CDC Shares ‘Pivotal Discovery’ on Covid-19 Breakthrough Infections that led to New Mask Guidance

A new study shows the Delta Covid-19 variant produced similar amounts of virus in vaccinated and unvaccinated people if they get infected — illustrating a key motivation behind the federal guidance that now recommends most fully vaccinated Americans wear masks indoors.

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.

CDC document warns Delta variant appears to spread as easily as chickenpox and cause more severe infectionCDC document warns Delta variant appears to spread as easily as chickenpox and cause more severe infectionThe 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.

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.

In a new report, the CDC shares details from a Covid-19 outbreak around Provincetown, Massachusetts, shown here in 2020.In a new report, the CDC shares details from a Covid-19 outbreak around Provincetown, Massachusetts, shown here in 2020.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.What's different about the Delta variant? Here's what's knownWhat’s different about the Delta variant? Here’s what’s knownOn 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.How does the new CDC mask guidance change how families stay safe from Covid-19? An expert's takeHow does the new CDC mask guidance change how families stay safe from Covid-19? An expert’s takeGandhi 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.