Excellent question, and I think a lot of us are wishing we had more data on this—unfortunately, there is very little data so far. But here's my take:
- If you had J & J for your first shot, I think there's enough evidence now to say it's probably (p = 0.7?) better to get Pfizer or Moderna for your booster.
- If you had Pfizer / Moderna for your first two shots, my instinct is that J & J might be the better choice, because there's an argument from microbiology that mixing types might produce a more robust response.
- If you had Pfizer / Moderna and want an mRNA shot for your booster, I don't think it'll make much difference which one you get: they're very similar.
There's an argument to be made that absent strong reasons to do otherwise, it's best to follow standard practice (in this case, to get the same brand of booster as the original shots) simply because you'll be in a larger, better-studied cohort.
A couple of sources, such as they are:
“But something has really become clear: The mixing really is most impactful when you have a DNA/adenovirus vaccine first followed by the mRNA vaccine,” Gandhi said. WaPo
The study’s researchers warned against using the findings to conclude that any one combination of vaccines was better. The study “was not powered or designed to compare between groups,” said Dr. Kirsten E. Lyke, a professor at the University of Maryland School of Medicine, who presented the data. NYTimes