[PaulDavisThe1st]

"wearing pants" is, i believe, an allusion to "engage in adult social behavior", not a literal exhortation to cover one's legs.

the vaccine technology is both new and old. most vaccines come with various levels of immediate side effects, and anecdotally, the COVID-19 ones seems to be at least as safe in this regard as any other vaccines that have been administered to hundreds of millions of people. It is true that the long term effects are unknown, and there could be some. However, most immunologists seem to be of the consensus that most after-effects from vaccines take place within a fairly narrow window, which we are now firmly out of for very large numbers of people. That doesn't rule out some hitherto unknown and unforeseen long side effect, but it does make it rather unlikely.

Consequently, taking the vaccine is reasonably considered "adult social behavior" at this point, and is therefore analogous to "wearing pants".