There isn't a magical number which means the vaccine is effective, and below that number it is ineffective. A vaccine is effective if it helps people deal with

There isn't a magical number which means the vaccine is effective, and below that number it is ineffective. A vaccine is effective if it helps people deal with a virus (i.e. lowering chance of hospitalisation/death). It doesn't magically become ineffective because it doesn't meet an arbitrary percentage of prevented cases. The Polio vaccine is "only" 99-100% effective. That means that it's not 100% effective (as in, there are cases where it doesn't prevent transmission/infection). According to the criterium laid out by GP this means it is an ineffective vaccine, just like the Covid vaccine.