22.3 percent with two lifetime sexual partners, and 31.5 percent with three or more lifetime sexual partners experience at least one HPV infection.

HPV infection is often asymptomatic, but it may lead to the presence of cervical lesions or warts in some individuals. HPV infection is considered transient and usually lasts between 4 and 20 months in healthy individuals (Trottier and Franco, 2006). High-risk types of HPV such as HPV16 and HPV18 carrying the greatest risk of persistent infection constitute the most important risk factor for cervical cell abnormalities and invasive cervical cancer (Molano et al., 2003). Various approaches such as cryotherapy, electrocautery, surgical excision, and topical therapies have been used to treat HPV-associated lesions and warts (CDC, 2002).

Research and development of an HPV vaccine was spurred by evidence that inactivated bovine papillomavirus (BPV) could immunize cattle against BPV infection in the 1980s (Jarrett et al., 1990). However, owing to the oncogenic nature of HPV, live attenuated or inactivated vaccines could not be safely developed for humans (Schiller et al., 2008). In the 1990s, researchers found that inoculation with virus-like particles (VLPs) developed from the L1 protein of specific papillomaviruses (PVs) could protect against PV infection, but the protection is not universal for all HPVs (Schiller and Lowy, 1996).

Currently, two vaccines are licensed in the United States to prevent diseases caused by HPV infection. The quadrivalent vaccine, Gardasil (Merck & Co., Inc.) (HPV4), was licensed in 2006 by the Food and Drug Administration (FDA) to protect girls and women age 9 through 26 against angogenital warts and cancers (vulvar, vaginal, cervical, and anal) caused by HPV6, 11, 16, and 18 (CDC, 2007). Each 0.5 mL dose contains 20 µg each of HPV 6 and HPV 18 L1 protein and 40 µg each of HPV 11 and HPV 16 L1 protein (CDC, 2007). It also contains 225 µg of amorphous aluminum hydroxyphosphate sulfate (adjuvant), sodium chloride, L-histidine, polysorbate 80, sodium borate, and water (CDC, 2007). In 2009, Gardasil was also approved for use in males aged 9–26 years for the prevention of anal cancer and genital warts; however, although it is licensed for use in the same schedule and composition, as of May 2010, the Advisory Committee on Immunization Practices did not recommend routine vaccination in this population (CDC, 2010b). The bivalent vaccine, Cervarix (GlaxoSmithKline Biologicals) (HPV2), was also licensed in 2009 by the FDA to protect girls and women age 10 through 25 against HPV 16 and 18 (CDC, 2010a). Each dose of Cervarix is 0.5 mL and contains 20 µg each of HPV 16 and HPV 18 as well as 500 µg of aluminum hydroxide, 50 µg of 3-O-desacyl-4’monophosphoryl lipid A (adjuvant), sodium chloride, sodium dihydrogen phosphate dehydrate, and water (CDC, 2010a). Both vaccines are recommended in a three-dose series of intramuscular



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