Don't call it a 'breakthrough infection'

The phrase became popular after COVID-19 vaccines became available, but it’s not accurate, says UCLA Health infectious diseases specialist Dr. Annabelle de St. Maurice.

The term “breakthrough infection” came into the lexicon after COVID-19 vaccines were introduced. As the virus mutated and new variants emerged, people who were fully vaccinated still sometimes got infected.

COVID-19 has continued to mutate, with omicron sub-variants proving particularly infectious, even in people who’ve received four shots.

Still, it’s not fair to call these cases “breakthrough infections,” says Annabelle de St. Maurice, MD, MPH, co-chief infection prevention officer for UCLA Health.

“There are some vaccines, like measles vaccine, that are really highly protective against infection. You’re unlikely to get infected with measles if you get the vaccine,” Dr. de St. Maurice says. “But for other respiratory viruses, like flu and COVID, we shouldn’t measure vaccine efficacy against infection, but against severe outcomes like hospitalization and death.”

The term “breakthrough infection” is misleading, she says, because it implies that the vaccine should provide total protection against all infections.

“I don’t think it’s a realistic outcome,” Dr. de St. Maurice says.

Instead, these cases should be described as “infection in a vaccinated individual,” she says.


While vaccination may not always prevent infection, it generally reduces severity of symptoms and protects against the worst outcomes. For example, in July, 2022 unvaccinated Californians were 3.6 times more likely to die from COVID-19 than people who received a primary series of shots, according to state health data. Nationally, during the same period, unvaccinated people were more than twice as likely to be infected with COVID-19 and had five times the risk of dying from the virus, according to the U.S. Centers for Disease Control and Prevention.

Dr. Annabelle de St. Maurice

Similarly, flu vaccines may not prevent infection with influenza. Studies show flu vaccines reduce the likelihood of doctor visits for flu by 40% to 60%, according to the CDC, especially when the vaccine is well-matched to circulating strains of the virus.

Notably, the term “breakthrough infection” hasn’t been used when people vaccinated against the flu contract the virus.

Most importantly, though, the flu vaccine offers significant protection against hospitalization and serious illness. And for children, flu vaccination reduces the risk of life-threatening influenza by 75%, according to a recent study.

“Those are the things that really matter long term,” Dr. de St. Maurice says. “I’ve had flu in the past after getting the vaccine and I had some bad symptoms, but I kept thinking, ‘Oh my gosh, it could have been so much worse.’”

Get a flu shot to protect yourself against serious illness.