(CNN) -- To prevent further spread of the Delta variant, the US Centers for Disease Control and Prevention updated its mask guidance on Tuesday to recommend that fully vaccinated people wear masks indoors when in areas with "substantial" and "high" transmission of Covid-19, which includes nearly two-thirds of all US counties.
"In recent days I have seen new scientific data from recent outbreak investigations showing that the Delta variant behaves uniquely differently from past strains of the virus that cause Covid-19," CDC Director Dr. Rochelle Walensky told a media briefing on Tuesday.
"This new science is worrisome and unfortunately warrants an update to our recommendations," she said. "This is not a decision that we or CDC has made lightly."
New unpublished data showing that vaccinated people infected with the Delta coronavirus variant can have as much virus as those who are unvaccinated is the primary driver for the CDC's latest mask guidance change, a source involved with the decision process told CNN.
Overall, vaccinated people still play a small role in transmission and breakthrough infections are rare.
In addition, the source noted two other factors that led to this decision: the prevalence of the Delta variant and low vaccine uptake.
When the CDC previously revised its guidance on May 13 for vaccinated people to unmask, Delta only represented 1% of reported infections. Now, according to the CDC, it represents at least 83% of cases.
The source also noted that the country's overall level of vaccination is lower than what was initially expected and that most transmission is happening in areas with vaccination levels below 40% of the population.
"When you get information about risks and how to mitigate risks, there's a public health obligation to let people know about it," a senior administration official said. The White House is hoping that the new guidance will give local officials "a lot of cover" to implement new mask mandates where appropriate.
Meanwhile, the guidance for unvaccinated people remains the same: continue masking until they are fully vaccinated.
Nearly half -- 46% -- of US counties currently have high transmission and 17% have "substantial" transmission, according to data from the CDC, as of Tuesday morning.
In two states, Arkansas and Louisiana, every county is currently listed as having "high" levels of community transmission of Covid-19, according to CDC data. Several other states -- including Missouri, Mississippi and Alabama -- also have "high" transmission in nearly every county. In Florida, every county was listed with high transmission on Monday; as of Tuesday morning, one -- Glades County -- has substantial transmission.
"Vaccinated individuals continue to represent a very small amount of transmission occurring around the country. We continue to estimate that the risk of a breakthrough infection with symptom upon exposure to the Delta variant is reduced by seven-fold. The reduction of 20-fold for hospitalizations, and deaths," Walensky said during Tuesday's briefing.
"As CDC has recommended for months, unvaccinated individuals should get vaccinated and continue masking until they are fully vaccinated," she said. "In areas with substantial and high transmission, CDC recommends fully vaccinated people wear masks in public, indoor settings, to help prevent the spread of the Delta variant and protect others -- this includes schools."
The CDC's latest guidance also recommends for community leaders to encourage vaccination and mask-wearing to prevent further outbreaks in areas of substantial and high transmission.
The agency recommends that local jurisdictions encourage universal indoor masking for all teachers, staff, students, and visitors to schools, regardless of vaccination status.
Earlier this month, the CDC's Covid-19 school guidance noted that fully vaccinated people do not need to wear masks, and then about a week later the American Academy of Pediatrics issued stricter guidance recommending that everyone older than 2 wear a mask in schools, regardless of vaccination their status.
Now the updated CDC guidance recommends everyone in schools wear masks.
"CDC recommends that everyone in K through 12 schools wear a mask indoors, including teachers, staff, students and visitors, regardless of vaccination status. Children should return to full-time, in-person learning in the fall with proper prevention strategies in place," Walensky said. "Finally, CDC recommends community leaders encourage vaccination and universal masking to prevent further outbreaks in areas of substantial and high transmission. With the Delta variant, vaccinating more Americans now is more urgent than ever."
The updated CDC guidance makes "excellent sense," Dr. David Weber, professor at the University of North Carolina School of Medicine in Chapel Hill and board member of the Society of Healthcare Epidemiology, told CNN on Tuesday.
"Breakthrough disease clearly occurs, and for those cases, we know they're much more mild in vaccinated people, but we don't know how infectious vaccinated people are," he said. "But clearly, if you want to protect your children under 12 or grandchildren, or protect immunocompromised people, as well as protect your own health -- from even mild disease -- then you should be wearing a mask, particularly in areas of high transmission when indoors."
Guidance has been in talks for days
The last time the CDC updated its mask guidance was two months ago, to say most people who are fully vaccinated could go without masks indoors. That guidance moved so quickly that administration officials were informed less than a day before -- and many public health experts were taken by surprise.
This time, the CDC's updated guidance has been expected for some days now.
As the highly contagious Delta coronavirus variant continues to circulate across the United States, top federal health officials debated whether to issue new guidance on masks, huddling on Sunday night to go over the new data and evidence regarding the transmissibility of the variant and breakthrough cases among vaccinated people, according to a person familiar with the talks.
The CDC emphasized on Tuesday that while some breakthrough cases have occurred, getting vaccinated still prevents severe illness, hospitalization and death. The CDC also noted that the highest spread of Covid-19 cases and severe outcomes is happening in areas with low vaccination rates and among unvaccinated people.
"There are many reasons for getting vaccinated -- keeping you from dying, keeping you from being hospitalized, protecting loved ones, as well as not ending up with permanent symptoms, potentially like loss of sense of smell and taste and confusion that may last for a prolonged period of time," Weber said.
"There are many reasons still to get vaccinated. Even if under certain circumstances, it's recommended you wear your mask," he said. "The CDC and others will be flexible and if we can achieve higher percentages of vaccination and drive the rate of infections back down again, then the recommendation will go back to saying if you're vaccinated, you don't need a mask -- but I don't see that for a substantial period of time."
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