| In this week's edition of the Covid Q&A, we look at the risk of mixing different types of vaccines. In hopes of making this very confusing time just a little less so, each week Bloomberg Prognosis is picking one question sent in by readers and putting it to an expert in the field. This week's question comes to us from Thomas in Havana, Cuba. Thomas is a senior in good health living in a nation that has developed its own vaccine. He asks: I can get a vaccine here in Cuba, but it doesn't use messenger RNA technology like Pfizer and Moderna. I would like to get that shot and then get one from Pfizer or Moderna later. Is there any risk? This question is probably on a lot of people's minds after the Johnson & Johnson vaccine was cleared for emergency use in the U.S. a week ago.
Before we get to the question, let's briefly talk about efficacy. The Pfizer and Moderna vaccines, which require two shots, were shown to have an efficacy of about 95% in clinical trials. The single-shot J&J vaccine, which uses a harmless adenovirus to build immunity instead of mRNA, was shown to have an estimated efficacy of 72%. Those numbers — 95% versus 72% — have made some people feel like the mRNA vaccines are preferable to J&J's. But those numbers can be a bit misleading because some of the variables used to determine them (like trial location and time) were different in tests of each vaccine. All of the approved vaccines are highly effective and all significantly reduce the chances that you'll get sick from Covid-19. Fewer sick people means less spread of the virus. With a limited supply of vaccines, there's no sense in waiting for one or the other.
Now back to the question at hand. "There are no studies that 'mix and match' different vaccines, and we will probably never have such a study that actually measures how well they work to prevent disease, as such studies require tens of thousands of patients," says Eric Rubin, an infectious-disease professor at Harvard University and editor-in-chief of the prestigious New England Journal of Medicine. "We might, however, learn from small numbers of patients how potent an immune response can be elicited by mixed vaccination." In other words — and we know people are tired of hearing this — more study is needed.  Doses of Pfizer-BioNTech Covid-19 vaccine are prepared for injection. Photographer: KENZO TRIBOUILLARD/AFP Rubin says it's not clear whether a booster shot would be needed for any of the approved vaccines, since all seem to provide good protection. Of course, new strains of the virus could change that equation.
"It's probably too early, though, to know if boosting is going to help," says Rubin. Ross Kedl, an immunologist at the University of Colorado Anschutz School of Medicine, says there is some prior evidence that mixing vaccines can be a boon. "We have used this method in animal studies for decades, and it is well known that this promotes a much better immune response than when immunizing with the same vaccine twice," says Kedl.
The gist, he says, is that if you have two different approaches to generating immunity against the virus, there's a greater chance that it will work. In the U.K., trials are actually already underway to test combining the Pfizer vaccine with one from AstraZeneca, which, like J&J's, uses an adenovirus to spark an immune response against the coronavirus. That said, far more people will need to be vaccinated before there is serious consideration of a multi-vaccine approach. "It's a bit too early to be getting greedy about taking multiple vaccines just yet," says Kedl. "Let's all just be content with one for now until everyone gets a shot." Thanks to all of you for writing in this week! Next Sunday, we'll be answering the best question we receive again. So if you have any, we want to hear from you. Write to us at CovidQs@bloomberg.net — Kristen V. Brown | 
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