What are breakthrough cases, and how concerned should you be?

Medically reviewed on November 2, 2021. To give you technically accurate, evidence-based information, content published on the Everlywell blog is reviewed by credentialed professionals with expertise in medical and bioscience fields.

The introduction and distribution of vaccines have been one of the most encouraging parts of the pandemic, but we’re not through the tunnel just yet. Things have become even worse thanks to a new strain of the initial SARS-CoV-2 virus known as the Delta variant.

The Delta strain is more transmissible than the original strain, which had a basic reproduction number of two to three. This means that someone infected with the initial COVID-19 virus spread it to two or three people on average. The Delta strain has a basic reproduction number of five to nine according to estimates from the CDC, and vaccinated people are not exempt from that [1].

What are COVID breakthrough cases?

A breakthrough case refers to someone who has been fully vaccinated testing positive for COVID-19. As the CDC states, COVID breakthrough cases of any form are expected. No vaccine is going to support 100 percent immunity, but vaccines are effective in reducing the severity and longevity of an infection, thereby reducing fatalities and hospitalizations. Since a fully vaccinated person can still contract the coronavirus, it is essential that anyone showing symptoms stays home in order to prevent the spread of this breakthrough infection.

State health departments report breakthrough cases to the CDC, which also monitors hospitalized and fatal breakthrough cases. They can then cluster breakthrough infections based on geographic location, patient demographics, vaccine type, and lineage of SARS-CoV-2 [2].

How many vaccinated people have gotten COVID-19?

Breakthrough cases are always possible, but the research shows that the number of breakthrough infections is nearly negligible. Data from the Kaiser Family Foundation shows that breakthrough cases affect about 0.01 to 0.29 percent of fully vaccinated people, and even in those cases, the infections are generally much milder for the vaccinated individual than in those who have not received a vaccination.

That still holds true for the Delta variant, as well. Full vaccination is currently about 96 percent effective in preventing hospitalizations from Delta, and it is 88 percent effective at preventing symptomatic forms of the disease. Currently, unvaccinated people comprise about 95 percent of COVID-19 hospitalizations. As you can see, the vaccine effectiveness for this severe disease is still high [1].

Are some individuals more at risk for severe breakthrough illness?

Prior to vaccines, older people were the most vulnerable to COVID-19. That still holds true with breakthrough cases, which are more common in older adults with underlying conditions, including kidney disease and diabetes, and those who are already immunocompromised [3].

Again, these cases are rare, and the reporting has been inconsistent. In about 28 percent of the hospitalizations tracked by the CDC, breakthrough cases were asymptomatic or didn’t play a role in the hospitalization. For example, say an older man ends up at the hospital after having a heart attack, and he tests positive for COVID-19, even though his case is asymptomatic. He is still counted as a hospitalized COVID patient, even if his heart attack was the reason for his hospitalization. This can apply to fatalities as well. A patient may have passed away with a COVID-19 diagnosis, but COVID-19 wasn’t the cause of the death [3].

Perhaps more concerning is the rise in cases among children. Children under the age of 12 still are not eligible for vaccination, and with kids returning to in-person learning, case numbers are only going up among this age group. While deaths remain low, many kids are developing long COVID symptoms, while others are developing a condition known as multi-system inflammatory syndrome (MIS-C). This causes severe inflammation of vital organs and tissues, and requires medical treatment. Kids who are symptomatic or have been exposed to someone with COIVID-19 should be screened with a test authorized for use in children. This condition requires treatment [4]. If you have kids at home, it would be a good idea to have a home collection COVID test on hand so you can be proactive as soon as a child begins to feel ill.

Breakthrough cases can be alarming, but they are expected and rarely severe. Every expert currently agrees that there is high vaccine effectiveness. The main concern is the potential to spread the virus, which is why you should continue wearing a mask, practicing social distancing measures even if you are vaccinated, and getting tested if you notice any symptoms.

Experiencing COVID-19 symptoms and looking for relief? Get COVID-19 treatment online via Everlywell's Virtual Care offering.

Why keeping a COVID test handy at home might actually be a good idea

COVID testing for variants: what you need to know

What is the Delta variant?


1. How the pandemic now ends. The Atlantic. URL. Accessed November 2, 2021.

2. Improving communications around vaccine breakthrough and vaccine effectiveness. Centers for Disease Control and Prevention. URL. Accessed November 2, 2021.

3. Rarely, Covid vaccine breakthrough infections can be severe. Who's at risk? NBC News. URL. Accessed November 2, 2021.

4. MIS-C: The COVID-Related Condition Parents Need to Know About. Michigan Health. URL. Accessed November 2, 2021.

Everlywell makes lab testing easy and convenient with at-home collection and digital results in days. Learn More