I live in the US. I'm 37 years old and fairly healthy though not in perfect health with regard to covid risk (I'm a little overweight and have a blood clot in my personal history). I've never gotten covid as far as I know. I got my second dose of Pfizer on April 21, 2021. I am tentatively planning to get a booster shot soon, but I'm not certain whether I should or not. When should I get my booster, which vaccine should I get for my booster, and why? [Edit 11/19/21: and should I get a booster at all?]
Considerations are as follows. A lot of the following comes from the comments and responses to this question about booster effectiveness and in particular the comments by Elizabeth. This other question about which booster shot to get is also relevant.
Getting any booster would almost certainly increase my immune defense to covid substantial and have no long-term or major direct side effects. Pfizer and Moderna boosters are approximately functionally equivalent. A J&J booster might be slightly worse or slightly better.
But getting a booster now might have indirect long-term side effects. In particular, Elizabeth suggested that there could be a limit to the number of mRNA shots that one can get in a lifetime, either against Covid or against all diseases in the aggregate, because the body could develop immune reaction to mRNA vaccines, or side effects could increase with the number of doses. If this is true, then it might make sense to wait on a booster, until a more effective booster comes out, for instance one specifically tailored to the delta variant. Or to wait until the timing of the booster is maximally efficient to improve immune function.
Also, there may be legal or administrative restrictions on getting more than one booster, so again it may make sense to wait until a better booster is available. Even if I were willing to lie to get around these restrictions, such lies might prevent me from having an accurate vaccination card, which could cause problems down the line.
And I've also seen the theory that mixing and matching vaccines may be more effective than getting all the same vaccine, because it would promote a more diverse immune response. But that the three vaccines available in the US are basically equivalent in this regard. So it might make sense to wait until another vaccine is available in the US or even travel to another country to get another vaccine (not a trip solely for that purpose, but I enjoy travel anyway and it would be a nice excuse to do more travel).
My inclination is that I'm probably overthinking this and am biased towards inaction because I don't relish putting up with a day or so of vaccine side effects. It's probably best to get a booster as soon as reasonably possible, and I should probably just get a Pfizer booster to match my initial Pfizer vaccination as I believe that's the current recommendation of the medical establishment. The immediate benefit of greater immunity now outweighs the concerns above, especially given that I have been returning to a more normal life (traveling, going to bars and restaurants, riding public transit, etc.) and therefore the extra immunity is immediately quite relevant. And we're unlikely to have much better evidence on the questions above anytime soon, nor are we likely to have a booster specifically for delta or other new variants anytime soon.
But I'm not sure, a part of me is tempted to wait because I feel like I'm pretty well protected with two shots of Pfizer, and I would be giving up an unknown and possibly quite large amount of future utility by getting a booster now. For instance, I could be using up my lifetime ability to take mRNA vaccines, and there could be a future mRNA vaccine against a much deadlier new future pandemic or against HIV or against cancer or against a much deadlier strain of Covid. [edit 11/27/21 -- original antigenic sin makes this last possibility (vaccine against a deadlier strain of covid) less relevant. Thank you, evhub for this comment pointing that out.]