The BEST COVID19 vaccines under ideal conditions with self-selected volunteers screened for potential complications, are 94% effective, meaning 6% of certified "healthy" vaccinated people can still get infected. Among the general population this is, of course, considerably higher.
Vaccines are effective in stopping epidemics by reducing the average transmission rate (how many people each infectee infects) to well below 1. For the Alpha variant, this meant that 70% reduction in transmission would cause an exponential decline in infections. This can be achieved by vaccinating 100% of people with a 70% vaccine, 70% of people with a 100% vaccine, or 85% vaccinated with an 85% vaccine.
OR, everybody distancing and wearing masks PROPERLY, 100% of the time, and never coughing, sneezing, singing or talking loud. (covering nose and mouth, with no gaps around the cheeks - apparently a concept beyond the average citizen).
BUT! I don wanna! WAAHH! cry the ostensibly adult babies
Enter Delta. Much more infectious, raising the bar. We have a moving target, with mutations driven by the unvaccinated. Now we need 90% of people vaccinated with a 90% vaccine, AND masks in crowded places.