The Gardasil vaccine has been available for some time now as a potentially useful tool to prevent HPV infections, and the increased risk of cervical cancer from those infections. In 2007 Australia became the first country to run a nation-wide vaccination program for young girls. Seven years later, the results became available as to how effective it was, published in the British Medical Journal.
In a nutshell, they found that women who have developed high-grade cervical lesions (meaning, they found advanced abnormal cells) were less likely to have received all 3 scheduled vaccines. This means that out of the thousands of women studied, there was a 46% effectiveness rate. Even better (57%) for younger girls aged 15-19. If only two of the scheduled vaccines were given, the effectiveness rate was only 21%. In the world of vaccines, that's not a particularly bad effectiveness rate, but it's clear that you can more than double it with just one more injection.
The study found that 22 girls would have to be vaccinated with all 3 courses in order to prevent 1 of them from having any kind of abnormality on her future PAP smears. That's a pretty impressive result!
However, it's worth to note that there was some conflict of interest declared in the study - some of the authors have worked for the makers of the vaccine. Nonetheless, given how large this study was, a conclusion in support of the full course can definintely be made. If you decide to do the vaccine (and we recommend you do you due diligence and research all your options first), it's definitely best to go through the whole course - the benefit is highest only with all 3, so don't stop after just one or two.
Here's the link to he Medscape article if you want to read more: http://www.medscape.com/viewarticle/821591?nlid=51085_1842&src=wnl_edit_medp_wir&uac=191531CY&spon=17