(CNN) While people putting their masks back on indoors will help reduce the spread of Covid-19 across the country, getting more Americans vaccinated is still central to ending the pandemic, US Surgeon General Dr. Vivek Murthy said Tuesday.
"The bottom line is the masks will help us reduce spread further, but the vaccinations remain the bedrock of ending this pandemic," Murthy told CNN's Wolf Blitzer.
The Centers for Disease Control and Prevention updated its mask guidance Tuesday to recommend fully vaccinated people wear masks indoors in areas with "substantial" or "high" transmission of Covid-19 -- more than half of all US counties.
"What's really important, also, is to say what has not changed," Murthy said. "And what has not changed is that vaccines still work. They still save lives. They still prevent hospitalizations at a remarkably high rate."
Much of the country remains unvaccinated, despite incentive programs and urging from health experts. Only 49.2% of the US population is fully vaccinated, according to the CDC. More than 34.6 million people in the United States have tested positive for the coronavirus.
Murthy said he doesn't want Americans to think the new mask guidelines mean that progress against Covid-19 is lost.
"That is not the case," he said, adding that vaccines have prevented many hospitalizations and deaths.
Dr. Anthony Fauci, the longtime director of the National Institute of Allergy and Infectious Diseases, said the situation could have been different.
"If we had the overwhelming proportion of the people vaccinated, we would not be having this conversation," he told PBS.
Fauci addressed the changing guidance from the CDC, which in mid-May said most fully vaccinated people don't have to wear masks indoors or outdoors.
"We are dealing with an evasive type of a virus. It evolves," Fauci said. "People need to understand. It's a painful realization, but it's true. We're dealing with a virus that's a wily character."
The CDC's May guidance was based on the Alpha coronavirus variant being dominant, he and Murthy said. It no longer is, so the CDC isn't flip-flopping, Fauci said; it's keeping up with the changing science.
The CDC updated the "When You've Been Fully Vaccinated" section of its website on Tuesday.
The guidance change also included the recommendation that everyone in and around K-12 schools wear masks, even if they are fully vaccinated.
CDC Director Rochelle Walensky said Tuesday on a telebriefing that the agency, through investigations of clusters of Covid-19 outbreaks, determined the Delta variant is more likely to infect even fully vaccinated people.
"What we've learned in that context is that when we examine the rarer breakthrough infections, and we look at the amount of virus in those people, it is pretty similar to the amount of virus in unvaccinated people," Walensky said.
She said the "vast majority" of transmission is through unvaccinated people.
Nearly two-thirds of US counties have high or substantial transmission of Covid-19, according to CDC data; 46% of counties have high transmission and 17% have substantial transmission.
The CDC defines high transmission as at least 100 new cases per 100,000 people or a test positivity rate of at least 10% in the past seven days. The threshold for substantial transmission is 50 new cases per 100,000 people or a test positivity rate of 8%.
Schools are filled with unvaccinated people who can spread Covid-19 to other settings, former US Surgeon General Dr. Jerome Adams said.
"Schools are a large reservoir of unvaccinated people -- of superspreaders, if you will -- who can take the Delta variant home and to other places," Adams told CNN's Anderson Cooper.
Adams said the risk of serious illness or death is low for children but the point is they also spread the virus to other people.
Florida is one state unlikely to require masks in schools. Gov. Ron DeSantis, responding through a spokesperson to the CDC recommendations, said parents know what's best for their children.
"Fortunately, the data indicate that Covid is not a serious risk to healthy children, which is why schools in most countries were among the first institutions to reopen," said spokesperson Christina Pushaw. "At the end of the day, the governor trusts parents to weigh the risks and benefits and make the best choices for their kids."
DeSantis' statement is in contrast to CDC evidence that shows Covid-19 can be a serious risk to children. CDC data indicate more children have died from Covid-19 -- 517 so far -- than are killed by the flu annually, even in a bad influenza year.
There were reports of more than 38,600 new cases in children for the period between July 15 and 22, according to the American Academy of Pediatrics.
Children have accounted for 1.3% to 3.6% of the hospitalizations, depending on the state, according to the organization, which represents pediatricians.
The US could see nearly four times the current rate of Covid-19 cases in the next four to six weeks as the Delta variant spreads and the population hits a wall on vaccinations, a former director of the Centers for Disease Control and Prevention told CNN.
"We're heading into a rough time. It's likely, if our trajectory is similar to that in the United Kingdom, that we could see as many as 200,000 cases a day," Dr. Tom Frieden said Monday, adding the US likely won't see the "horrific death tolls" of earlier in the pandemic thanks to the number of vulnerable people who are vaccinated. Frieden was CDC director during the Obama administration.
But, he said, "You will see a steady increase in deaths, and these are preventable deaths."
Cases have jumped as the Delta variant has spread.
The US averaged more than 57,300 new daily cases over the last week -- an average that's generally risen since the country hit a 2021 low of 11,351 daily on June 22, according to Johns Hopkins University data.
As of Tuesday, cases have risen by 50% or more in 35 states over the past seven days compared to the week before.
The last time more than 200,000 US cases were reported in a day was in January, according to Johns Hopkins.
The Delta variant is believed to be more transmissible than other strains -- so much so, Frieden said, that it is essentially finding people who are unvaccinated.
Vaccination rates have hit a wall, and that could have serious consequences, said Dr. Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia.
"Now we are at a point where there is a solid 25 or 30% of the population that's saying they don't want to get vaccinated, that they are okay with allowing this virus to continue to spread, continue to do harm and, worst of all, continue to possibly create variants that are going to be resistant to vaccine-induced immunity," Offit said.
And vaccinated people will likely pay a price for those choices, CNN medical analyst Dr. Leana Wen said. Even though vaccines offer strong protection against the virus, being surrounded by unvaccinated people could lead to infection spillover, and vaccinated people could get sick or pass on the infection to their loved ones, she said.