ARI SHAPIRO, HOST:
Just how well are vaccinated people protected from getting sick with COVID-19? We know that the vaccine does a good job preventing hospitalization and death. And yet at the same time, a recent CDC report found that hundreds of vaccinated people were infected with the coronavirus on Cape Cod, Mass., at the start of last month. To help us understand how common and how serious breakthrough infections are, NPR science correspondent Joe Palca is here.
JOE PALCA, BYLINE: Hi, Ari.
SHAPIRO: Health officials keep telling us the evidence shows the vaccine works. At the same time, this outbreak on Cape Cod led to new guidance about vaccinated people wearing masks. So what exactly did scientists learn from that outbreak in Provincetown, Mass.?
PALCA: Well, if they needed a reminder that the vaccine wasn't 100% effective, they got one. But National Institutes of Health Director Francis Collins says most of the time you won't get infected. Here he is speaking this morning on Morning Edition.
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FRANCIS COLLINS: If you do get infected as a vaccinated person, the chances are extremely good that you're going to have either no symptoms or mild symptoms like a cold. So yeah, I am a little worried that - as there's been a lot of buzz about breakthrough infections, that people begin to think, oh, maybe the vaccines aren't really protecting me. Oh, yes, they are.
SHAPIRO: You say most of the time you won't get infected, so explain how all these breakthrough infections happened.
PALCA: Well, you have to put that number in - 346 - in perspective. I was in touch with Dr. Collins earlier today. Those 346 cases were mostly among people attending large parties and other crowded public gatherings. And by some estimates, there were nearly 250,000 people at these events. So suddenly, 346 out of 250,000 doesn't sound like all that much. And the other thing you have to keep in mind is that only four of those 346 were hospitalized, and none died. So the vaccine is doing its job. It's keeping people from getting so sick that they need to go into the hospital. And if they do wind up in the hospital, it's keeping them from dying.
SHAPIRO: This CDC report says three-quarters of the cases that were documented were among vaccinated people. I mean, how does that figure if the vaccine is working?
PALCA: Well, in a strange way, Ari, that's good news. It suggests that most of the people at these gatherings were vaccinated. Think about it. Health officials know the vaccine isn't perfect, so some people will get infected. But if most people are already infected and the virus is circulating and if you do the math, it's quite possible for more people who are vaccinated to get infected. I mean, think of it this way. If everyone were vaccinated and 346 people got infected, then you'd say 100% of the people who got infected were vaccinated. But that wouldn't mean the vaccine wasn't working.
SHAPIRO: So tell us how this leads to the new CDC guidance on masks for vaccinated people.
PALCA: Well, it is true that a fully vaccinated person can have high levels of virus in their nose and throat, but they still will most likely not become very sick. Now, one reason for that is the nature of these vaccines. The Moderna, Pfizer and Johnson & Johnson vaccines produce what's called systemic immunity. So interior organs in your body, like the lungs and the heart and the kidney, especially the lungs, they're not going to be infected. So you don't wind up with breathing difficulties and on a respirator.
But these vaccines don't produce a strong immune response where the virus enters the body. This is the nose and throat, so you can have a localized infection there. And depending on the individual, you may not even notice that you have an infection, but there still may be enough virus there to spread to someone else just by breathing on them. And that's one of the insidious things about the coronavirus that causes COVID-19. People with no symptoms can spread the disease.
SHAPIRO: That's NPR science correspondent Joe Palca explaining breakthrough infections.
Thank you so much.
PALCA: You're very welcome.
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