How many people had a breakthrough case of COVID-19? It's difficult to know

Government COVID-19 data shows that people who are fully vaccinated against COVID-19 can still get infected. That data shows infections occur rarely, even with the rise of the highly transmissible Delta variant. However, several issues make it challenging to assess the accuracy of those numbers.

First, in May, the Centers for Disease Control and Prevention (CDC) stopped tracking all reported breakthrough cases that don’t result in death or hospitalization. Second, for breakthrough cases to show up in any dataset, vaccinated people need to get tested if they think they contracted the virus. There isn’t a government data set or survey that measured the likelihood vaccinated people would get a test if they were symptomatic.

The CDC defines breakthrough cases as the detection of COVID-19 in someone more than two weeks after completing vaccination, whether it’s a two-shot Pfizer or Moderna vaccine or the single-shot Johnson and Johnson vaccine. From January 1 to April 30, there were 10,262 breakthrough cases across 46 states and territories. With 101.4 million people fully vaccinated by that point, 0.01% of fully vaccinated people had contracted COVID-19. But this data was from before the Delta variant took hold in most of the US.

The CDC is still tracking hospitalizations and deaths of people with breakthrough cases of COVID-19. Through August 2, there were 7,525 breakthrough hospitalizations, or 0.004% of all fully vaccinated Americans. There were 1,503 breakthrough deaths, representing 20% of these severe COVID-19 cases.

Some states also publish cases on their breakthrough cases. The states vary on the metrics reported and the dates those metrics cover. For example, as of July 31, Massachusetts reported 7,737 breakthrough cases among 4.4 million fully vaccinated people. That’s one breakthrough case for every 570 people fully vaccinated. One hundred of the breakthrough cases resulted in death.

The CDC released a report finding that between July 3 and July 17, there were 469 new COVID-19 cases among people who traveled to Barnstable County, Massachusetts. Seventy-four percent of the cases occurred in fully vaccinated people. Among the cases tested, 90% involved the Delta variant, which is more transmissible than older strains of COVID-19.

Out of 1.6 million California cases between January 1 and July 26, 21,083 people were fully vaccinated. About 20.7 million Californians completed their vaccines by July 26. There was one breakthrough case per 982 people fully vaccinated.

In Washington state, there were 4,241 breakthrough cases out of 4.4 million people fully vaccinated. As of July 24, 0.1% of fully vaccinated Washingtonians got COVID-19. There were 52 deaths, with a median age of 82.5, among the breakthrough cases.

Through July 30, Virginia identified 1,566 breakthrough cases out of 4.6 million people vaccinated, or one for every 2,966 fully vaccinated. Among the breakthrough cases, 145 people were hospitalized, and 42 people died.

South Carolina counted 542 breakthrough cases as of July 23. With 2 million people completing their shots at that point, that amounts to one case per 3,814 people fully vaccinated.

To learn more about the state of the US during the pandemic, visit the cases and deaths map, the vaccine tracker, and the COVID-19 Impact and Recovery Hub.