(CNN)The CDC's decision to change its mask guidance this week was prompted by scientists' determination -- only days old -- that vaccinated people with breakthrough Delta coronavirus variant infections can pass the virus to others, the center's director said Wednesday.
Dr. Rochelle Walensky said such breakthrough infections are rare, and stressed that Covid-19 vaccines generally prevent hospitalizations and deaths even if vaccinated people are infected.
But "with prior variants, when (vaccinated) people had these rare breakthrough infections, we didn't see the capacity of them to spread the virus to others," Walensky, the US Centers for Disease Control and Prevention's director, told CNN's "New Day."
Now, outbreak investigations conducted in the last couple weeks showed vaccinated people who happen to contract the highly contagious Delta variant "can actually now pass it to somebody else," Walensky said Wednesday.
That realization came only "in the last several days," and more information will be published in the coming days, she said.
Walensky had told reporters Tuesday that for Delta variant infections of vaccinated people, the amount of virus found in them was "pretty similar to the amount of virus in unvaccinated people.
Still, she said the "vast majority" of transmission is through unvaccinated people.
The CDC updated its mask guidance Tuesday to recommend fully vaccinated people wear masks indoors in parts of the country with "substantial" or "high" transmission of Covid-19.
The CDC also recommended that everyone in and around K-12 schools wear masks, even if they are fully vaccinated.
As the Delta variant recently increased its prominence in the US, and with the CDC saying only 49.3% of the US population had been fully vaccinated as of Tuesday, coronavirus case rates have jumped.
The US averaged more than 61,300 new daily cases over the last week -- an average that's generally risen since the country hit a 2021 low of 11,299 daily on June 22, according to Johns Hopkins University data.
As of Wednesday, cases have risen in all but one state in the past seven days compared to the week before, according to Johns Hopkins.
US Surgeon General Dr. Vivek Murthy said the US is seeing just how dangerous the variant is. "This is actually what you want to happen with science. You want science to be dynamic, you want recommendations to reflect the latest science, and that's what you see in the recommendations that were issued today," Murthy told CNN's Wolf Blitzer on Tuesday.
Almost three-quarters of US residents live in counties where indoor masking is recommended for everyone
More than 71% of the US population -- about 237 million people -- live in counties considered to have "substantial" or "high" Covid-19 transmission, according to a CNN analysis of data published Wednesday by the CDC.
About 48% are in "high" transmission counties, and 23% are in counties with "substantial" transmission.
This is up from a week ago, when 50.5% of Americans lived in such counties.
Only 1% of the population lives in areas with "low" transmission.
The CDC considers a county to have "high" transmission if there have been 100 or more cases of Covid-19 per 100,000 residents in the past week, or a test positivity rate of 10% or higher during the same time frame.
For "low" transmission, those numbers must be fewer than 10 new cases per 100,000 or a test positivity rate under 5%.
Vaccinated people don't yet need a booster, surgeon general says after Pfizer news
Pfizer released new data Wednesday suggesting that a third dose of its vaccine can "strongly" boost protection against the Delta variant -- beyond the protection afforded by the standard two doses.
The data, which included 23 people, has not yet been peer-reviewed or published.
But Murthy told CNN on Wednesday that fully vaccinated people don't need to get a booster at this point, if ever. And any decision on whether that will change will be made by agencies such as the CDC and the US Food and Drug Administration, he said.
"This data from Pfizer, we've been in talks with them about what they're seeing with regard to their studies related to boosters," Murthy told CNN's "Newsroom" when asked about the data release. "But at this point, I want to be very clear: People do not need to go out and get a booster shot."
Murthy also said whether it'd be ethical to recommend a third shot while there is a major vaccine supply shortage in the developing world was a "critical question." The ability to reduce the likelihood of future variants developing depends on tamping down spread around the world, he said.
Pfizer anticipates submitting data on a third dose of its coronavirus vaccine to the FDA as soon as next month, one of its research and development leaders said during a company earnings call Wednesday.
Visitors to the Grand Central Market in Los Angeles are mostly masked on Tuesday.
Vaccinations are still the 'bedrock' of ending the pandemic
While masking up will help reduce the spread of Covid-19 in the US, getting vaccinated is still "the bedrock" of ending the pandemic, Murthy said.
"Vaccines still work. They still save lives. They still prevent hospitalizations at a remarkably high rate," he added.
Over the past seven days, the rate of Americans getting their first vaccine shots has gone up. It was 35% higher than the previous seven-day period and the highest it has been in three weeks, according to CNN analysis of CDC data.
But vaccination rates are still not so high as to get enough of the US inoculated against the virus to slow or stop its spread, experts have said. Many experts have advocated for vaccine requirements as one way to increase vaccination rates in the US.
Los Angeles officials announced Tuesday that the city will require all of its employees to show proof of vaccination or submit to weekly testing.
"The fourth wave is here, and the choice for Angelenos couldn't be clearer -- get vaccinated or get Covid-19," Mayor Eric Garcetti said. "We're committed to pursuing a full vaccine mandate. I urge employers across Los Angeles to follow this example."
The move comes after the number of people hospitalized with the coronavirus in Los Angeles County nearly doubled in the past two weeks. There are currently 745 people hospitalized with the virus, compared to 372 people two weeks ago, according to the Los Angeles County Department of Public Health.
Such requirements by local entities are "very reasonable," Murthy said Tuesday.
Some US hospitals and federal agencies are mandating that employees get vaccinated against Covid-19 or submit to regular testing. Murthy noted that many private institutions are considering following suit.
"Those are decisions the federal government is not going to make," Murthy said. "It's going to be institutions that make them, but I do think that they are very reasonable, because this is a time when we've got to take all steps possible to protect not just ourselves, but the people around us, from Covid-19."
