Both require two shots. We don’t get a choice which to take, we get what’s available in our area.
But if I get the Pfizer vaccine now, then 20 some days later…I go for my second one…does it HAVE to be the Pfizer one, or can it be the Moderna one?
I assume there are controls or records that show which one I had previously, but I wonder if there will be cases where the ‘other’ vaccine is all they have for the second shot.
I very much doubt that the people administering the vaccines will allow you to mix them. Unless there’s a formal decision to go with a single dose protocol, I think second doses will be prioritized. And slight variation in the timing of the second dose introduces less uncertainty than mixing different vaccines.
They work in a similar way, with the same antigen, so a priori you’d guess they might be interchangeable. But why introduce the uncertainty, both about efficacy and about risk, by doing something for which we have no data at all?
Yeah. While mixing would likely work, it hasn’t been studied and probably won’t be. My understanding is that when you get the first dose, the second is set aside for you.
With normal vaccinations, the drug approval is for a specific administration regime. So no mixing. Then, there is also an instruction for the real world: what do you do if previous history is unknown? Or if the interval has been too long? Etc.
And one of the things you’ll sometimes see is instructions for what to do if the patient has had a previous incomplete course of something else. Perhaps in the form “not known to cause problems”, or “only use one dose”.
Instructions like that are not necessarily an approved use. Sometimes they are a guide for a doctor to make an informed medical decision.
No kidding! See, for example the CDC recommended protocol for the two pneumonia vaccines. It’s an elaborate flowchart to decide which vaccines should be given, and the timing of them, and depending on whether you already had the PPSV23 before the PCV13 was approved and how long ago (because you’re supposed to get the PCV13 first), and how old you are and . . .
From the CDC: Interim Clinical Considerations for Use of mRNA COVID-19 Vaccines Currently Authorized in the United States
Either of the currently authorized mRNA COVID-19 vaccines can be used when indicated; ACIP does not state a product preference. However, these mRNA COVID-19 vaccines are not interchangeable with each other or with other COVID-19 vaccine products. The safety and efficacy of a mixed-product series have not been evaluated. Both doses of the series should be completed with the same product.
Bolding mine. This is consistent to what I have have seen and heard from various reporters. Bottom line: if your first dose is Pfizer your second dose should be as well. Same for Moderna.