What Is a Breakthrough COVID-19 Infection?

While COVID-19 vaccines are pretty effective at combating the novel coronavirus, breakthrough cases can happen. But what does that mean, exactly?

Photo: Design by Jo Imperio

One year ago, many people were envisioning what summer 2021 might look like after the early throes of the COVID-19 pandemic. In a post-vaccinated world, maskless gatherings with loved ones would be the norm, and return-to-office plans would be underway. And for a little while, in some places, that was the reality. Butfast-forward to August 2021, however, and it feels as if the globe has taken a giant step backward in combatting the novel coronavirus.

Although 164 million people in the United States have been vaccinated against COVID-19 there are rare cases in which fully vaccinated folks can contract the novel coronavirus, called "breakthrough cases" by the Centers for Disease Control and Prevention. (

But what constitutes a breakthrough COVID-19 infection, exactly? And how common — and dangerous — are they? Let's dive in.

What Are Breakthrough Infections?

Breakthrough infections occur when someone who is fully vaccinated (and has been for at least 14 days) contracts the virus, according to the CDC. Those who experience a breakthrough case despite being vaccinated for COVID-19 may experience less severe symptoms or may be asymptomatic, according to the CDC. Some symptoms associated with breakthrough COVID-19 infections, such as a runny nose, are less severe than the notable symptoms often linked to COVID-19, such as shortness of breath and difficulty breathing, according to the CDC.

On that note, even though breakthrough cases do happen, the number of breakthrough cases that result in serious illnesses, hospitalizations, or death are extremely low, according to the Cleveland Clinic — only about 0.0037 percent of vaccinated Americans, according to their calculations.

While it's not considered a breakthrough case, it's worth noting that if a person is infected with COVID-19 prior to or shortly after vaccination, there is still a possibility they could come down with the virus, according to the CDC. That's because if a person hasn't had enough time to build protection from the vaccine — aka the antibody proteins your immune system creates, which takes about two weeks they could still fall ill.

Does This Mean the Vaccines Aren't Working?

Actually, breakthrough cases were expected to happen among vaccinated people. That's because no vaccine is ever 100 percent effective in preventing illness in those who are vaccinated, according to the CDC. In clinical trials, the Pfizer-BioNTech vaccine was found to be 95 percent effective at preventing infection; the Moderna vaccine was found to be 94.2 percent effective at preventing infection; and the Johnson & Johnson/Janssen vaccine was found to be 66.3% effective, all according to the CDC.

That said, as the virus continues to mutate, there may be new strains that aren't prevented as effectively by the vaccine, such as the Delta variant (more on that in a sec), according to the WHO; however, mutations should not ever make the vaccines completely ineffective, and they should still offer some protection. (

How Common Are Breakthrough Cases?

As of May 28, 2021, a total of 10,262 breakthrough COVID-19 cases had been reported in 46 U.S. states and territories, with 27 percent reportedly asymptomatic, according to CDC data. Of those cases, 10 percent of patients were hospitalized and 2 percent died. Newer CDC data (last updated July 26, 2021), has counted a total of 6,587 breakthrough COVID-19 cases in which patients were hospitalized or died, including 1,263 deaths; however, the organization isn't 100 percent certain how many breakthrough cases exist. The number of COVID-19 vaccine breakthrough infections reported to the CDC is likely "an undercount of all SARS-CoV-2 infections among" the fully vaccinated, according to the org. Given symptoms of a breakthrough infection can be confused with that of the common cold — and given the fact that so many breakthrough cases can be asymptomatic — people may feel they don't need to get tested or seek medical attention.

Why, exactly, are breakthrough cases happening? For one, the Delta variant is posing a particular problem. This new-ish strain of the virus appears to spread more easily and come with a higher risk of hospitalization, according to the American Society for Microbiology. Plus, preliminary research shows that the mRNA vaccines (Pfizer and Moderna) are only 88 percent effective against symptomatic cases of the Delta variant versus their 93 percent effectiveness against the Alpha variant.

Consider this study released by the CDC in July detailing a COVID-19 outbreak of 470 cases in Provincetown, Massachusetts: Three-quarters of those infected were fully vaccinated, and the Delta variant was found in most of the genetically analyzed samples, according to the organization's data. "High viral loads [the amount of the virus an infected person may have in their blood] suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people with Delta can transmit the virus," said Rochelle Walensky, M.D., and director of the CDC, on Friday, according to The New York Times. Indeed, a Chinese study claims the delta variant viral load is 1,000 times higher than earlier strains of COVID, and the higher the viral load, the more likely it is that someone will spread the virus to others.

In light of these findings, the CDC recently implemented updated mask guidance for the fully vaccinated, suggesting people wear them indoors in areas where transmission is high, since vaccinated people can still get sick with and transmit the virus, according to the CDC.

What to Do If You Think You Have a Breakthrough Infection

So, what happens if you were exposed to someone who tested positive for COVID-19 but you yourself are fully vaccinated? It's easy; get tested. The CDC advises getting tested three to five days after potential exposure, even if you have no symptoms. On the flip side, if you feel sick — even if your symptoms are mild and you think it's just a cold — you should still get tested.

Although COVID-19 is still evolving — and, yes, breakthrough cases are possible — the vaccines remain the greatest protectors in combatting the pandemic. That, plus practicing reasonable personal hygiene (washing your hands, covering your sneezes and coughs, staying home if you're sick, etc.) and following updating CDC guidelines on mask wearing and social distancing to keep both you and others safe.

The information in this story is accurate as of press time. As updates about coronavirus COVID-19 continue to evolve, it's possible that some information and recommendations in this story have changed since initial publication. We encourage you to check in regularly with resources such as the CDC, the WHO, and your local public health department for the most up-to-date data and recommendations.

Was this page helpful?
Related Articles