The original strain upon which the current vaccines are based has basically died out and Delta carries a lot more viral load (400x or so?), which changes the game a bit.

In comparison, smallpox virus was pretty much the same across centuries, so it was comparably easy to target and eradicate. No animal hosts helped, too.

Pfizer themselves have performed studies that show immunity declining. I do know that they tell from the levels of antibodies seen in blood but I don’t know if the effect of variants has been measured (if it can be measured at all without unethical experiments).