Earlier quoted context omitted.
That's a long way of saying that you're changing the definition of X between assertions. "Effective" doesn't mean the same thing in both contexts.
Vaccines are highly, but not perfectly, effective. The definition is the same, it's the adjective that's changing. For true immunity without anyone else needing to be vaccinated they'd need to be perfectly effective. Since they're not, more of the community needs to be vaccinated. The less effective they are (or the more widespread or infectious the virus is) the larger the proportion of the population that needs to…
If you have the polio vaccine, you basically cannot get polio and have lifelong immunity. That's an effective vaccine. It doesn't require everyone else to get frequent booster shots. The current covid vaccine does not measure up to the polio vaccine.