What matters is whether the R0 > 1 or < 1. Vaccine help reduce the R0, but if they are not effective enough to reduce transmission so that R0 become < 1, the e

What matters is whether the R0 > 1 or < 1. Vaccine help reduce the R0, but if they are not effective enough to reduce transmission so that R0 become < 1, the epidemic go on and almost everyone will still get infected (albeit more slowly). Let's take an original R0 of 10 to make the number round : meaning that one person would normally transmit to 10 other people. Let's say they vaccine are 90% effective at reducing transmission (1 vaccinated person transmit it to only one whereas it would have transmitted it to 10 if non-vaccinated). It mean that even if you have 100% vaccinated population one person will still transmit to one other people and the epidemic can go on (combined R0 of 1). We are dangerously close to the threshold where vaccine alone won't suffice to stop the epidemic. And the more active cases you currently have, the more chance there are for vaccine resistant mutation, or higher transmissible mutation to occur. Humanity is currently running on luck, we are one "Antibody-dependent enhancement" mutation away from turning the vaccine from our greatest weapon to our greatest weakness, and by vaccinating while not taking the necessary measures to reduce the number of active cases we are pressure selecting for it to happen.