What Does the Unpublished Evidence Cited by the CDC’s New Face Mask Guidance Actually Show?
Two and a half months ago, when the Centers for Disease Control and Prevention (CDC) began telling people vaccinated against COVID-19 that they generally did not need to wear face masks in public places, CDC Director Rochelle Walensky said the new advice was based on two factors. First, vaccinated people were rarely infected by the coronavirus; second, such “breakthrough” infections typically involved relatively low viral loads, meaning that vaccinated carriers were less likely to transmit the virus.
When the CDC changed its advice again this week, telling vaccinated people to resume wearing masks if they live in “areas of substantial or high transmission,” Walensky alluded to evidence that casts doubt on the second point, at least as it relates to the especially contagious delta variant that now accounts for the vast majority of newly identified cases in the United States. But the CDC has not yet published that evidence, leaving unanswered questions about what it actually shows and its relevance to the role vaccinated people might be playing in the current case surge.
“Emerging evidence suggests that fully vaccinated persons who do become infected with the Delta variant are at risk for transmitting it to others,” the CDC says in a Morbidity and Mortality Weekly Report article published on Tuesday. The article cites “unpublished data” from the CDC COVID-19 Response Team. An unnamed “federal official knowledgeable about the research” told The Washington Post the results will be “published imminently.”
Walensky described the CDC’s reasoning in an interview with SiriusXM Doctor Radio on Tuesday. She noted that in May, the last time the CDC revised its mask advice, the alpha variant of the coronavirus accounted for most COVID-19 cases, while the delta variant was involved in “about 1 percent.” At that point, she said, “we had evidence both of the vaccine was working against alpha and preventing severe disease and death, but also that if you happen to be one of those very few breakthrough infections, there was very little evidence that you could actually give it to someone else.” Specifically, “the virus that would be circulating in your nasal pharynx was actually a very low level of virus.”
Today the delta variant accounts for about 83 percent of new U.S. cases. “We know and have known that the amount of virus in your nasal pharynx when you’re infected and unvaccinated with delta is much more than it was with alpha,” Walensky said. “Over the last several weeks,” she added, the CDC’s “outbreak investigations” in “many different places in the United States” have found that vaccinated people with breakthrough delta infections “have the same amount of virus as the unvaccinated people.” That evidence “is very much leading us to believe that it is probably the case that those vaccinated breakthrough infections, rare as they might be, have the potential to infect others.”
Walensky also alluded to data from other countries. “Studies from India with vaccines not authorized for use in the United States have noted relatively high viral loads and larger cluster sizes associated with infections with Delta, regardless of vaccination status,” the CDC’s new guidance says. It cites a preprint study of breakthrough infections in vaccinated Indian health care workers that found “higher respiratory viral loads compared to nondelta infections.” That study described the “respiratory viral loads” as “significant” but did not say they were comparable to those seen in unvaccinated carriers. The senior author of that study, microbiologist Ravindra Gupta, told the Post the ability of vaccinated people with breakthrough infections to transmit the virus has not been “formally measured in a rigorous way,” although he agreed with the CDC’s new mask advice.
“They’re making a claim that people with delta who are vaccinated and unvaccinated have similar levels of viral load, but nobody knows what that means,” Gregg Gonsalves, an associate professor at the Yale School of Public Health, told the Post. “It’s meaningless unless we see the data.”
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