Sep 02, 2021 1:00 PM

Author: University of Utah Health Communications

Breakthrough cases are defined by the Centers for Disease Control and Prevention (CDC), as the “detection of SARS-CoV-2 RNA or antigen in a respiratory specimen collected from a person more than 14 days after all recommended doses of an FDA-authorized COVID-19 vaccine.” Simply put, a breakthrough case is when someone tests positive two or more weeks after completing all recommended doses of the COVID-19 vaccine.

Breakthrough Infections Do Not Mean That COVID-19 Vaccines are Not Working

Misinformation is circulating that breakthrough cases demonstrate that COVID-19 vaccines are not effective. However, this is untrue. Breakthrough infections do not mean that the vaccines are failing.

Breakthrough infections are to be expected because no vaccine is 100% effective. You can get COVID-19 after being vaccinated, but it rarely happens and symptoms are likely to be mild—like having the common cold or the flu. Data from the U.K. shows that, after full vaccination, the Pfizer vaccine was 88% effective in preventing symptomatic COVID-19 from the Delta variant. 

The CDC explains the benefits of vaccination clearly: “The number of COVID-19 cases, hospitalizations, and deaths that will be prevented among vaccinated persons will far exceed the number of vaccine breakthrough cases.”


When it comes to breakthrough cases, testing positive and getting sick are two different things.

Experts at U of U Health answer other common questions about breakthrough cases. 

Q: Can you transmit the virus to others if you have a breakthrough case?

A: The short answer is yes. However, because the COVID-19 vaccines prime the body’s immune system, vaccinated people clear infection more quickly, explains Stephen Goldstein, PhD, an evolutionary virologist at U of U Health. If you are vaccinated and have a breakthrough case, you won’t be infected for as long as if you were unvaccinated, and any transmission will occur for a shorter amount of time. 

Q: How likely is it for those who have had COVID-19 to be re-infected by the Delta variant?

A: There is some degree of “immune evasion with this variant,” Goldstein says. Prior infection by SARS-CoV-2 provides some protection from the Delta variant, but “if you were infected previously and you got vaccinated, then your protection—from an immunological standpoint—is off the charts.”

Q: How do we know if someone has been infected with the Delta variant?

A: There is no clinical test that identifies which virus variant a person has been infected with. However, based on samples provided for RT-PCR tests that have also undergone sequencing by departments of health across the country, we know that 95% of positive cases in the United States are caused by the Delta variant, says Emily Spivak, MD, MHS, an infectious disease physician at U of U Health.

Q: Do we know if people with breakthrough cases can develop “long-haul” symptoms?

A: Emerging evidence shows that vaccination reduces the risk of developing "long COVID". Moreover, the symptoms of long COVID should be a motivator to get vaccinated since unvaccinated people are more likely to get infected and therefore would be more likely to develop long-term symptoms. “To decrease your risk of long-term symptoms or disability, vaccinations are the answer,” Spivak says.

“The number of COVID-19 cases, hospitalizations, and deaths that will be prevented among vaccinated persons will far exceed the number of vaccine breakthrough cases.”

Where We Are Now

“Based on sequencing, we are seeing that the vast majority of positive cases—95%—are due to the Delta variant, and 85% of cases are in those that are unvaccinated,” Spivak says.

Since the Delta variant is the dominant strain of the virus that causes COVID-19, we are dealing with a much more transmissible disease than we did in 2020. However, now we have vaccines. “Vaccines remain extremely effective at preventing symptomatic infection and are even better at preventing hospitalization and death from this virus,” Spivak says. “The best way to prevent new variants from emerging is to get vaccinated.”

covid19 vaccines coronavirus breakthrough cases health delta variants

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