Dr. Fauci Says This Is What Happens If You Combine the Moderna & Pfizer Shots

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If you've chosen to get either the Moderna or Pfizer COVID vaccines, you can expect to receive two doses spaced three to four weeks apart. And while most people will be given two doses of the same brand, others may find themselves in a rare situation that requires mixing and matching the two. In January, the Centers for Disease Control and Prevention (CDC) updated its recommendations to allow for this uncommon scenario, ruling that if someone doesn't know their first dose type, or if that same type isn't available for a second dose, vaccination sites can pair Moderna and Pfizer shots to complete the vaccine series.

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However, the CDC's determination leaves some questions unanswered—including whether mixing Moderna and Pfizer vaccines could trigger additional side effects or alter the vaccines' efficacy. White House COVID Adviser Anthony Fauci, MD, appeared on a recent episode of Jimmy Kimmel Live to field these questions and more.

"They're both the same type of vaccines, they're both mRNA vaccines," Fauci said, referring to the Moderna and Pfizer COVID shots. "We recommend you get the same type, but if one is not available, we feel there really isn't a major difference as long as they're the same type," he explained, adding, "If you started off with one and ended with the other, you should be in good shape and get a good response." In other words, there's currently no evidence to suggest that mixing the two will put you at additional risk or lower your levels of protection.

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Researchers are only just beginning to formally investigate the impact of combining vaccines, yet many are hopeful that this practice may not only be safe, but could heighten a person's immune response. This week, a team in the U.K. revealed that they are studying whether one dose of the AstraZeneca vaccine paired with a follow-up dose of the Pfizer vaccine might confer greater protection against COVID than the AstraZeneca vaccine alone.

"There is absolutely a possibility—and I think it's quite an exciting possibility—that if you combine these two different vaccines you can broaden that immune response. You can strengthen that immune response," Helen Fletcher, PhD, a professor of immunology at the London School of Hygiene and Tropical medicine told ABC News.

However, experts caution that, until there is a solid body of conclusive data on vaccine-mixing, it's important to abide by current recommendations. "From a regulatory perspective, strictly speaking, any combination of two vaccines should be first tested in a clinical trial. The purpose of doing that will be to make sure that this combination is safe and the benefits they bring far outweigh the potential safety concerns," explained Bali Pulendran, PhD, an immunologist at Stanford.

So, if two doses of the same vaccine are available to you, the CDC still says that you should make "every effort" to complete your vaccine series with a single brand. However, as Fauci will tell you, it's OK to combine Moderna and Pfizer doses if the choice is between a different mRNA shot and no second shot at all, or getting your second shot outside the recommended timeframe.

RELATED: These 2 Vaccines Are Effective Against the New India Variant, Study Finds.