Why does Covid-19 vaccine protection wane?

Covid is surging across Europe, but what's causing it, and will the UK follow suit, or are Europe hot on our heels? What's the distinction between booster and 3'rd doses of vaccines and how does this factor into travel passes? Should vaccines be mandatory, and would vaccine passports work? Why does Covid-19 vaccine protection wane with time, and are we looking at a relentless cycle of boosters indefinitely, or will one be sufficient? Dr Chris Smith joins RNZ's Kim Hill for a catch-up all about the current state of Covid-19...