Corrections & clarifications: This story has been updated to clarify the number of COVID-19 vaccine shots administered in the U.S.
The Centers for Disease Control and Prevention has had a busy week.
Only a few days after announcing updated mask guidelines, the agency on Friday released new scientific data on the delta variant that gives a snapshot of how the highly contagious strain triggered a wave of coronavirus cases.
The much-anticipated report comes a day after a presentation compiled by a doctor with the agency was leaked to the media and detailed the dangers of the delta variant and how mask-wearing is essential to bring it under control.
In a briefing Tuesday, CDC director Dr. Rochelle Walensky said the new data spurred the agency to take immediate action by recommending fully vaccinated people to wear mask indoors in public settings where coronavirus transmission is high.
Mapping CDC's new guidelines:High transmission areas where you need to wear a mask indoors
"The delta variant is showing every day its willingness to outsmart us and be an opportunist in areas where we have not shown a fortified response against it," she said earlier this week. "This new science is worrisome and unfortunately warrants an update to our recommendations."
Here's everything to know about the delta variant and how it impacts fully vaccinated people.
'Pivotal discovery': What the new data says about delta variant, transmission
Fully vaccinated people made up nearly three-quarters of COVID-19 infections that occurred in a Massachusetts town during and after Fourth of July festivities, according to a CDC study published Friday in the agency’s Mortality and Morbidity Weekly Report.
Out of 469 cases that were identified in Barnstable County, Massachusetts, from July 3 to 17, the agency found 74% occurred in fully vaccinated people. The CDC sequenced samples taken from 133 patients and discovered 90% were caused by the delta variant.
“High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with delta can transmit the virus,” Walensky said in a statement sent to USA TODAY on Friday. “This finding is concerning and was a pivotal discovery leading to the CDC’s mask recommendation.”
Health officials continue to reiterate the majority of COVID-19 transmission occurs among the unvaccinated, not fully vaccinated people.
“Vaccinated individuals continue to represent a very small amount of transmission occurring around the country,” Walensky said. “We continue to estimate that the risk of breakthrough infection with symptoms upon exposure to the delta variant is reduced by sevenfold. The reduction is twentyfold for hospitalizations and death.”
Four fully vaccinated people between the ages of 20 and 70 were hospitalized, two of whom had underlying medical conditions. No deaths were reported.
The study found 79% of patients with breakthrough infection reported symptoms including cough, headache, sore throat, muscle pain, and fever.
Of the 346 breakthrough infections, 56% of people were vaccinated with the Pfizer-BioNTech vaccine, 38% with Moderna and 7% with Johnson & Johnson. As of Friday, over 190 million doses of the Pfizer vaccine has been administered in the U.S., nearly 140 million of Moderna and 13.3 million of Johnson & Johnson, according to the CDC.
Health experts say the reason why more breakthrough infections occurred in the mRNA vaccines compared to the Johnson & Johnson vaccine is because more people in the U.S. received the Pfizer-BioNTech and Moderna vaccines.
"When you look at the data, it may concern some people that there appears to be a higher rate of breakthrough COVID infections in people fully vaccinated with the Pfizer vaccine, however, as a percentage of people who are fully vaccinated, more people have been vaccinated with the Pfizer vaccine," said Dr. Teresa Murray Amato, chair of emergency medicine at Long Island Jewish Forest Hills in Queens, New York.
"It still appears that all three of the current vaccines with emergency use administration authorization in the United States are safe and effective against the delta variant of the COVID-19 virus," she added.
While study authors say evidence suggests fully vaccinated people exposed to the delta variant can contract and spread the virus, it is not sufficient to determine the vaccines' effectiveness against the highly contagious strain.
Delta substantially more contagious than other variants
Although the study didn't specify if fully vaccinated people can transmit the virus to other fully vaccinated people, health experts say they should wear a mask and socially distance largely to protect those who haven't been vaccinated or who have a weakened immune system and can't get full protection from the vaccine.
"The data makes a pretty compelling justification for why we need to go back to mask wearing and other public health measures," said Dr. Charles Chiu, an infectious disease specialist at the University of California, San Francisco. "I do think it's because of the delta variant."
The delta variant is known to be substantially more contagious than other variants – as contagious though deadlier than chicken pox, according to the CDC presentation. Among common infectious diseases, only measles is more contagious.
People may also be infectious for longer with the delta variant, 18 days instead of 13, the presentation says.
Vaccines remain effective at preventing hospitalization and death from COVID-19, though they worked better against the original strain and the alpha variant than they do against delta, data finds.
What do the CDC mask guidelines say?
The CDC is urging fully vaccinated Americans to wear masks indoors in areas of high or substantial coronavirus transmission.
They're also recommending universal indoor masking for all teachers, staff, students and visitors inside schools from kindergarten to 12th grade, regardless of vaccination status. That aligns closely with guidelines from the American Academy of Pediatrics, which recommended this month that anyone older than 2 be required to wear a mask in school.
The CDC and the AAP are still urging that children return to full-time in-person learning in the fall.
The goal behind the guidance may be to protect both the fully vaccinated and the unvaccinated, health experts say, especially vaccinated people who may be immunocompromised and children under 12 who aren’t yet eligible to get their shot.
But the reality is there’s hardly any transmission among fully vaccinated people to truly affect community spread, they say.
"It makes sense why they did it, but I don't think it's going to make a major difference in the large surge that we're having," said Dr. Ashish Jha, dean of the Brown University School of Public Health in Providence, Rhode Island. "The real issue still is unvaccinated people who are not going around masked up. I have no reason to think that this guidance will get unvaccinated, unmasked people putting on masks. And that's what we really need."
Is there a test for the delta variant?
A traditional PCR test alone cannot differentiate the delta variant from the original virus.
The delta variant has distinctive mutations that serve as biological markers that can only be detected through genome sequencing.
Many U.S. laboratories sequence a small – but nationally representative – number of positive samples for epidemiological purposes. According to the CDC, more than 175,000 sequences have been collected through the agency’s surveillance program since Dec. 20.
People who test positive for COVID-19 aren’t made aware if they were infected by the delta variant, even if their sample was sequenced.
"Our patients will not learn if they have a variant or not," said Dr. Christina Wojewoda, chair of College of American Pathologists Microbiology Committee. "It is for epidemiology purposes only and currently, there is no medical use for that result."
However, the CDC said more than 80% of sequenced samples have the delta variant, which means people sick with COVID-19 were most likely infected with the highly contagious strain.
“It is safe to assume in most places, if you are infected now, it is likely delta," Wojewoda said.
Contributing: Ken Alltucker and Karen Weintraub, USA TODAY. Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT.
Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.
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