Why breakthrough COVID-19 infections don’t mean the vaccine isn’t working

July 21, 2021

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(WASHINGTON) — With more than 161 million people now fully vaccinated in the U.S., experts say we are bound to see reports of breakthrough infections, meaning people will test positive for COVID-19 despite being fully vaccinated.

These breakthrough COVID-19 cases aren’t proof the vaccines aren’t working, experts said, but are normal and expected. All evidence suggests that even in the face of the new, highly-transmissible delta variant, COVID vaccines are still working as they should to dramatically decrease the risk of hospitalization and death.

“When you hear about a breakthrough infection, that doesn’t necessarily mean the vaccine is failing,” Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, said before Congress on Tuesday.

COVID-19 vaccines are highly effective, but they do not block the virus 100% of the time, meaning that some breakthrough infections occur after vaccination.

“I think people need to appreciate when you talk about breakthrough infections that the original data from the clinical trial — the efficacy data was based on preventing clinically apparent disease, not preventing infection, such as a symptomatic infection,” Fauci said.

Despite many high-profile cases of breakthrough infections with mild or no symptoms, including among Olympic athletes and some politicians, the overall number is very low compared to the number of people vaccinated.

And the number of people who have been hospitalized or died after being fully vaccinated is even lower, according to the U.S Centers for Disease Control and Prevention (CDC) and state health departments. This demonstrates that vaccinated people are far less likely to die of COVID-19 compared to unvaccinated people.

That doesn’t mean severe illness as the result of an infection isn’t possible, but this tends to happen in people who are elderly or otherwise immune-compromised, experts said.

“Out of 157 million fully vaccinated in the US, there were 4,909 hospitalizations and 988 deaths,” Dr. Carlos del Rio, infectious disease physician and professor of Medicine in the Division of Infectious Diseases at Emory University School of Medicine, said during a press briefing on Monday.

“Of course we will see some breakthrough infections that lead to severe illness, more in vulnerable populations with underlying chronic conditions who couldn’t mount a response to vaccines because they couldn’t,” Dr. John Brownstein, an infectious disease epidemiologist at Boston’s Children’s Hospital and an ABC News contributor, told ABC News.

Although studies on this aren’t completed, Fauci said last week that the risk of a vaccinated person spreading COVID to someone else is assuredly far less than an unvaccinated person spreading COVID.

“You could make a reasonable assumption that the rate of transmissibility from the asymptomatic vaccinated person to an uninfected person would be less likely than if the person was unvaccinated,” Fauci said at a White House COVID-19 Response Team briefing last week.

The overall number of breakthrough infections is rising, but that could be because more people overall are getting vaccinated, resulting in more breakthrough cases, Dr. Shobha Swaminathan, an associate professor and infectious disease expert at the Rutgers New Jersey Medical School, told ABC News.

“As the number of infections in the U.S. increases, there may be a slight increase in the number of ‘breakthrough’ infections,” Swaminathan said. “However, the majority of infections continue to be reported among those who have not been vaccinated.”

Experts said the delta variant could be contributing to these cases, but for now, research is ongoing.

“If it causes an increased rate of breakthrough infections, that’s unknown,” adds Brownstein.

But experts feel reassured by what they do know, that even with the highly-transmissible delta variant sweeping the country, more than 99% of COVID-19 deaths are among people who are unvaccinated.

Alexis E. Carrington, M.D. is an ABC News Medical Unit Associate Producer and a rising dermatology resident at George Washington University. Sony Salzman is a Coordinating Producer for ABC News Medical Unit.