There has been some confusion among community members regarding whether or not adults who have completed only their initial vaccination cycle (ie, no booster) more than six months ago can enter Italy without having to self-isolate.
I spoke to an operator at the health ministry who told me that for entry purposes, the vaccination certificate validity is 270 days from the date of completion of the initial vaccination cycle (ie, the one dose of J&J or the second dose of a two-dose vaccine).
For entry purposes for people vaccinated abroad, the vaccination certificate validity is not extended beyond 270 days, even if it is accompanied by a negative Covid antigen or PCR test.
Self-isolation requirements, however, vary based on the country that the visitor is traveling from. For example, visitors from countries on List C (EU) can enter Italy with a basic green pass; that is, a negative result on a Covid rapid antigen or PCR test. However, visitors from countries on List D have to show both a negative result on a test as well as a valid vaccination certificate (or recovery certificate for visitors from Canada, Japan or the US).
To check entry regulations into Italy, go to the foreign ministry's interactive survey here.