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There have been a number of recent reports of fully vaccinated people testing positive for the coronavirus — at the 

. And with the fast-spreading delta variant driving up infections, hospitalizations and deaths, a lot of people are wondering whether the vaccines are as protective as we thought.

 doesn't surprise or alarm public health experts. "I haven't seen any signals in the U.S. indicating that they are occurring at the levels that would give us concern that the [vaccine] effectiveness is going down," said 

, director of the Yale Institute for Global Health. 

Breakthrough infections, the delta variant and more

In all of these settings, from Capitol Hill to professional sports, nearly everyone is vaccinated, he points out, and people are subject to frequent, asymptomatic testing. "It is not surprising to see a substantial number of cases in the vaccinated when the vaccination rate is high," he said.

Still, people who've been vaccinated may rightly be asking: Are breakthrough cases becoming more common because of the delta variant? Could I get sick or get a family member sick?

Here's what to know about breakthrough cases in the context of delta, and what scientists are doing to track the vaccines' efficacy:

You can get COVID-19 even if you're vaccinated, but it's rare and likely to be mild

Bottom line: Don't panic. So far, research shows the current vaccines are holding up well against the delta variant. For instance, a June 

 found that the Pfizer vaccine is 96 percent effective against hospitalization from the delta variant after two doses.

If you do get infected (which is not likely but possible), the vaccine should help you keep from getting seriously sick. "Breakthrough infections, they tend to be mild — they tend to be more like a cold," said 

, professor of medicine and infectious disease epidemiology at Emory University.

 three weeks ago, are accompanied by lower viral loads and less — much less — symptoms," added Dr. 

, infectious disease physician at the University of California, San Francisco. In particular, the study "showed that if you get a mild breakthrough with any variant, you have a 40 percent lower viral load in your nose after vaccination than you do if you had a natural infection," she said.

Severe cases among vaccinated people are possible, but extremely rare — the vaccines dramatically reduce the risk of serious illness that leads to hospitalization or death. And 97 percent of those currently hospitalized with COVID-19 are unvaccinated, 

, director of the Centers for Disease Control and Prevention.

For context, as of July 12, out of 159 million fully vaccinated people, the 

 5,492 cases of fully vaccinated people who were hospitalized or died from COVID-19, and 75 percent of them were over age 65. It's not clear how many of these breakthrough infections were caused by the delta variant, but that's now by far 

The chances of getting seriously ill after being vaccinated are higher for those with 

 president and CEO of Houston Methodist, said that at his hospital, 90 percent of the patients with COVID-19 are unvaccinated. The small percentage of vaccinated patients who do end up hospitalized, he said, "have underlying significant health risks — like cancer, like [organ] transplants — that probably prevented them from mounting a full immune response to the vaccine."

Some argue asymptomatic breakthrough cases shouldn't even count

"I think we are misusing the term breakthrough," said Dr. 

, director of the Vaccine Education Center at Children's Hospital of Philadelphia. "If someone who is fully vaccinated is subsequently hospitalized or killed by the virus, that's a breakthrough case." He said he wouldn't call an "asymptomatic or relatively mild case" a "breakthrough case."

What matters, he said, is "the vaccine is still doing what it is designed to do — keep people out of the hospital and out of the morgue."

 of asymptomatic vaccinated people for the coronavirus. As Gandhi of UC San Francisco explains, positive tests might just be picking up "dead viral particles in your nose," she said. "Vaccinated people may get it in their nose, but they're going to kill it — that's actually what the immune system does."

CDC can't track all breakthroughs, but it's using modeling to estimate the risk  

There's no exact number of breakthrough cases, for several reasons. First of all, there would be no way to count the asymptomatic breakthrough cases, because the U.S. isn't testing nearly enough to catch them all. And in fact, the CDC stopped keeping a 

running tally of mild breakthrough cases in May

But that's not necessarily a problem, said 

, epidemiology professor at the University of Michigan. "You don't want to test everybody — you don't need every positive to be identified," she said. "You just need to understand how the positives that you're finding represent the whole pie that you're not seeing." 

The CDC runs carefully designed surveillance systems (which try to find pie slices that are representative) and does burden estimation (calculations you use to fill in the rest of the pie). It's a process that's already done every year with the flu, Martin said, to assess how well the flu shot is working when the vast majority of people who get the flu are never tested.

Now, the CDC is doing the same thing to monitor COVID-19. Hospitals and health departments are sending the CDC detailed information about certain cases of vaccinated patients who are hospitalized with COVID-19. 

"We'd like to have a sample of the virus so that we can understand the viral load, so that we can sequence it, we can understand their symptoms and their risks that potentially put them in that situation," the CDC's Walensky told a 

At the same time, the agency is doing ongoing 

 at long-term care facilities, academic medical centers, hospitals, and among health care and essential workers. "We're doing many of those studies across the nation," Walensky said, explaining that these allow the CDC to get an accurate picture of the real rate of breakthroughs among vaccinated people and whether that's changing over time.

"That is a more systematic way of measuring breakthrough cases," Martin said. Realistically, health officials can't count every case, so this careful, long-term surveillance, along with calculations to extrapolate those findings, gives a fuller picture of how much virus is really out in the community. 

The number of breakthrough cases is growing, and that's expected

There's basic arithmetic at play: As more people get vaccinated, even if breakthroughs are rare, a rising number of cases will be among the vaccinated.

"Even with a 95 percent efficacious vaccine, you will have one in 20 vaccinees who are exposed get the disease," said Dr. 

, director of the Center for Vaccine Development and Global Health at the University of Maryland. The important thing to note is that "the overwhelming number [of cases] are among the unvaccinated," she said.

Certainly, with the delta variant taking over — which spreads about 

 than the original strain — there will be more cases among everyone, vaccinated and unvaccinated, said 

, professor of microbiology and immunology at Weill Cornell Medicine. 

"But its ability to infect fully vaccinated people is much less than those who are not vaccinated," he said. "In other words, the vaccines still work, just a bit less well."

Epidemiologist Martin added that there's no clear data yet to show that delta is to blame for a big rise in breakthrough cases in the United States. "We are still watching carefully though," she added.

It's not known yet whether you can spread the coronavirus if you get a breakthrough infection

In general, people who are vaccinated and get infected have less virus. "That lower viral load makes it less likely for you to transmit — not impossible by any means but less likely," Gandhi said. 

Dr. Anthony Fauci, chief medical adviser on COVID-19 for President Joe Biden and the nation's top infectious disease expert, 

 that research on "whether or not the transmission occurs is a large study that's ongoing right now," referring to 

, a study of transmission among about 12,000 college students vaccinated with the Moderna vaccine.

While there aren't yet results from that study, Fauci added, the lower viral load in breakthrough cases suggests "it would be less likely that that vaccinated breakthrough person would transmit, compared to an unvaccinated person."

, a growing chorus of health experts is recommending using masking and other mitigation strategies to prevent spread just in case. "I think that the prudent thing at this point in time — and what I'm doing personally," Emory's del Rio said, "if I'm going to a place that it's crowded, where there are multiple individuals, including people who are likely not vaccinated, I am wearing a mask." 

Is it time to mask up again?

If you do get sick, vaccines likely lessen the chance of getting long COVID-19

"We have not seen reports yet of breakthrough cases causing long COVID, thank goodness," said Gandhi of UC San Francisco. One reason for that could be that 

 involves "a very dysregulated inflammatory response that you get with natural infection," she said. "What you get with an infection after you've been vaccinated, it's what we call a more regulated response or adaptive immunity."

 of the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada noted the data on this is limited so we can't completely rule it out.

"The safest thing to do is to avoid being infected altogether, and that's why I think it is appropriate for people to employ additional mitigation measures," she said. 

That might mean — in addition to vaccination — breaking out masks again, reupping good hand hygiene and getting more air circulating when gathering indoors. "The mitigation measures that we have put in place previously will still work against the delta variant — it's not being transmitted by some other route," she added.

NPR's Allison Aubrey contributed to this report.

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COVID-19

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