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How common are breakthrough infections?

As the Delta variant continues to dominate COVID-19 infections in the U.S., so-called breakthrough cases have gained more attention. These stories are in the news, our social media feeds and in the minds of many of those who got inoculated. But how common are they?

Dr. Monica Gandhi, an infectious disease specialist and professor of medicine at the University of California, San Francisco, discusses the chances of getting a breakthrough infection, and who is more likely to be affected.

Video transcript

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ROCHELLE WALENSKY: [INAUDIBLE] on the Delta variant from several states and other countries indicate that, in rare occasions, some vaccinated people infected with the Delta variant after vaccination may be contagious and spread the virus to others. In areas with substantial and high transmission, CDC recommends fully vaccinated people wear masks in public indoor settings to help prevent the spread of the Delta variant and protect others.

MONICA GANDHI: So I think that, essentially, part of the reason they made that decision is because in a Provincetown outbreak. And this was about 60,000 people that descended on Provincetown at a time that we didn't need to wear masks inside, happened to be raining that weekend, really high kind of stress environment for the vaccines, lots of inside time, no masks, closed windows, mixed population, some unvaccinated, and really a lot of intimacy as well. All of that put together, there was a high rate of symptomatic breakthroughs in that outbreak study. About 3/4 of people had some symptoms.

But luckily, what we really need to do is quantitate breakthrough infections under more normal circumstances, not a high stress situation. Just what are people doing every day? And really what the data gathered from Virginia and from Washington state and from Utah, putting it all together, the real risk is about one in 5,000. In more high incidence regions, in places like San Francisco, where I live, lower event, so probably even lower in Vermont about one in 10,000.

The question is, how much can you spread if you're vaccinated? And I think there's two ways to think about it, If you have symptoms and you don't have symptoms. So if you have symptoms and you have like a cold or a flu or mild symptomatic breakthrough, can you spread? Well, the Provincetown study was just one study, one point in time, looked like the viral load was the same in vaccinated and unvaccinated. But again, it didn't do serial viral loads.

There was a study from Singapore that showed us that among Delta symptomatic breakthroughs, for one point in time, the viral load may be the same. But then the viral load comes down very quickly in vaccinated individuals, as compared to unvaccinated that it takes a while. And that makes sense because your immune system is going to fight that virus, bring down the viral load. So you're likely not as transmissible.

There are some people who have had severe breakthroughs and have needed to be hospitalized after vaccination. The CDC actually does track this data. And one thing a lot of people are asking the CDC to do is please tell us all the characteristics of those severe breakthroughs because those are likely the people who are going to need a third shot.

Immunocompromised is definitely going to be on the list. I think it's really important for the CDC to give us the demographic data of those severe breakthroughs. And I actually think our messaging for the last month has been really not positive. And really, what's going on is that vaccinated people are really very protected from severe disease, really high, over 96% in many studies, and that this risk of these mild breakthroughs depends on how much community transmission is in your area but is really quite low if you're just doing your normal things. And so one in 10,000.

And so because of that, I think we may have been giving a message that sounds like the vaccines aren't working very well, which, to me, could scare the vaccinated. And actually doesn't make the unvaccinated think they should get a vaccine. So I have been, yes, very upset by the messaging and think we need to be way more positive.