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Home » COVID vaccine breakthrough infections – let’s do some simple math

COVID vaccine breakthrough infections – let’s do some simple math

Let’s talk about COVID-19 vaccine breakthrough infections. That is individuals who contract COVID-19 despite being fully vaccinated by the Pfizer, Moderna, or JNJ vaccines, the only ones with Emergency Use Authorizations in the USA.

This article is going to be short and, hopefully, sweet, because even though the number of COVID-19 vaccine breakthrough infections appear large, it really isn’t. It’s just simple math.

What are breakthrough infections?

Well, it’s kind of straightforward. A breakthrough infection is a case of illness in which a vaccinated individual becomes sick from the same illness that the vaccine is meant to prevent. In the case of a COVID-19, the person who received the vaccine will subsequently contract the disease.

This happens when the vaccine fails to provide immunity against the pathogen they are designed to target. This isn’t just an issue with the COVID-19 vaccine, but breakthrough infections occur with almost every vaccine from HPV to hepatitis B.

There are several causes of these breakthrough infections – improper administration or storage of vaccines, mutations in viruses, and antibody blocking. In general, no vaccine is 100% effective.

COVID-19 vaccine breakthrough infections
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COVID-19 vaccine breakthrough infections

In a recent article, the Centers for Disease Control and Prevention reported that, between 1 January and 30 April 2021, a total of 10,262 SARS-CoV-2 vaccine breakthrough infections had been reported from 46 U.S. states and territories. Of course, over 10,000 breakthrough infections in just four months may seem like a lot, but let’s take a closer look.

However, as of 30 April 2021, As of April 30, 2021, approximately 101 million persons in the United States had been fully vaccinated against COVID-19. That means that 100,990,000 people didn’t have breakthrough infections. In other words, this data implies that these vaccines are 99.99% effective!

OK, to be fair, the article states that there are two limitations with this data:

  1. We are probably undercounting the number of SARS-CoV-2 infections because of asymptomatic infections or individuals who don’t go to a healthcare setting because of any number of reasons.
  2. SARS-CoV-2 sequence data are available for only a small proportion of the reported cases.

So, given the limitations, there could be a lot more breakthrough infections in vaccinated individuals. However, given that the effectiveness of the COVID-19 vaccines being around 90-95%, we could expect around 5-10 million breakthrough infections, and we just aren’t seeing levels that high.

Again, 10,000 breakthrough infections may seem like a lot, but given the number of fully vaccinated individuals in the USA, I am not sure that this is worthy of too much concern. Obviously, the CDC and the FDA (along with other regulatory agencies across the world) will continue to monitor these infections as a leading indicator of the effectiveness of these vaccines, especially with new variants.

unrecognizable medic showing ampoule containing vaccine in bright laboratory
Photo by Alena Shekhovtcova on



Michael Simpson

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