The Gardasil vaccine is extremely effective at preventing infection from four strains of the human papilloma virus that cause cervical cancer and genital warts, according to WebMD. Gardasil is 100 percent effective at preventing infection of HPV-16 and HPV-18, which are the strains most likely to cause cancer.
If women have already been exposed to HPV-16 or HPV-18, Gardasil does not protect them against infection and possible cervical cancer, explains WedMD. While Gardasil does not protect women against all strains of HPV, there is evidence that the vaccine may offer protection from strains other than HPV-16 and HPV-18. The Gardasil vaccine has been shown to be effective against strains 16 and 18 for at least four years.