What is vaccine breakthrough?

This article was updated January 17, 2022, to reflect new information from the CDC.

Every day in the U.S., the number of people being vaccinated against COVID-19 grows.

As more people roll up their sleeves to help end the pandemic, the U.S. Centers for Disease Control and Prevention (CDC) continues to study vaccine effectiveness, to ensure COVID-19 vaccines are working as expected.

That review continues whether a vaccine has received full approval from the U.S. Food and Drug Administration (FDA) or is under emergency use authorization (EUA). The Pfizer vaccine received full approval on August 23, 2021, for use in people age 16 and older. Vaccines from Moderna and Johnson & Johnson continue to be administered to people age 18 and older under the FDA’s EUA Vaccination of children age 5 to 15 with Pfizer also is under the FDA’s EUA.

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As part of the review, one of the things the CDC monitors closely is vaccine breakthrough. Douglas Kasper, MD, the section head of infectious disease at the University of Illinois College of Medicine Peoria and a leader in the OSF HealthCare response to COVID-19, defines “vaccine breakthrough” as someone contracting COVID-19 at least two weeks after receiving the full course of the vaccine.

When is it considered vaccine breakthrough?

For a case to be considered vaccine breakthrough, the timing between vaccination and infection is the key.

“What’s important here is that a sufficient amount of time has passed from when they received the vaccine,” Dr. Kasper said. “So if somebody receives the vaccine on a Monday and contracts COVID-19 the following day, that does not count as a vaccine breakthrough. There has not been sufficient time for the body to create the protective immune response that we know will occur as usually two to four weeks pass after receiving the vaccine.”

According to the CDC, a “breakthrough” case is when a person tests positive for COVID-19 at least two weeks after becoming fully vaccinated (either receiving a one-dose vaccine or the second dose of a two-shot vaccine). While the CDC is investigating thousands of reported vaccine breakthrough cases, it is still a very small percentage of those who have received the full dose.

More importantly, the number of breakthrough cases that result in serious illnesses, hospitalizations, or death remain much lower when compared to the same outcomes in unvaccinated individuals. The CDC is tracking those numbers, and the most recent data shows that unvaccinated individuals are five times more likely to become infected with COVID-19 and 29 times more likely to be hospitalized from COVID-19 infection as compared to fully vaccinated persons.

Vaccine breakthrough is expected

Breakthrough cases are expected for any vaccine, Dr. Kasper said. That’s because each person is unique and will respond to the virus and the vaccine differently.

“Any immune response is not uniform. How all of us interact with a virus will be different,” he said. “So because of that, vaccines or natural immunity are not 100% protective. We strive for 100%, but what we understand is that some number of people, even when they receive the vaccine, may become ill with the virus.

“What we know is that people who receive the vaccine, even if they become ill, tend to have more mild cases of disease than those who have not received the vaccine.”

Breakthrough infections and variants

Breakthrough cases being reported in the United States are due to the variant strains of COVID-19 that have emerged over the last year.  These variant strains are proving to be more transmissible than the original virus, making it more likely for those who come in contact to become infected.

The highly transmissible rate of variant strains can be blamed for surges in COVID-19 cases seen throughout the United States and the world.  Completing a course of COVID-19 vaccination, including a booster, remains the best option to protect an individual from developing serious health effects from COVID-19 infection, Dr. Kasper said.

No reason to avoid vaccination

For Dr. Kasper, the existence of COVID-19 vaccine breakthrough cases is not a reason for anyone to avoid the vaccine. The opposite, he said, is true. Continuing research shows the benefits of getting vaccinated far outweigh the risk of infection.

“Vaccines remain highly effective at preventing severe effects from COVID-19 infection.  Additionally, vaccines have shown to be very safe across all approved age groups,” Dr. Kasper said. “This should reassure our community that timely vaccination is of paramount importance to controlling the COVID-19 pandemic.

“Even with the emergence of concerning variants, the most effective tool for prevention against getting an infection is vaccination.”

Get vaccinated

Currently, anyone age 5 and older is eligible to receive the COVID-19 vaccine, and can find an appointment with OSF HealthCare. Boosters are available to people age 12 and older. Boosters are given five months after receiving either of the two-dose Pfizer or Moderna vaccines or two months after receiving the one-dose Johnson & Johnson.

Individuals who are immunocompromised should get a third shot of Pfizer or Moderna 28 days after their second dose, followed by a booster five months later.

Dr. Kasper urges everyone, whether they’re vaccinated or unvaccinated to continue wearing masks, especially indoors with others around, and to follow other public health guidelines to slow the spread of COVID-19.

Last Updated: January 17, 2022

About Author: Lisa Coon

Lisa Coon is a Writing Coordinator for OSF HealthCare, where she has worked since August 2016.  A Peoria native, she is a graduate of Bradley University with a degree in journalism. Previously, she worked as a reporter and editor at several newspapers in Iowa and Illinois.

She lives in Groveland with her husband and son. In her free time she likes to cook, bake and read. She freely admits that reality TV is a weakness, and she lives by the quote, “The beach is good for the soul.”

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