| In this week's edition of the Covid Q&A, we look at whether it's OK to mix doses of different Covid-19 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 Phillip, who has received his first dose of a Covid-19 vaccine from Pfizer-BioNTech. As vaccine supplies dwindle, he wonders whether it's safe to receive the Moderna vaccine instead as his second dose. He asks:
What would happen if only Moderna doses are available? Are they interchangeable? Is it safe to switch?
This is a great question, and one that many other newsletter readers raised this past week. The Moderna and Pfizer vaccines are remarkably similar. Each uses a technology called messenger RNA (mRNA) to teach cells how to make a protein that will trigger an immune response to the virus, preparing the body for a potential encounter with it in the wild. "The Pfizer and Moderna vaccines carry exactly the same viral genetic material, and they differ only in how they get delivered to our cells once they are introduced in our body," says Ramon Lorenzo Redondo, a molecular virologist at Northwestern University's Feinberg School of Medicine. The first dose primes the immune system, while the second one helps to rev it up.  Shots of the Moderna vaccine are prepared in Garden City, New York. Photographer: Johnny Milano/Bloomberg "You can think of a paperback and a hardcover book. They are different in the outside but contain exactly the same information," he says. "This genetic material, once introduced in our cells, makes them produce identical viral proteins. Therefore, it is expected that they will generate the same immune response."
The issue, though, is that mixing vaccine doses from different drugmakers has yet to be studied. "There is no data on this, so you need to use judgment," says John Moore, a vaccine researcher at Cornell University. "We would like to have data rather than use judgment. But we don't have the luxury of data."
In fact, just last week, the U.S. Centers for Disease Control and Prevention issued updated guidance advising that in "exceptional situations" it is acceptable to administer a different vaccine at a minimum interval of 28 days between doses.
Moore says that if there were no alternative, he wouldn't hesitate to mix the two vaccines. "We have to accept that we are in somewhat of an abnormal situation at the moment," he says.
Postscript: Barbara from Pitcairn, Pennsylvania, wrote in with a follow-up question to last week's newsletter. A reader had wondered whether it was problematic if their second dose of is delayed. Tony Moody, an infectious-disease expert at Duke University, said it was probably fine. "We know that the timing between vaccinations is more about the minimum time between doses, not maximum," he said.
Barbara's dilemma, she wrote, is that she was scheduled to get her second shot before the three weeks recommended for the Pfizer vaccine. "Is it alright to take it four days before the three-week recommendation?" she asked. We followed up with Moody. "Vaccine trials are generally run with windows of time around each shot, and the data are analyzed together," he said. "So we don't have a good way to know if 17 days versus 21 days really makes any difference." There simply isn't data to definitely answer Barbara's question, Moody said. Still, he said, getting a second dose just a few days early should be fine. "There shouldn't be anything wrong about being a bit early, and certainly better to be early than not at all," he said. "I'm not sure I'd push much earlier than that."
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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