This Is What Happens When You Mix Pfizer With Another Vaccine, Study Says

Researchers said combining these two vaccines had the effect they were hoping for.

With a handful of vaccines on the market across the globe, the possibility of getting two different shots may have crossed your mind. And you're not alone: Researchers believe that combining two vaccines could be the key to bolstering immunity in some recipients. One recent study sought to find out the effect of mixing two vaccines, Pfizer and AstraZeneca, the latter of which is not currently approved in the U.S. Researchers concluded that administering a Pfizer dose after an AstraZeneca dose significantly improved immune response.

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

The study, which was discussed during an online presentation on May 18, found that giving a Pfizer shot after an AstraZeneca shot "greatly enhanced" immunity. The CombivacS clinical trial from Carlos III Health Institute examined 663 people who received an AstraZeneca shot followed by a Pfizer shot, and a control group that didn't receive any booster shot. The control group experienced no shift in antibody levels, while those who received Pfizer—at least eight weeks after their initial AstraZeneca shot—gained a more potent immune response.

The AstraZeneca vaccine is a one-dose viral vector vaccine, similar to the Johnson & Johnson vaccine that has been approved for emergency use in the U.S. The jolt that Pfizer gave to AstraZeneca recipients' immune response led to much higher levels of antibodies than they previously had. As Nature notes, that these were the results the researchers had hoped for. Zhou Xing, PhD, an immunologist at McMaster University, told Nature that the antibody response to the Pfizer booster seems to be stronger than what people generally have after two doses of the AstraZeneca vaccine, based on earlier data. However, it's currently unclear how the antibody response compares to someone who received two doses of the Pfizer or Moderna vaccines.

According to Nature, mixing vaccines in this manner—known as a heterologous prime and boost—is a strategy that's proven effective against other diseases such as the Ebola virus. A handful of European countries have begun to recommend that people who received the AstraZeneca vaccine get a second dose of an mRNA vaccine (like Pfizer or Moderna) in the hopes of triggering a stronger immune response. According to Nature, double doses of the AstraZeneca vaccine don't seem to be worthwhile, as viral vector vaccines tend to become increasingly less effective over time.

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It's worth noting that mixing two different vaccines could result in more side effects, according to a study published on May 12 in The Lancet. Researchers found that people who received mixed vaccines were more likely to report mild and moderate side effects, including headaches, chills, and muscle pain.

While the Centers for Disease Control and Prevention (CDC) does not yet advocate for combining vaccines, saying "both doses of the series should be completed with the same product," the agency could shift its recommendation as more studies are completed.

RELATED: It'll Be "At Least" This Long Before You Need Another COVID Shot, Doctor Says.

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