I don't quite understand the concern of the person whose booster is 6 months and 3 days before their trip to Italy. We are doing back-to-back tours of Italy in September and October. Our booster will be much older than 6 months by then. What is this requirement?
Most credible medical professionals that I've heard/read, citing data from Israel (where they are now giving 2nd boosters out), say that protection from boosters starts dropping after 3 months, and continues to drop quickly with each passing week. CDC is, I believe, mulling 2nd boosters after 5 months, but I've heard some epidemiologists saying that's too long to wait, instead recommending 3 months, and urging no more than 4 months, until further boosters. There is no clear consensus yet on the proper interval from the policy-makers. But lots of medical professionals are saying studies show it should not be 6 months until a 2nd booster, at least not for those vulnerable or in high-risk-of-exposure roles.
My spouse is a front line medical professional. She got the vaccine among the very first recipients in the USA, before Christmas of 2020 (we joke that she got her first Pfizer dose before President Biden did, and it's true). Her 2-shot regimen was completed in mid-January 2021. She got boosted late August 2021 (I'm a little behind her, but also an early adopter). I figure she is due for her 2nd booster NOW (over 4 months since her last jab). She expects to get that soon, I'll take mine as soon as they're available.
Point being: if you're feeling bullet-proof because you got your second jab, and got boosted too...but that was 6, 8, 9 months ago...stop feeling like you are still highly protected. Yes, you still have some level of protection, you're probably not going to get badly sick, hospitalized, etc. But you're not nearly as protected as you were soon after that second shot (or first booster).
I am not a medical professional and am not giving medical advice. But...check the medical professionals, you will see the issue of "waning protection" is real. Check the Israeli studies and see what they're doing.
OK now, back to concerns about where you will sit on the bus...