As someone with two doses of Moderna, plus a Moderna booster, I'm looking for a non-mRNA vaccine for my second booster.

I'm mostly focused on how well T-cell immunity will work on any new variants, and my reasoning goes as follows: T-cell reactivity is based on bits and pieces of an 'invader' presented to the T-cells by the cell under attack (cool little tendril-thing on the cell wall). The mRNA vaccines prompt cells to present analogs of the spike protein, which is just one part of the virus Both convalescent and attenuated/killed-virus vaccines, a more-random selection of the bits and pieces of the virus are presented, conveying an immunity to a broader selection of the virus. From testing, it appears that there's lower antibody effectiveness in the J&J — but I would expect T-cell immunity to a broader set of variants - just because it's to those random bits and pieces, and strain variability seems to be mostly in the spike.

I'm no kind of bio or medical pro, just know enough to realize that one cannot simply reason from first principles - so I could easily be (and probably am) wrong.