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Why Are Some Vaccinated People Still Getting COVID-19?

Older man receiving a COVID test.

Ergin Yalcin / Getty Images

Key Takaways

  • A limited number of people who have been vaccinated may still get a "breakthrough" COVID-19 infection.
  • However, research shows that vaccinated people are much less likely to become seriously ill or need to be hospitalized compared to unvaccinated people.
  • Most breakthrough infections are likely related to the lifting of pandemic protocols like mask-wearing and social distancing, as well as the highly transmissible Delta variant.

At the start of the summer, fully vaccinated people were able to drop some of the precautions that were put in place during the early days of the pandemic, such as wearing a mask and social distancing. However, as cases and deaths rise nationwide, the Centers for Disease Control and Prevention (CDC) is now urging vaccinated people living in areas with high rates of transmission to mask up again.

While the possibility of getting COVID-19 after being vaccinated may come as shock to some, public health experts say this is expected. And these "breakthrough" cases of COVID-19 are less common and severe than those experienced by the unvaccinated.

What Is a Breakthrough COVID-19 Case?

A breakthrough COVID-19 infection occurs when someone who is fully vaccinated against COVID-19 (at least 14 days after all recommended doses of an FDA-authorized COVID-19 vaccine) tests positive for the virus.

How Vaccines Work

David Dowdy, MD, associate professor of epidemiology at Johns Hopkins Bloomberg School of Public Health, tells Verywell that the COVID-19 vaccines are working. They are very effective against the virus, as well as the Delta variant that is now the dominant strain in the U.S.

But confusion arises when people misunderstand how a vaccine works. Dowdy says that a vaccine is “not steel armor"—it works by giving your immune system a head start at recognizing a virus. It primes your body, helping it fight off the virus faster during possible future encounters.

Sten Vermund, MD, PhD, dean of the Yale School of Public Health, tells Verywell that “all vaccines— every single one of them—work to prepare the immune system to see the protein antigen on the virus that represents the invader before the invasion actually happens."

However, Dowdy says this “doesn't mean that this virus can't still get into and start to expand in your system." Breakthrough infections are to be expected with any vaccine.

“People think that you get vaccinated, you won't get infected. That's not true at all," Vermund says. "You get infected, but your immune system responds with such vigor and such specificity that you don't get seriously ill.”

Like a Seatbelt, Not Armor

You can think of vaccines working like seatbelts or airbags in cars, Vermund adds.

Having these protections in your car does not that mean you will not get into an accident. It also doesn't ensure that if you are in a crash, you won't get injured. However, you might walk away with minor injuries compared to what it would have been like if your car didn't have them at all.

Using that analogy, Vermund says that “being unvaccinated is like having no seatbelt, or airbag."

Researchers find that people who are vaccinated but get COVID-19 anyway tend to have a much milder illness compared to unvaccinated people.

Sten Vermund, MD, PhD

The number of vaccinated people who have become seriously ill is vanishingly small.

— Sten Vermund, MD, PhD

“If you do a survey at Yale New Haven Hospital—or whatever your hometown is—and you find out who is in the ICU right now, you're going to find most likely nobody who's vaccinated,” Vermund says. "The number vaccinated people who have become seriously ill is vanishingly small."

That's the key point to understand; Vermund says that vaccines are "transforming a potentially lethal virus into something like a mild flu or a cold."

What This Means For You

Vaccines jumpstart the immune system and help them quickly recognize and fight a virus, but they do not prevent a virus from getting into the body in the first place. That's why some people who have been fully vaccinated against COVID-19 have gotten a "breakthrough" infection. Getting vaccinated is still the best way to protect yourself not just from the virus, but from severe illness if you do get sick.

The Threat of the Delta Variant

The Delta variant of the COVID-19 virus is spreading rapidly, nearly doubling cases every 10 days, Dowdy says. The variant is highly transmissible, causing spikes in hospitalizations in states with low vaccination rates.

Because the Delta variant is more easily transmitted, Dowdy explains, vaccinated people are more likely to come into contact with unvaccinated people infected with the variant, leading to a breakthrough infection.

Still, this variant doesn't tell the whole story.

“It's important to somewhat separate the Delta variant from just increased transmission as a whole,” Dowdy says. “The Delta variant has come on the scene, but at the same time we as a society have been living life a little more freely."

Many states have loosened protocols from the early days of the pandemic—like mask-wearing and social distancing. This summer, people are also gathering and traveling more.

“How much of the increase in infections is due to the Delta variant versus due to our behavior is not entirely known, but my bias is that it's more due to our behavior than to the variant,” Dowdy says, noting that other countries have been dealing with the Delta variant longer than the U.S., but have not had the same spike in deaths.

Breakthrough COVID-19 cases are more common because infections are more common, Vermund adds. The cases will be more common in states with low vaccination rates that are keeping transmission high and less common in areas with high vaccination uptake.

“All of us are more likely to be in contact with someone who is infected, and perhaps even to contact them more closely than we would have before, meaning there might be more of the virus transmitted than before," Vermund says. "And the more of those events that happen, the more likely it is going to happen that the virus makes it past that head start we've given our immune systems with the vaccine.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

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  1. Centers for Disease Control and Prevention (CDC). Interim Public Health Recommendations for Fully Vaccinated People. Updated July 27, 2021.

  2. Centers for Disease Control and Prevention. COVID-19 Vaccine Breakthrough Infections Reported to CDC — United States, January 1–April 30, 2021. May 25, 2021.

  3. Centers for Disease Control and Prevention (CDC). COVID Data Tracker. Updated July 27, 2021.