could be an important strategy for boosting the vaccination rate and bringing the virus under control.
Starting August 2, workers at 11 New York City hospitals and clinical workers with the city's health department will be required to either get vaccinated or take weekly Covid-19 tests, Mayor Bill de Blasio said Wednesday.
The 11 hospitals in question belong to the city's public Health + Hospitals system. About 60% of H+H staff are vaccinated now, and the new requirement probably will increase that rate, CEO Dr. Mitchell Katz said.
Additionally, Banner Health, a nonprofit health service that is the largest private employer in Arizona, is requiring employees to be vaccinated against Covid-19 in order to keep their jobs.
"With limited exceptions, all team members have until November 1 to be fully vaccinated," the company said Tuesday in a news release.
Banner Health cited the rise of the Delta variant as a reason for the mandate, along with the need to prepare for the upcoming flu season. Specifics about how employees could request an exemption to the requirement will be released later, the company said.
"We are taking this step to reduce risk for our patients, their families, visitors and each other," President and CEO Peter Fine said. "Safety is an absolute top priority and the Covid vaccine mandate reflects that commitment."
Banner Health employs about 52,000 people in Arizona, California, Colorado, Nebraska, Nevada and Wyoming, it said.
Such measures could become more common when the vaccines get full FDA authorization, experts have said.
Despite the vaccination push, a poll published Tuesday by Axios-Ipsos showed a majority of unvaccinated Americans said they are not at all likely to get vaccinated -- regardless of outreach efforts.