They reinforce anticovid-19 measures in the United States 2:57
(CNN) -
A new study shows that the delta variant of covid-19 produces similar amounts of virus in vaccinated and unvaccinated people if they become infected.
This discovery illustrates a key motivation behind the federal guidance, which now recommends that most fully vaccinated Americans wear masks indoors.
Experts say vaccination makes you less likely to get COVID-19 in the first place.
But, for those who do, this data suggests that they might have a similar tendency to spread it as unvaccinated people.
"Elevated viral loads suggest an increased risk of transmission and raise concerns that, unlike other variants, vaccinated people infected with delta can transmit the virus," said Dr. Rochelle Walensky, director of the Centers for Disease Control. and the US Disease Prevention (CDC) in a statement Friday.
The study, released by the CDC on Friday, describes 469 Massachusetts residents who were infected in a July outbreak in Barnstable County, which includes the summer vacation destination of Provincetown.
No deaths were reported between them.
How was the study
About 74%, or 346 cases, were from fully vaccinated people.
Of those cases, 79% reported symptoms.
The genetically sequenced cases revealed the delta variant as the main culprit.
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The researchers found evidence that viral loads were similar between 127 fully vaccinated people and another 84 who were unvaccinated, partially vaccinated, or whose vaccination status was unknown.
A "fundamental discovery"
Viral load is an indicator of how likely someone is to transmit the virus to other people.
On Tuesday, Walensky previewed these findings and released guidance that determined that people in areas with "high" or "substantial" COVID-19 transmission should resume wearing mask indoors.
More than 75% of the US population lives in these areas.
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The finding that the delta variant resulted in similar viral loads "was a pivotal discovery that led to the CDC's update of the mask-wearing recommendation," Walensky said Friday.
"The recommendation to wear masks has been updated to ensure that the vaccinated public does not unknowingly transmit the virus to others, including their unvaccinated or immunosuppressed loved ones."
Questions remain
Before the delta variant, vaccination was known to affect the factors likely to influence transmission.
Not only did vaccinated people tend to have lower viral loads, they also had milder symptoms and were sick for less time.
But research has shown that the delta variant spreads more aggressively.
Earlier this month, for example, Chinese scientists described viral loads that were roughly 1,000 times higher with the delta variant than with previous strains.
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On Thursday, an internal CDC document indicated that the delta variant was about as transmissible as chickenpox, while an early strain was closer to the common cold.
This means that, under certain conditions, an infected person may have transmitted the virus to two or three people, on average, at the beginning of the outbreak.
But now, with the delta variant, that number could be five to nine.
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An expert opinion
"It's one of the most communicable viruses that we know of. Measles, chicken pox, this, they're all there," Walensky told CNN Thursday night.
The CDC document also cited reports that the variant could also cause more serious illness.
But, even with similar viral loads, it is not a foregone conclusion that vaccinated people are necessarily as contagious as unvaccinated people.
More studies are required
"This is intriguing data, it's important. But I'm not sure you're equally infectious if you're vaccinated," said Dr. Monica Gandhi, an infectious disease specialist at the University of California, San Francisco, who was not involved in the research. .
The discovery was key in the decision to update the guide on the use of masks
Gandhi said that there are multiple parts of the immune system, including antibodies and T cells, that raise important questions about the use of viral load, which is measured by PCR tests, as an indicator of how contagious a person is.
The new report says that "microbiological studies are required to confirm these findings" of similar viral loads among advanced infections.
It also notes that "asymptomatic breakthrough infections may be underrepresented," because they are less likely to be detected.
Earlier this week, Walensky said ongoing outbreak investigations will help uncover more about what happens when these infections occur.
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"We are now continuing to follow those groups to understand the impact of forward transmission from vaccinated people," he said.
"But again, I want to reiterate, we believe that the vast majority of transmission occurs in unvaccinated people and through unvaccinated people."
Even more reasons to get vaccinated
Even with the delta variant, covid-19 vaccines still reduce the spread of the virus.
Also, vaccinated people are much less likely to end up in the hospital.
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The internal CDC document estimated that vaccines reduce the risk of serious illness or death tenfold or more, and reduce the risk of infection threefold.
"Vaccination continues to prevent serious illnesses, hospitalizations and deaths, even with the delta variant," Walensky said this week.
More details of the study
Among the Massachusetts residents in the latest study, none died and only five were hospitalized.
Of those five, one person was unvaccinated and had underlying medical conditions, and four were vaccinated.
Among the vaccinated group, one had received the Pfizer vaccine and the remaining three had received Johnson & Johnson.
The vaccinated people were in their 20s and 70s, and two had underlying health problems.
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Additional infections were identified among visitors from at least 22 other states who were in the area from July 3-17.
Thousands of people gathered for the summer festivities, and infected people reported attending "densely packed indoor and outdoor events in venues that included bars, restaurants, guest houses and rental homes," the study says.
Although these findings prompted the CDC to update its guidance for areas with the highest levels of viral transmission, the study notes that Barnstable County was not one of those areas until the outbreak.
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Between July 3 and 17, daily new cases increased from an average of 14 days from 0 to 177 cases per 100,000 inhabitants.
The study suggests that "even jurisdictions without substantial or high COVID-19 transmission could consider expanding prevention strategies, including wearing face masks in closed public settings regardless of vaccination status."
Extreme measures
"The measures that we need to get this under control are extreme. The measures that are needed are extreme," Walensky told CNN Thursday night.
The study also notes that new infections are expected, especially as the portion of the vaccinated population grows.
"As vaccination coverage increases at the population level, vaccinated people are likely to account for a higher proportion of COVID-19 cases," the authors wrote.
CNN previously reported on the Provincetown-related outbreak.
In all, at least 882 cases have been linked to the group so far, about 60% of which were Massachusetts residents, according to local officials.
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Provincetown City Manager Alex Morse told CNN on Friday that the city appeared to be taking a favorable turn, with an indoor mask mandate in place and a test positivity rate that is in the slope.
"We think we are going in the right direction, but we are obviously concerned about what happened here in the last few weeks," Morse told CNN's John Berman.
A source familiar with the CDC's decision to update its recommendations previously told CNN that, in addition to the viral load findings, the overall prevalence of the delta variant and lower-than-expected vaccine uptake played a key role in the latest version of the guide.
Contributed to this report Elizabeth Cohen
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