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.
Health agencies in France and Germany are already encouraging people who’ve gotten the AstraZeneca vaccine to consider getting one of the mRNA vaccines for their second shot.
“So there’s a practical reason why you would want to mix two different types of vaccine. But there is also a scientific reason as well,” Fletcher says.
Basically, all vaccines work by showing people’s immune systems something that looks like an invading virus but really isn’t. If the real virus ever comes along, their immune systems will recognize it and be prepared to fight it off.
Using two different vaccines is a bit like giving the immune system two pictures of the virus, maybe one face-on and one in profile.
“If you give two different types of vaccine, then you tend to get a better immune response than if you give the same vaccine twice,” Fletcher says.
Some vaccine manufacturers have embraced this approach and are making vaccines of two different types by design.