Experts say that vaccination makes it less likely that you'll catch COVID-19 in the first place -- but for those who do, this data suggests they could have a similar tendency to spread it as unvaccinated folks.
"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," Dr. Rochelle Walensky, director of the US Centers for Disease Control and Prevention, said in a statement Friday.
The study, published by CDC Friday, describes 469 Massachusetts residents who were infected in a July outbreak in Barnstable County, which includes the summer vacation destination Provincetown. No deaths were reported among them.
About 74% -- or 346 cases -- had been fully vaccinated. Of those cases, 79% reported symptoms. Genetically sequenced cases revealed the delta variant as the main culprit.
CDC COVID-19 Transmission Levels by U.S. County
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The researchers found evidence that viral loads were similar among 127 fully vaccinated people and 84 others who were unvaccinated, partially vaccinated or whose vaccination status was unknown. Viral load is a proxy for how likely someone might be to transmit the virus to others.
On Tuesday, Walensky previewed these findings while unveiling guidance that people in areas with "high" or "substantial" COVID-19 transmission should resume wearing masks indoors. Over 75% of the US population live in these areas.
The finding that the delta variant resulted in similar viral loads "was a pivotal discovery leading to CDC's updated mask recommendation," Walensky said Friday.
"The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones."
Prior to delta, vaccination was known to impact factors that likely influence transmission. Not only did vaccinated people tend to have lower viral loads, but they also had milder symptoms and were sick for less time.
But research has shown that delta spreads more aggressively. Earlier this month, for example, Chinese scientists described viral loads that were about roughly 1,000 times higher with delta than earlier strains.
On Thursday, a CDC internal document said the delta variant was roughly as transmissible as as chickenpox, whereas an early strain was closer to the common cold. This means that, under certain conditions, an infected person may have transmitted to two or three people, on average, early in the outbreak. But now, with delta, that number could be five to nine.
"It's one of the most transmissible viruses we know about. Measles, chickenpox, this -- they're all up there," Walensky told CNN late Thursday.
The CDC document also cited reports indicating the variant might cause more severe disease, as well.
But even with similar viral loads, it's not a foregone conclusion that vaccinated people are necessarily as contagious as unvaccinated people.
"This is intriguing data, it's important, but I'm not positive that you're equally as infectious if you're vaccinated," said Dr. Monica Gandhi, an infectious disease specialist at University of California, San Francisco, who was not involved in the research.
Gandhi said there are multiple parts to the immune system -- including antibodies and T cells -- that raise important questions around using viral load, which is measured by PCR tests, as a proxy for how contagious someone is.
The new report says that "microbiological studies are required to confirm these findings" of similar viral loads among breakthrough infections. It also notes that "asymptomatic breakthrough infections might be underrepresented" because they are less likely to be detected.
Earlier this week, Walensky said that ongoing outbreak investigations will help uncover more about what happens when these breakthrough infections do occur.
"We are now continuing to follow those clusters to understand the impact of forward transmission of those vaccinated people," she said. "But again, I want to reiterate, we believe the vast majority of transmission is occurring in unvaccinated people and through unvaccinated people."
Even more reason to get vaccinated
Even with delta, COVID-19 vaccines still reduce the spread of the virus. Moreover, vaccinated people are far less likely to end up in the hospital.
The CDC internal document estimated that vaccines reduce the risk of severe disease or death 10-fold or greater, and they reduce the risk of infection three-fold.
"Getting vaccinated continues to prevent severe illness, hospitalization and death -- even with delta," Walensky said this week.
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. The vaccinated individuals ranged in age from 20 to 70, and two had underlying health conditions.
Additional infections were identified among visitors from at least 22 other states who visited the area from July 3 through 17. Thousands gathered for summer festivities, and infected people reported going to "densely packed indoor and outdoor events at venues that included bars, restaurants, guest houses, and rental homes," the study says.
Although these findings motivated CDC to update its guidance for areas with higher levels of viral transmission, the study notes that Barnstable County was not one of those areas until the outbreak. Between July 3 and 17, daily new cases rose from a 14-day average of 0 to 177 cases per 100,000 residents.
The study suggests that "even jurisdictions without substantial or high COVID-19 transmission might consider expanding prevention strategies, including masking in indoor public settings regardless of vaccination status."
"The measures we need to get this under control -- they're extreme. The measures you need are extreme," Walensky told CNN late Thursday.
The study also notes that breakthrough infections are expected, especially as the slice of the population that's vaccinated grows larger. "As population-level vaccination coverage increases, vaccinated persons are likely to represent a larger proportion of COVID-19 cases," the authors wrote.
CNN previously reported on the outbreak connected to Provincetown. In total, at least 882 cases have been linked to the cluster so far, about 60% of whom were Massachusetts residents, according to local officials.
The town manager of Provincetown, Alex Morse, told CNN Friday that the town appeared to be taking a favorable turn -- with an indoor mask mandate in place and a test positivity rate that's on the decline.
"We think we're heading in the right direction, but obviously concerned about what's 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 delta and lower-than-hoped vaccine uptake played key roles in the latest iteration of the guidance.
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