Tech & Science dictionary

breakthrough case

[ breyk-throo keys ]

What does breakthrough case mean?

A breakthrough case is an instance in which a person becomes sick with a disease despite having received the vaccine for that disease. In other words, the vaccine fails to prevent the person from becoming infected.

To be clear: In breakthrough cases (and in general), the infection doesn’t come from the vaccine—the person becomes infected (through one of the usual routes) despite the presence of the vaccine in the body.

Awareness and use of the term breakthrough case increased dramatically in 2021 amid reports of breakthrough cases of the COVID-19 virus. In particular, the rapid spread of the more contagious Delta variant of COVID-19 led to an increase in reported breakthrough cases.

Please note: No vaccine is 100% effective, but breakthrough cases are extremely rare. In the US, breakthrough cases of COVID-19 are reported to have occurred in less than 1% of all those vaccinated—as low as .01% in some studies. And breakthrough cases among people vaccinated for COVID-19 are reported to be less severe and less likely to require hospitalization.

The term breakthrough case is sometimes used to refer to the person who becomes infected, as in I’m a breakthrough case, but my symptoms are very mild. 

A breakthrough case may sometimes be called a breakthrough infection, though that term is also used in a more specific way (typically among medical professionals) to refer to an instance of a vaccinated person becoming infected regardless of whether they actually get sick (in this sense, a breakthrough infection is considered milder than a breakthrough case).

The term breakthrough disease most often refers to a sickness that happens despite any type of medical treatment or prevention, not just vaccination.

A breakthrough case is sometimes called a breakthrough for short. The word breakthrough is also used more generally to refer to any illness or condition that manages to thwart prevention or treatment.

To learn more about breakthrough cases, please visit the Centers for Disease Control and Prevention (CDC).

Related words

ChAdOx1 nCoV-19, anti-masker, vaccine hesitancy, Delta variant, double mutant variant

Where does breakthrough case come from?

The first records of the use of the term breakthrough case in the medical community come from around 2000, and it’s likely it has been used since even earlier. The term combines the word breakthrough, in the sense of “an act or instance of removing or surpassing an obstruction or restriction,” and the word case, in the sense of “an instance of a disease requiring medical attention.” In the context of vaccines and disease, a breakthrough case is one that “breaks through” the defenses created in the immune system by the vaccine.

The term breakthrough case was used generally before the COVID-19 pandemic to refer to any instance of a disease making a person significantly ill despite being vaccinated against it. Around the time of the emergence of the more contagious Delta variant of the COVID-19 virus in 2021, the US Centers for Disease Control and Prevention repeatedly used the term breakthrough case in its reporting and advice regarding the virus. This led to widespread use and awareness of the term breakthrough case in reference to COVID-19.

Examples of breakthrough case

—Not sure about you, but to me a breakthrough case of COVID-19 sniffles is far preferable to a classic case of COVID-19 respiratory failure.
@DrHedrick, July 30, 2021
The CDC is continuing to follow those breakthrough cases to understand whether this higher-than-expected viral load leads them to infect other people.
Selena Simmons-Duffin and Rob Stein, NPR, July 29, 2021

Who uses breakthrough case?

Breakthrough case is a general term that can apply to any infection that happens despite vaccination, but use and awareness of the term increased in 2021 due to breakthrough cases of COVID-19 around the emergence of the Delta variant. Still, breakthrough cases are extremely rare—and often less severe than cases in those who have not been vaccinated.

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This is not meant to be a formal definition of breakthrough case like most terms we define on, but is rather an informal word summary that hopefully touches upon the key aspects of the meaning and usage of breakthrough case that will help our users expand their word mastery.