(CNN) -- The change in CDC guidance recommending all Americans wear a mask indoors in areas with high Covid-19 transmission is a sign of the change the Delta variant has carved into the pandemic landscape, Dr. Anthony Fauci told CNN.
"We're not changing the science," the director of the National Institute for Allergy and Infectious Disease told CNN's Chris Cuomo. "The virus changed, and the science evolved with the changing virus."
Before Tuesday, the US Centers of Disease Control and Prevention advised only unvaccinated people to wear masks indoors. But with the spread of the Delta variant -- believed to be at least twice as transmissible as the Alpha variant, which was dominate in the US in the spring -- and vaccination rates remaining low while infection rates on the rise, the CDC updated its guidance to advise that everyone in high transmission areas wear a mask when indoors.
Currently, only 49.2% of the US population is fully vaccinated against Covid-19, according to the CDC.
Some experts point to unvaccinated Americans as an important factor in the mask guidance change, saying the measure had to be implemented to get them to mask up.
"Eighty million American adults have made a choice. They made a choice not to get the vaccine, and those same people are not masking and that is the force that is propagating this virus around this country," CNN Medical Analyst Dr. Jonathan Reiner told CNN.
But others, including the CDC, said the decision had more to do with new data showing that, unlike with other strains, vaccinated people who are infected with the Delta variant can still get high viral loads, making it more likely they could spread the virus.
"Unlike the Alpha variant that we had back in May, where we didn't believe that if you were vaccinated you could transmit further, this is different now with a Delta variant," CDC Director Dr. Rochelle Walensky said, citing information investigators found when looking at outbreak clusters.
With nearly all 50 states undergoing a surge of new cases averaging at least 10% more than the week before, according to data from Johns Hopkins University, US Surgeon General Dr. Vivek Murthy said the US is seeing just how dangerous the variant is in real time.
"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 Tuesday.
But one thing hasn't changed, Murthy added, saying data is still showing current vaccines are highly protective against infection, severe illness and death from the Delta variant.
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" to ending the pandemic, Murthy said.
"Vaccines still work. They still save lives. They still prevent hospitalizations at a remarkably high rate," he added.
Vaccination rates are still not where they need to be 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," said Mayor Eric Garcetti in a statement. "We're committed to pursuing a full vaccine mandate. I urge employers across Los Angeles to follow this example," he added.
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 told CNN's Wolf Blitzer. "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."
Officials call for more vaccinations as hospitals are overwhelmed
The impact of the Delta variant and increasing cases can be seen in the data and in the strain on hospitals.
After decreases over the past couple of months, cases of Covid-19 among children and teens are on the rise again, with more than 38,600 infected last week, the American Academy of Pediatrics reported Tuesday.
More than 4.13 million kids have tested positive for Covid-19 since the start of the pandemic. Kids represent more than 14% of the weekly reported cases.
In Springfield-Greene County in Missouri, the CoxHealth hospital system is having to expand its morgue capacity due to an increase in Covid-19 related deaths, President and CEO Steve Edwards said Tuesday.
"Last year we did expand it and we are expanding it further. We've actually brought in a portable piece of technology that allows bodies to be cooled and placed outside the morgue. We have had to expand that because the mortality has gone up so much lately," Edwards said during an update in the county on behalf of CoxHealth.
In explaining what he called the "severity of the disease", Edwards said, "We've had over 4,000 admissions for Covid. And with 549 deaths that means thirteen and a half percent of our admissions have died. And when we look in our ICUs, about 40% of patients that are in the ICU don't make it out of the ICU."
In New Orleans, as cases have gone up, hospitals have become strapped for resources and started turning people away, Communications Director for the City of New Orleans Beau Tidwell said Tuesday.
"For God's sake, get your vaccine," he added.
The CDC called on doctors and public health officials to act urgently to get more Americans vaccinated.
"COVID-19 cases have increased over 300% nationally from June 19 to July 23, 2021, along with parallel increases in hospitalizations and deaths driven by the highly transmissible B.1.617.2 (Delta) variant," the CDC said in Tuesday's health alert.
Without more vaccinations, the US could see increased morbidity and mortality related to Covid-19, which could continue to overwhelm healthcare facilities, the CDC said.
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