Offsetting behaviour: vaccine edition

I'm more inclined to call stupidity over risk homeostasis on this one:

Some adolescent girls who get the HPV vaccine to prevent cervical cancer wrongly think they no longer need to practice safe sex, U.S. researchers said on Monday.

The study, published in the Archives of Pediatric & Adolescent Medicine, shows the need for better education about the vaccines and their limitations.

...The authors said the study was limited in that subjects came from a single urban clinic serving low-income clients so the findings may not apply to more general populations.

I wonder if anybody's running effects of vaccination on STD and condom use rates. It would be surprising if there weren't some drop-off in condom use with vaccination; the risk has indeed gone down. But I'd love to see numbers on it. Unfortunately, the kinds of things that would predict vaccination likely also predict sexual behaviour regardless of whether vaccination has taken place. Finding an exogenous instrument could be tough.