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Vaccine breakthrough cases rising with Delta: Here’s what that means


At a recent town hall meeting in Cincinnati, 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,” President Biden said at the event, which was broadcast live on CNN.

delta variant of coronavirus Andriy Onufriyenko

“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, 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 Dr. 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.

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 1,000 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, 1,101, 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.


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