While getting vaxxed remains the best protection against severe COVID-19, infections are still possible, and new data suggest that older and sicker adults are particularly vulnerable.

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You got vaccinated, and for a moment in time it appeared the nation might be turning the corner on COVID-19. But lots of people hesitated or flatly refused to get their jabs, giving the virus plenty of opportunity to spread and mutate. And now the super-infectious Delta variant is running amok.

While vaccination remains largely protective against severe disease, Delta is proving to be much more challenging than other iterations of the SARS-CoV-2 virus, which causes COVID-19. A CDC internal memo obtained by the Washington Post suggests it's even more transmissible than chickenpox, the news organization reported in late July.

Not only are fully vaccinated people capable of becoming infected—developing so-called "breakthrough" cases (usually mild or asymptomatic)—but now there's evidence that even the vaccinated can pass the virus on to others, something that hadn't been seen until Delta came along.

As long as COVID is circulating in the community, infections in fully vaccinated people "cannot be completely eliminated," says the US Centers for Disease Control and Prevention (CDC).

So what does it mean to get a breakthrough case of COVID, who's at risk, and what should fully vaccinated people do it they happen to develop symptoms? We asked experts in public health and infectious disease to help people make sense of it all. Here's what we know so far.

What is a breakthrough infection?

By the CDC's definition, a breakthrough infection is a COVID case that occurs in someone who is fully vaccinated, meaning 14 or more days after completing the recommended doses of an authorized vaccine. (Remember, it takes time for your immune system to build protective antibodies against COVID, so you're not fully vaccinated until two weeks after you've finished your vaccine regimen.)

Let's say you were fully vaxxed up by late April, but you test positive for COVID in August. That clearly meets the definition of breakthrough.

But Jaimie Meyer, MD, an infectious disease physician at Yale Medicine, doesn't like calling it that. "It makes it sound like the vaccine somehow failed, and it didn't," she tells Health.

We've known all along that the vaccines were highly protective against severe disease and somewhat less effective against symptomatic infections, "so no one ever said this vaccine was a hundred percent protective against infection at all," says Dr. Meyer, who prefers to use the term "post-vaccination infections."

As in many places across the country, 99% of people being hospitalized for COVID in Rochester, New York, are unvaccinated, says Angela Branche, MD, co-director of the University of Rochester Medical Center Vaccine Treatment and Evaluation Unit. "One of the good things about vaccines and being vaccinated is that you can feel confident that you are very protected against getting severely ill, being hospitalized or dying from COVID," she tells Health.

How common are breakthrough infections?

That's something public health officials continue to grapple with. As of August 30, CDC reports that 49 states and territories have reported nearly 13,000 breakthrough cases resulting in hospitalization or death. But the CDC acknowledges that number is likely an undercount of all COVID infections, especially mild or asymptomatic cases, among people who've been fully vaccinated,

In a media briefing in July, CDC Director Rochelle Walensky, MD, said the agency is studying breakthrough infections in more than 20 different "cohorts," or groups of people, across many different states involving tens of thousands of people. In some cohorts, the agency is collecting PCR (COVID test) data from every person each week.

The CDC estimates that getting vaccinated reduces the risk of a breakthrough infection with symptoms from the Delta variant by sevenfold, Dr. Walensky noted.  "And the reduction is twentyfold for hospitalizations and deaths."

Who is likely to get a breakthrough infection?

The data we have so far suggest the worst outcomes are among older folks and people with other health conditions.

Of the nearly 13,000 breakthrough infections recorded though August, CDC reports that 87% of cases ending in death were among adults 65 or older. Among breakthrough infections resulting in hospitalization, 70% were in the 65-and-older demographic.

Separately, a pre-print study from the CDC examines characteristics of vaccinated and unvaccinated people who landed in the hospital with COVID between January and June. Among a sample of more than 4,700 cases, the median age of fully vaccinated folks was 73 (versus 59 among the unvaxxed). The people with breakthrough infections were also more likely to be sicker—having three or more underlying health conditions.

Why are we suddenly seeing more breakthrough infections?

Delta isn't the first variant to make waves in the US. The CDC monitors multiple variants of SARS-CoV-2. Four of them—Alpha, Beta, Gamma, and Delta—are classified as "variants of concern," meaning they pose an increased risk of transmission and severe disease. Delta is now the dominant strain in the US, accounting for 99% of sequenced cases, per CDC data.

But in the spring, Alpha led the way, and the belief was if you were vaccinated, you couldn't transmit it further, Dr. Walensky told reporters during the July briefing. Not so with Delta. "We're seeing now that it's actually possible, if you're a rare breakthrough infection, that you can transmit further."

Data published in July demonstrated just how transmissible the virus appears to be. A total of 469 COVID cases were identified among Massachusetts residents who traveled to Barnstable County for summer events and large public gatherings. With a 69% vaccination rate at the time, Massachusetts has been among the more highly vaccinated states. Yet 74% of cases occurred in people who were fully vaccinated with either two doses of Pfizer or Moderna or the single-dose Johnson & Johnson vaccine, according to the report in CDC's Morbidity and Mortality Weekly Report.

Genomic testing involving specimens from 133 patients identified Delta as the culprit in 90% of cases, according to the report. Overall, 79% of vaccinated patients with breakthrough infections had symptoms. Of five COVID patients who were hospitalized, four were fully vaccinated. But no deaths were reported.

A small, pre-print study from China also added to worries about Delta. Researchers followed 62 people who were quarantined after exposure to the Delta strain and tested their "viral load" daily to see how it changed, according to an article in Nature. Then they compared their infections to 63 people who were infected early in the 2020 pandemic. According to the study, made public prior to undergoing peer review, people with Delta had viral loads of up to 1,260 times higher than those with the original strain, according to the study.

It shows that "people have extraordinarily high levels of virus from Delta variant in their upper airways," says Dr. Meyer. "And that's because Delta is so efficient at replicating."

What symptoms can you expect if you develop a breakthrough infection?

Here's the thing about COVID breakthrough infections: you might have mild symptoms, mimicking seasonal allergies or the common cold. Or you could be asymptomatic, so you wouldn't necessarily know you're infected unless you get tested.

"Many of the post-vaccination infections have no symptoms at all," says Dr. Meyer, who is also an associate professor of medicine and public health at Yale. Others may experience fever, headache, or diarrhea, she says. You might even experience shortness of breath, "but not to the point that you're so sick that you would need to go to the hospital."

What should you do if you get a breakthrough infection?

First, keep in mind that vaccination remains the best weapon we have against the virus, to ward off severe illness and thwart further transmission and new mutations.

"The vaccine protects you," says Dr. Branche. "But because there's this possibility now of a mild or asymptomatic breakthrough infection, it means you have now the potential to pass the virus to other people in your community and other people in your home who may not be vaccinated," she says. You can be part of that "train of transmission" even though you're vaccinated.

Because of this possibility, the CDC recommends that even fully vaccinated folks mask up in indoor public spaces in areas where there is substantial or high COVID transmission. You might opt to wear a mask even if you're not in one of those hot spots, says the CDC, like if you are immune compromised or if you have contact with other people who are unvaccinated or at risk of severe illness.

Pamela Aaltonen, PhD, RN, professor emerita at Purdue University School of Nursing in West Lafayette, Indiana, and past president of the American Public Health Association, never disposed of her mask. She still wears one while grocery shopping. And with the Delta variant, "I'm even more vigilant," she tells Health.

If you happen to have been around someone who has COVID, the CDC recommends that you get tested three to five days after your exposure, even if you don't have symptoms. Mask up indoors in public areas for 14 days or until you get a negative test result.

Anyone who is fully vaccinated but develops symptoms is advised to get tested and stay home and away from others. And if you test positive? Isolate at home 10 days, says the CDC.

The recommendations may be a bitter pill to swallow for the 164 million Americans who are fully vaccinated. But people need to remain open to changing advice as we learn more about the virus and its variants, says Aaltonen.

"They sometimes forget that science is kind of like a journey. It's not that we come to a conclusion and it stays that way forever. Especially with the virus—it keeps producing variants."

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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