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Mixing different kinds of COVID-19 vaccine might help boost immune responses, but the idea has been slow to catch on.
Typically, if you get a COVID-19 vaccine that requires two doses, you should get two of the same vaccine. Two Pfizer shots, or two Moderna shots. Not one and then the other.
But in the future, that could change, either by necessity or by design.
This idea of using two types of vaccines isn't a new concept. It's known as heterologous vaccination, although there's a more colloquial term.
"In the U.K. at the moment, we're sort of calling it 'mix and match,' " says Helen Fletcher, a professor of immunology at the London School of Hygiene & Tropical Medicine. She says shortages of a vaccine or concerns about side effects may induce health officials to adopt a mix-and-match strategy.