What information are you using to determine that HPV “doesn’t work that way”? I searched around for herd immunity information regarding HPV, and can find no indication that herd immunity wouldn’t be a factor with HPV.
It’s not a communicable disease for one, Charles.
And, unlike tetanus, a small segment of the population is responsible for the vast majority of the infections. Herd immunity relies on certain assumptions, one of which that there exist no significant variances in disease vectors.