Vaccine Breakthrough Cases Rising With Delta: Here's What That Means

Brenda Goodman, MA

July 23, 2021

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

At a recent town hall meeting in Cincinnati, Ohio, President Joe Biden was asked about COVID-19 cases, hospitalizations, and deaths rising in response to the Delta variant.

Touting the importance of vaccination, "We have a pandemic for those who haven't gotten a vaccination. It's that basic, that simple," Biden said at the event, which was broadcast live on CNN.

"If you're vaccinated, you're not going to be hospitalized, not going to the ICU unit, and not going to die," he said, adding "you're not going to get COVID if you have these vaccinations."

Unfortunately, it's not so simple. Fully vaccinated people continue to be well protected against severe disease and death, even with Delta, but so-called breakthrough cases, where a person gets infected despite being fully vaccinated, are on the rise. Because of that, many experts continue to advise caution, even if fully vaccinated.

"I was disappointed," Leana Wen, MD, MSc, an emergency physician and visiting professor of health policy and management at George Washington University's Milken School of Public Health in Washington, DC, told CNN in response to the president's statement.

"I actually thought he was answering questions as if it were a month ago. He's not really meeting the realities of what's happening on the ground," she said. "I think he may have led people astray."

Vaccines Still Work

Recent cases support Wen's claim. Fully vaccinated Olympic athletes, wedding guests, healthcare workers, and even White House staff have recently tested positive. So what gives?

The vast majority of these illnesses are mild, and public health officials say they are to be expected. 

"The vaccines were designed to keep us out of the hospital and to keep us from dying. That was the whole purpose of the vaccine and they're even more successful than we anticipated," says William Schaffner, MD, an infectious disease expert at Vanderbilt University in Nashville, Tennessee.

As good as they are, these shots aren't perfect. Their protection differs from person to person depending on age and underlying health. People with immune function that's weakened because of age or a health condition can still become seriously ill, and, in very rare cases, die after vaccination.

When people are infected with Delta, they carry approximately 1000 times more virus compared with previous versions of the virus, according to a recent study. All that virus can overwhelm even the strong protection from the vaccines.

"Three months ago, breakthroughs didn't occur nearly at this rate because there was just so much less virus exposure in the community," said Michael Osterholm, PhD, director of the Center for Infectious Disease Research and Policy at the University of Minnesota in Minneapolis.

Breakthroughs by the Numbers

In Los Angeles County, where 69% of residents over age 12 have been fully vaccinated, COVID-19 cases are rising, and so, too, are cases that break through the protection of the vaccine.

In June, fully vaccinated people accounted for 20%, or 1 in 5, COVID cases in the county, which is the most populous in the United States. The increase mirrors Delta's rise. The proportion of breakthrough cases is up from 11% in May, 5% in April, and 2% in March, according to the Los Angeles County Department of Public Health. 

In the United Kingdom, which is collecting the best information on infections caused by variants, the estimated effectiveness of the vaccines to prevent an illness that causes symptoms dropped by about 10 points against Delta compared with Alpha (or B.1.1.7). 

After two doses, vaccines prevent symptomatic infection about 79% of the time against Delta, according to data compiled by Public Health England. They are still highly effective at preventing hospitalization, 96% after two doses.

Out of 229,218 COVID infections in the United Kingdom between February and July 19, 28,773 — or 12.5% — were in fully vaccinated people. Of those breakthrough infections, 1101, or 3.8%, required a visit to an emergency room, according to Public Health England. Just 474, or 2.9%, of fully vaccinated people required hospital admission, and 229, or less than 1%, died.

Unanswered Questions

One of the biggest questions about breakthrough cases is how often people who have it may pass the virus to others.

"We know the vaccine reduces the likelihood of carrying the virus and the amount of virus you would carry," Wen told CNN. But we don't yet know whether a vaccinated person with a breakthrough infection may still be contagious to others.

For that reason, the Centers for Disease Control and Prevention says that fully vaccinated people still need to be tested if they have symptoms and shouldn't be out in public for at least 10 days after a positive test.         

How should fully vaccinated people behave? That depends a lot on their underlying health and whether or not they have vulnerable people around them. 

If you're older or immunocompromised, Schaffner recommends what he calls the "belt-and-suspenders approach," in other words, do everything you can to stay safe.

"Get vaccinated for sure, but since we can't be absolutely certain that the vaccines are going to be optimally protective and you are particularly susceptible to serious disease, you would be well advised to adopt at least one and perhaps more of the other mitigation measures," he said.

These include wearing a mask, social distancing, making sure your spaces are well ventilated, and not spending prolonged periods of time indoors in crowded places.

Taking young children to visit vaccinated, elderly grandparents demands extra caution, again, with Delta circulating, particularly as they go back to school and start mixing with other kids.

Schaffner recommends explaining the ground rules before the visit: Hugs around the waist. No kissing. Wearing a mask while indoors with them.

Other important unanswered questions are whether breakthrough infections can lead to prolonged symptoms, or "long covid." Most experts think that's less likely in vaccinated people.

And Osterholm said it will be important to see whether there's anything unusual about the breakthrough cases happening in the community.

"I think some of us have been challenged by the number of clusters that we've seen," he said. "I think that really needs to be examined more."

Follow Brenda on Twitter: @ReporterGoodman.

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