People vaccinated against COVID-19 in high-risk parts of the United States should resume wearing masks indoors, the Centers for Disease Control and Prevention (CDC) said Tuesday.
The shift comes as the Delta variant of the coronavirus is driving a sharp surge in cases, with hotspots in regions that have lagged behind in vaccinations.
"In areas with substantial and high transmission, CDC recommends fully vaccinated people wear masks in public indoor settings," said CDC director Rochelle Walensky, explaining the change was needed because of the Delta variant that is rapidly spreading throughout the country.
While vaccine efficacy remains high against the Delta variant, new data "indicate that, in rare occasions, some vaccinated people... may be contagious and spread the virus to others," she added.
"This new science is worrisome and unfortunately warrants an update to our recommendation."
According to the latest CDC data, much of the southern United States is experiencing high or substantial transmission, while highly vaccinated parts of the Northeast are mostly experiencing moderate rates of community transmission.
Substantial transmission is defined as being between 50 to 100 daily cases per 100,000 people over a 7-day average, while high is defined as more than 100 cases per 100,000 people over the same average.
To stem the spread of the Delta variant, the CDC will recommend schools adopt universal masking, including teachers, staff, students and visitors, regardless of vaccination status, added Walensky.
As recently as last week, the CDC defended its May decision that vaccinated people do not have to wear masks indoors in most circumstances, with exceptions including public transit and hospitals.
But infection numbers are now swelling, thanks to the Delta variant, which accounts for around 90 percent of cases.
The latest seven day average of daily cases is more than 56,000, similar to levels last seen in April.
Forty-nine percent of the US population is fully vaccinated, but the vaccination rate is heavily skewed between politically liberal and conservative parts of the country.
"This is not a reflection on the quality or level of protection of the vaccine," Eric Cioe-Pena, director of Global Health at Northwell Health, told AFP.
"This is a solution to a problem that exists because we have high levels of people that are not vaccinated."
According to a recent paper in the journal Virological, the viral load in the first tests of patients with the Delta variant was 1,000 times higher than patients in the first wave of the virus in 2020.
Not only does the variant replicate more rapidly inside its host compared to past strains, but people who are infected shed much more of it in the air, greatly increasing the likelihood it will be transmitted.
Last month, Israel reinstituted mask mandates, just 10 days after lifting them, due to the Delta variant, and local US jurisdictions, including Los Angeles County, have taken similar steps.