On Thursday night, the Washington Post resolved some of the mystery of why the Centers for Disease Control and Prevention had suddenly changed its mask guidance with little explanation. It turned out that an internal CDC document had indicated that the delta variant of COVID-19 was likely both more dangerous and more contagious than previously thought.
The document warned that there was evidence that vaccinated people who became sick from the delta variant shed as much of the virus as the unvaccinated. On Friday afternoon, the Post reported that a CDC study bore that out: Vaccinated people carried just as much virus as unvaccinated people. The study found that three-quarters of an outbreak in Provincetown, Massachusetts—an outbreak that began with July 4 celebrations—had been vaccinated.
“This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation,” the CDC director said in a statement.
To make sense of just how concerning this news is, Slate spoke with Dr. Larry Corey, a virologist at the Fred Hutchinson Cancer Research Center. This interview has been condensed and edited for clarity.
Slate: How surprising is this news?
Larry Corey: I would call it a teaching moment. And a learning moment that density is the fuel of the virus. Potentially density is the enemy of the human. And indoor crowding, especially celebrations in which you’re speaking, drinking, and unmasked, are still high-risk situations. There’s a threshold for everything.
How should people understand this news?
OK, we have an outbreak here. That reminds us that all vaccines, no matter how good they are, can be overpowered. There can be limitations. Having said that, it’s still an infrequent event. When we look at the data out of the United States, for the last month in which delta has been the overpowering variant, 99 percent of the hospitalizations were in unvaccinated persons.
Here we have some hospitalizations in which people were vaccinated. But having said that, they did well from a hospitalization perspective. And no one died. So, I do think that it is a sort of teaching moment that we’re not invulnerable, that delta is a formidable variant, and that high-risk indoor exposures can overpower vaccination. I don’t even look at it as a durability issue [with the vaccine], because it appears that the vaccination was within less than 100 days of the second dose for the vast majority of people.
So in terms of how vaccinated people should modify their behavior, you’re mainly saying to avoid crowds indoors?
To avoid indoor events with high density. I will say personally, when I’m going to the grocery store now, or indoor shopping malls, I’m wearing my mask.
What would you say to people who are worried less about their own personal safety and more about giving COVID to someone else?
The acquisition of asymptomatic infection is possible with vaccination. How frequently that occurs, how frequently you actually get infected and transmit it, we actually don’t know. We have a study or two in the field that may allow us to grab a handle on that, but that’s a couple months away. So I have to say, I don’t think we have the data to quantitate that. I think that keeping an eye on the ball, vaccines are meant to reduce the severity of disease and the medical complications of the disease. The vaccines against the initial variant even prevented you from getting ill at all at 94 percent. So, OK, with delta, that may drop into the 80s, to 85 percent. But that’s still 98 percent against hospitalization and death. We’ve got to understand that’s the goal. That was always the main goal of vaccination.
Would you say you have any other sort of big, outstanding questions?
I also say that at the moment I don’t think this outbreak says anything about booster shots, because even if we boosted, we wouldn’t know whether it would change any of this. Would it protect you from nasal colonization even better? We don’t actually know that at the moment. I don’t think this outbreak reflects upon that one way or the other.
How worried should people be right now?
I don’t think that the sky is falling. I think it is a moment to understand that you need to be prudent, even if you’re vaccinated. Vaccines work. They clearly do work, and the data from the United States continue to show that if you’re vaccinated, the likelihood of you getting hospitalized or dying from COVID goes down by 99 percent. But nothing is infallible. You take an enormous number of people and pack them into an indoor room, we can get transmission.