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Can we Mix and Match COVID Vaccines?

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The answer is: Not yet, but hopefully soon, at least with some.  We don’t have enough information at this time, but there are ongoing studies to answer this question.

Recent shortages of vaccine and the urgency of increasing vaccination rates have brought this question even more into the limelight.  If mixed vaccine combinations work, they may provide needed flexibility when production of one vaccine falters.  And there’s the chance that mixing doses of two different vaccines may boost protection.  For example, AstraZeneca’s vaccine uses a chimpanzee adenovirus vector for both its prime and booster doses.  In theory, the use of the same vector for both shots could mean the immune response triggered by the first vaccine could cripple the booster vaccine if the body recognizes and destroys the chimpanzee adenovirus vector in the booster dose prior to it getting where it needs to go.  

Studies are beginning to address this question, including a mixed vaccine trial already underway examining matching a dose of the Sputnik V vaccine made by Russia’s Gamaleya Research Institute of Epidemiology and Microbiology with a booster dose of AstraZeneca’s vaccine.  A second trial, the Com-COV trial, is just getting underway in the UK, mixing AstraZeneca’s vaccine with Pfizer’s.  In this trial, there will be 820 participants who will be allocated at random to receive 2 doses of the same vaccine, or one dose of one followed by a booster dose of the other, with booster doses given at 4 weeks or at 12 weeks after the initial vaccine dose.  Between 5 and 9 routine blood tests will be taken over the course of a year to look at the immune responses to the vaccine, and participants will complete an online diary for up to 28 days following each of the two vaccinations.

As we wait for more information from these studies, the Centers for Disease Control and Prevention currently discourages mixing vaccines until we know more, unless there are “exceptional circumstances,” such as shortage of the vaccine received for dose #1 because of production or distribution hiccups.

Jennifer Abrams, MD, April 5, 2021