Good visual on the impact of boosters in the UK. This chart shows the estimated chance of catching COVID over a 1-year period (based on current UK prevalence) adjusted for the time since the 2nd dose and after a booster:
https://twitter.com/user/status/1461374201474998275
So you can see waning protection up to 6 months and then the 3rd dose gives fantastic immunity that is hopefully more durable.
Related to that, here's a Moderna Phase 1 participant. He received his first round of vaccinations (low dose Moderna) in March/April of 2020 and then a booster a year later. Now 8 months after his 3rd dose, his antibody levels are still above 2,500 U/ml (which is in line with the average response soon after a 2nd dose):
https://twitter.com/user/status/1460630344672034824