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Do you have any opinions on the difference twixt the two?

Moderna seems to perform a bit better, but that may actually be because they waited 4 weeks instead of 3. (My Moderna was 5 weeks because Texas froze and screwed up all the flights.) Immunologist agree: 16 weeks would have been better. But the vaccine makers were in a hurry, and lots of folks dying every day, so they made the interval short. Some immunologists consider them a three shot series, if you get a booster six months after the second one. The immune system creates germinal centers in the lymph nodes, where they evolve the antibody genes to make every possible antibody against all corona viruses (current or future!). That takes time to get going. If you wait the 16 weeks between vaccinations, it's all ready and the second exposure to antigen gives you a lot of antibodies and a lot of kinds of antibodies. So new variants can't escape them very much. Same thing for infection followed later by vaccination: Excellent immunity for people with working immune systems.

Booster brand probably makes little difference. But I would hold out for mRNA if I got J&J before. That rare thrombocytopenia side effect of J&J and AstraZeneca seems to be from the adenovirus they use to carry the spike RNA. mRNA has no adenovirus.

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