My wife and I got the first HPV vaccine recently. We're not sure we ever got it previously since we were slightly too old when it first came out to get it in our regular shots as kids, but she's never slept with anybody else, and I've only slept with one other person, once, so we're both in a rare demographic of people over 30 for whom the vaccine might help.

We're seeking to open our relationship soon, and tbh I'm chomping at the bit to start, but also I want to be safe. I can't find anything on the efficacy of the vaccine after the first or second shots in the series. Obviously the third is supposed to make it effectively permanent, but I'm trying to understand what happens in between. Shouldn't it be perhaps equally effective but for a shorter time period after the first, or do I need all three to have any real protection?