I think omicron was and is a game-changer. Prior to that, two doses of vaccine, for example, had very high efficacy against infection, and therefore transmission, as well as against severe outcomes.
When omicron came along, protection from two doses really waned over time and saw a real decline over time to 20% or less after six months. It is an immune-evasive variant. A third dose gives you a boost back up to 60% on average for protection against infection.