COVID-19 vaccines are highly effective but don't always provide perfect protection. Some vaccinated people later exposed to the virus still get sick. Why and how often that happens is under study.

Transcript

STEVE INSKEEP, HOST:

Millions of people are getting vaccinated against COVID-19 every day in the United States now. Those vaccines are extremely effective, but not effective for every single person every single time. When you vaccinate millions, some still get sick, and hundreds of vaccinated people have been infected. Here's NPR health correspondent Rob Stein.

ROB STEIN, BYLINE: Ginger Eatman thought she was good to go after getting her second Pfizer shot in February, but she kept being careful anyway - wearing her mask, using hand sanitizer, wiping down the carts at the grocery store. But a few weeks later, after she was considered fully immunized, she noticed a scratchy throat.

GINGER EATMAN: By Wednesday morning, St. Patrick's Day, I was sick. I had congestion, a lot of congestion, and some coughing.

STEIN: Her doctor thought it might be allergies. But then Eatman, who's 73 and lives about an hour outside of Atlanta, started feeling sicker and then suddenly lost her sense of smell. So she got tested. It came back positive.

EATMAN: I was shocked. I almost cried. It's like, no, that can't be.

STEIN: Eatman is far from alone in this experience. It's a phenomenon known as vaccine breakthrough.

SAAD OMER: Essentially, these are cases that you see amongst vaccinated individuals during a period in which you expect the vaccines to work.

STEIN: Saad Omer is a vaccine researcher at Yale. He says vaccine breakthroughs happen with every vaccine. The COVID-19 vaccines are at least 80% effective at preventing infections and at least 90% effective at preventing severe disease. But that's not 100%, so it's no big surprise some people still catch the virus.

OMER: So the bottom line is it's expected, no need to freak out.

STEIN: So far, more than 72 million people have gotten fully vaccinated in the United States. It's not clear how many people have gotten infected anyway, but Michigan, Washington and other states have reported hundreds of cases. Most have only gotten mildly ill, but some have gotten very sick, and some have even died. But at a recent White House briefing, Dr. Anthony Fauci said such cases appear to be extremely rare, and the deaths seem to be happening primarily among frail, elderly people with other health problems.

(SOUNDBITE OF ARCHIVED RECORDING)

ANTHONY FAUCI: There's nothing there yet that's a red flag. We obviously are going to keep an eye out on that very, very carefully. But I don't see anything that changes our concept of the vaccine and its efficacy.

STEIN: And definitely no reason for anyone not to get vaccinated. But it is a reminder why it's important for people to continue being careful after getting vaccinated - because there's no guarantees, and scientists are trying to figure out why it happens. Dr. Alexander Greninger at the University of Washington is studying people who experience breakthrough infections.

ALEXANDER GRENINGER: Right now we don't have a great understanding of exactly, you know, where we're open, our sort of Achilles' heel. It's a little dramatic, but, like, you know, you want to get a well-rounded immune response.

STEIN: So is it how much virus someone is exposed to, one of the variants that can evade the immune system?

GRENINGER: Is it people in the lower part of the vaccine response mixing with the variants? I'd say it's a little bit more, honestly, mysterious.

STEIN: Solving that mystery could help scientists improve the vaccines to prevent more breakthrough infections. For her part, Ginger Eatman is still glad she got vaccinated. She recovered after about 10 days, is back singing with the Georgia Festival Chorus, but she has friends who weren't nearly as lucky. They didn't get vaccinated.

EATMAN: We've had people at our church, a couple of them - one that, once she got COVID, it took her fast, and then another lady, just a little bit older than I am, wound up in the hospital, had the pneumonia, in ICU, out of ICU, back in ICU, and she went home to be with the Lord.

STEIN: So Eatman is urging her friends to get vaccinated, even if the vaccines aren't perfect.

Rob Stein, NPR News.

(SOUNDBITE OF YEARS OF RICE AND SALT'S "ESKIMO KISS") Transcript provided by NPR, Copyright NPR.

Tags: COVID-19 vaccine  coronavirus pandemic  Treatments  Series: The Coronavirus Crisis  Coronavirus  COVID 19