The WHO would prefer you not to take the vaccine if you are at low risk. While not a personal medical argument it's still in good faith. I am being cheeky by extending their booster logic to the whole vaccine - the WHO are probably thinking it but too ashamed to say it out loud.

https://www.cnbc.com/2021/09/21/who-repeats-warning-against-...

There are multiple vaccines too! The Australian regulator recommends against some vaccines for the young and up until recently they were the only ones available.

https://www.health.gov.au/initiatives-and-programs/covid-19-...

I'm sure there are plenty of other reasons - God forbid you might even have a religious objection to vaccines.