How effective are the human papillomaviruses (HPV) vaccines?

Gardasil and Cervarix are highly effective in preventing infection with the types of human papillomaviruses (HPV) they target. Studies have shown that both Gardasil and Cervarix prevent nearly 100 percent of the precancerous cervical cell changes caused by the types of HPV targeted by the vaccine for up to 4 years after vaccination among women who were not infected at the time of vaccination.

This answer is based on source information from the National Cancer Institute.