Suppose there are two vaccines available. Suppose also that:

• Both vaccines have been shown to be 90% effective in double-blind clinical trials.
• Vaccine X has rather unpleasant side effects, which disappear after about 24 hours. Vaccine Y appears to have no short term side effects at all.
• Both vaccines are identical in all other relevant ways you can think of — cost, probability of long-term side effects, possibility of collateral benefits, etc.

Which vaccine do you prefer to receive, X or Y?

I’ll give my answer in a few days, or chime in sooner if someone else gives my answer first.

Hat tip to the ever-thoughtful Romans Pancs, who emailed me the relevant analysis.

Edited to add: Well, that didn’t take long. Jim Ancona nailed it in comment #2 — and expressed it so clearly that I feel no need to explain it in any words other than his.

I also want to commend the first of the two answers in Dave’s comment #1, which brings up another factor that hadn’t occurred to me. Of course (in Dave’s scenario) you won’t be the only one thinking this way, so it’s not clear that in equilibrium you’ll prefer X, but it is clear that some people will prefer X for Dave’s reason. Once enough of them have chosen X, you can be indifferent between X and Y.