Understanding Breakthrough Cases

I have been asked several times about breakthrough cases. Why do they happen? How often do they happen? What do these occurrences mean in regards to vaccine effectiveness?

The term breakthrough case typically indicates when a fully vaccinated individual later gets the disease they were vaccinated against. Note that testing positive for the cause of the disease doesn’t necessarily mean one actually has the disease. The person may remain asymptomatic. For SARS-CoV-2 and COVID-19, we define a breakthrough case as anyone fully vaccinated who tests positive, regardless whether they have symptoms, as such information is useful to have the person isolate in order to prevent spread to someone else.

First, let’s look at some data on the occurrence of breakthrough cases.

• The Vermont Department of Health reports that there are over 206,000 fully vaccinated Vermonters. Of these individuals, approximately 125 subsequently tested positive for COVID-19. Compare that with the more than 10,000 Vermonters who have tested positive for COVID-19 over the past 90 days.
• Similarly, 90 million people in the United States are vaccinated. Among them, 5,800 have tested positive for COVID-19. Meanwhile, more than 7 million people in the U.S. have tested positive for COVID-19 just in the past 90 days.
• As you might have suspected, your chances of testing positive for COVID-19 after being fully vaccinated are extremely low. Only 0.008% of cases of COVID-19 occur in vaccinated individuals.
• These numbers are completely in line with our expectations, meaning the vaccines are extremely effective at preventing you from getting COVID-19.

Second,

• Most of those who are vaccinated and develop COVID-19 have mild or no symptoms.
• Importantly, they are also much less likely to spread the virus to others.

Third, the prevalence of SARS-CoV-2 matters.

• If there is a high percentage of the population vaccinated, then the chances that a vaccinated individual will contract COVID-19 is negligible. However, the chances of an unvaccinated individual eventually getting COVID-19 is moderate.
• If there is a low percentage of the population vaccinated, the chances of a vaccinated individual eventually getting COVID-19 is low, and the chances of an unvaccinated individual eventually getting COVID-19 is high.

For instance, assume 50% of the population is vaccinated. In this situation, everyone encounters the virus frequently as they go about life and interacting with others.

• If an individual is unvaccinated, they will quickly become infected, as they are encountering the virus many times per week and have no protection.
• If an individual is vaccinated, they may still eventually become infected, as they are bumping into it many times per week and at some point, they succumb. About 90 – 95% of the time, the vaccine wins, but 5 – 10% of the time, the virus wins.)
• In other words, the lower the percentage of the population vaccinated, the more frequent one encounters the virus, and the greater the likelihood that even a vaccinated individual will eventually get the virus.

However, let’s look at where we want to be as a society, a situation in which 75% or more of the population is vaccinated. Now everyone encounters the virus far less frequently.

• If an individual is unvaccinated, they are still likely to become infected over time, as even though they are encountering the virus only a few times per month, they have no protection.
• If an individual is vaccinated, they are extremely unlikely to ever become infected, have good protection against those few times they happen to run into the virus.

It all boils down to the simple understanding that the higher the percentage of the population vaccinated, the less frequent the chance of encountering the virus.  This makes it is extremely unlikely that a vaccinated individual will ever get the virus, while the unvaccinated person still has a real chance of becoming ill.

Visit the Center for Disease Control and Prevention’s website to learn more about the small possibility of COVID-19 illness after vaccination.

Trey Dobson, MD, is an emergency medicine physician, the chief medical officer at Southwestern Vermont Medical Center in Bennington, and the medical director of the Dartmouth-Hitchcock Putnam Physicians.