75 Percent of Vaccinated People Who Get Severe COVID Have This in Common

Serious breakthrough infections may be more likely to occur among certain groups.

People who are fully vaccinated are the most protected against COVID, but that doesn't mean there isn't any risk. Breakthrough infections are being reported more and more, as overall infections in the U.S. have increased due to the Delta variant—and while most of these breakthrough cases have been mild, there have been a handful of serious cases. The Centers for Disease Control and Prevention (CDC) warned early on that no vaccine is 100 percent effective, and that a very small number of vaccinated individuals could still develop a COVID infection requiring medical care. According to data from the agency, however, the risk is not the same for everyone. The CDC has found that there is one similarity that can be seen among 3 in 4 vaccinated people who do end up developing severe COVID.

RELATED: 40 Percent of People Who Get Severe COVID After Pfizer Have This in Common.

As of July 12, there have been nearly 5,500 hospitalized or fatal vaccine breakthrough cases reported to the CDC out of more than 159 million fully vaccinated individuals. "Vaccine breakthrough cases are expected," the CDC notes. "No vaccines are 100 percent effective at preventing illness in vaccinated people. There will be a small percentage of fully vaccinated people who still get sick, are hospitalized, or die from COVID-19."

But according to the CDC's data, people who experience these severe breakthrough infections tend to have one thing in common: They're older adults. Fully vaccinated people 65 years and older account for 75 percent of breakthrough COVID cases that lead to hospitalization or death.

"Throughout the pandemic, people who died of COVID-19 were most likely to be older, and that continues to be true with breakthrough cases," a spokesperson for the Massachusetts Department of Public Health told NBC News.

As we get older, our immune system gets weaker and we are more at risk for a number of medical conditions. A July 6 study published in the Clinical Microbiology and Infection journal found that out of 152 severe breakthrough infections in Israel, only 6 percent had occurred in people with no underlying health conditions or weakened immune systems. Instead, the majority of those who got severe COVID after vaccination had multiple comorbidities, such as hypertension, diabetes, and heart failure. And 40 percent were immunocompromised.

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Severe breakthrough infections are still rare, however. The CDC has stopped collecting data on all breakthrough infections, but a recent analysis by NBC News of data available for the 27 states that continue tracking these cases found that while 65,000 cases of COVID after full vaccination have been reported, only around 5,500 of those cases have been severe.

The most serious risk still lies with those who are not vaccinated. During a July 16 White House briefing, CDC director Rochelle Walensky, MD, confirmed that more than 97 percent of people currently hospitalized for COVID were unvaccinated individuals. And older vaccinated adults are also much more protected than older unvaccinated adults. A CDC study from April found that fully vaccinated adults 65 and older were 94 percent less likely to be hospitalized for COVID than those of the same age who were not yet vaccinated.

"COVID-19 vaccines are highly effective and these real-world findings confirm the benefits seen in clinical trials, preventing hospitalizations among those most vulnerable," Walensky said in a statement at the time.

RELATED: Vaccinated People Who Get the Delta Variant Have This in Common, WHO Says.

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