What Happens if You Accidentally Get Different COVID-19 Vaccines? Doctors Explain

More than 134 million Americans have received at least one dose of the COVID-19 vaccine—exciting progress in the fight to end the pandemic. But remember: You need a second dose of an mRNA vaccine, either from Pfizer or Moderna, in order to be considered fully vaccinated against the novel coronavirus.

Recently, there have been reports of people getting their second dose from the wrong vaccine manufacturer. A woman in Oregon said she was accidentally given a dose of the Moderna COVID-19 vaccine after receiving her first shot of the Pfizer-BioNTech vaccine.

She said she received her Pfizer dose in mid-January, when she was seven months pregnant. “I felt horrible for a few days. I was light-headed, having chills, and being seven months pregnant, that worried me,” she said. Burgess didn’t want to experience that again while pregnant, so she delayed getting her second dose until April 5, after she had her baby.

Soon after, she realized she was given the wrong vaccine. “At that point, I immediately started Googling,” she said. “In my head, I’m freaking now, like, that’s not right.”

Burgess said she called both her primary care doctor and the Centers for Disease Control and Prevention (CDC). Both told her they’d never seen this before. As a precaution, they recommended that she stop breastfeeding her three-week-old son.

Burgess’s story (and others like hers) raise the question: What happens if you accidentally get two different COVID-19 vaccines? Here’s what you need to know.

First, a recap on how the Pfizer-BioNTech and Moderna vaccines work.

The Pfizer-BioNTech and Moderna vaccines are similar—both are messenger RNA (mRNA) vaccines. An mRNA vaccine works by encoding a portion of the spike protein found on the surface of SARS-CoV-2, the virus that causes COVID-19, according to the CDC. These vaccines use pieces of the encoded protein to prompt an immune response in your body, and antibodies to the virus are developed.

Here’s the full list of ingredients in the Pfizer-BioNTech vaccine, per the Food and Drug Administration (FDA):

And here’s the full list of ingredients for the Moderna vaccine, according to the FDA:

“These vaccines are very similar,” says Jamie K. Alan, Pharm.D., Ph.D., associate professor of pharmacology at Michigan State University. “They differ in the inactive ingredients, but the mode by which they work is nearly identical.”

Even though the Pfizer and Moderna vaccines are similar, mixing them is not recommended.

The CDC specifically says in interim guidance that the COVID-19 vaccines are “not interchangeable,” adding that, “the safety and efficacy of a mixed-product series have not been evaluated.” Instead, the agency says, both doses of the vaccine series with an mRNA vaccine should be completed with the same product.

However, the CDC does say that in “exceptional situations,” where the first dose of the vaccine can’t be determined or is no longer available, “any available mRNA COVID-19 vaccine may be administered at a minimum interval of 28 days between doses to complete the mRNA COVID-19 vaccination series.”

What happens if you accidentally mix the vaccines?

It’s unclear at this point, but you probably won’t have unusual side effects, says infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security. Plus, you’ll likely still get the benefits of being fully vaccinated, he says.

William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine, agrees that mixing vaccines is probably safe and effective. However, he also emphasizes that “mixing and matching has not been explicitly studied.”

He also noted that accidentally getting the second dose from the wrong maker is likely to continue happening. “This won’t be the first time, by any means—whether inadvertently or because somebody got their vaccine at one place and then wound up at a different place that had a different vaccine,” he says.

What should you do if you accidentally get different COVID-19 vaccines?

If you end up getting doses of two different mRNA vaccines, you don’t need additional doses of either one, the CDC says.

“Because the vaccines that are being interchanged use the exact same technology and are very close to being identical, people will have very similar immunity after they’re fully vaccinated,” Dr. Adalja says.

The CDC notes: “In situations where the same mRNA vaccine product is temporarily unavailable, it is preferable to delay the second dose (up to six weeks) to receive the same product than to receive a mixed series using a different product.”

How to avoid accidentally mixing vaccines

In most cases, pharmacies, hospitals, and doctors offices “should have protocols in place so that this doesn’t happen,” says Richard Watkins, M.D., an infectious disease specialist and a professor of internal medicine at the Northeast Ohio Medical University. They should be able to access your electronic health records to verify that you’re receiving a second dose of the same vaccine.

Once you arrive for your second dose appointment, Dr. Schaffner recommends paying attention to the type of vaccine you’re about to receive. Before the vaccine administrator injects you, ask them which vaccine you’re about to get.

Can’t remember which vaccine you got the first time and your electronic records aren’t available? Dr. Adalja says to consult your vaccination card. “The type of vaccine you received should be clearly listed on it,” he says.

You can also sign up for VaxText, a free text-message-based platform that records your vaccination date and COVID-19 vaccine name, plus sends out second-dose reminders.

Overall, accidentally getting the wrong vaccine for your second dose isn’t something to stress about. “Most of the time, this doesn’t happen,” Dr. Adalja says.

This article is accurate as of press time. However, as the COVID-19 pandemic rapidly evolves and the scientific community’s understanding of the novel coronavirus develops, some of the information may have changed since it was last updated. While we aim to keep all of our stories up to date, please visit online resources provided by the CDC, WHO, and your local public health department to stay informed on the latest news. Always talk to your doctor for professional medical advice.

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