NOEL KING, HOST:
If you get a COVID-19 vaccine that requires two doses, the recommendation is you get two doses of the same vaccine, so two Pfizers, two Modernas, no mixing. But that could change in the future, either by necessity or by design.
Here's NPR's Joe Palca.
JOE PALCA, BYLINE: The Pfizer and Moderna vaccines are very similar, both based on the same technology, so switching between them is probably not that big a deal. But in the United Kingdom, researchers have begun to study whether using two completely different vaccines is as good or even better than using the same one twice. Technically, this is known as heterologous vaccination. But there's a more colloquial description
HELEN FLETCHER: In the U.K. at the moment, we're sort of calling it mix-and-match.
PALCA: Helen Fletcher is a professor of immunology at the London School of Hygiene and Tropical Medicine. She says shortages of a vaccine or concerns about side effects may induce health officials to adopt a mix-and-match strategy. That could mean starting with what's called a viral vector vaccine, like the one made by AstraZeneca, and following with an mRNA vaccine, like Pfizer or Moderna. Health agencies in France and Germany have already begun recommending people consider doing that.
FLETCHER: 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.
PALCA: Here's why - 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.
FLETCHER: 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.
PALCA: Some vaccine manufacturers have embraced this approach and are making vaccines of two different types by design. Andrew Allen is CEO of a company called Gritstone. He says the immune system has two components, an antibody-based component and a cellular-based component. His company's strategy is to start with a viral vector vaccine that stimulates the cellular response and follow with an mRNA vaccine.
ANDREW ALLEN: The RNA makes a really good antibody response. And so potentially by combining these, you kind of get the best of both worlds.
PALCA: Allen says Gritstone's approach is already being tested in human volunteers. Stanford University's Bali Pulendran says this mix-and-match approach has been tried with vaccines for a variety of diseases.
BALI PULENDRAN: Diseases such as HIV, malaria, TB, even influenza - so there's ample evidence for the benefits of such strategies.
PALCA: So if it's such a good idea, why isn't it used routinely? Pulendran speculates there are two main reasons. One has to do with the way new vaccines are approved.
PULENDRAN: Regulatory authorities love simplicity. The simpler a vaccine, simpler a regimen, the more palatable they find this to be.
PALCA: The other reason is that while mix-and-match may make scientific sense, it doesn't always make business sense.
PULENDRAN: For example, if Company A makes one vaccine and Company B makes another vaccine, unless there's some overarching incentive for the two companies to enter into some sort of a marriage, I think either company would, in general, prefer to go along on their own.
PALCA: Of course, if a small company like Gritstone shows that a mix-and-match strategy really leads to a dramatically better vaccine, you can bet other pharmaceutical companies will find a way to solve the business problem.
Joe Palca, NPR News.
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