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.

Change to masking guidance based on new data

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 s