@wishgranter14 @BowsacNoodle @Holohoots @TrevorGoodchild @vic This is not accurate. The only real 'one and done' vaccine is tetanus. All the others have failure rates including the big ones(mmr *and especially polio*). There were multiple kinds of polio vaccines with different levels of protection, even.

Many vaccines require multiple doses, for example the recommended DTaP series is 5 doses.