Posted on June 26th, 2013 by
Two doses of the human papillomavirus (HPV) vaccine Gardasil may be just as effective in younger girls (preteens) as the recommended three doses given to older teens, according to the results of a study published in the Journal of the American Medical Association.
HPV is the most common sexually transmitted infection in the United States. There are more than 100 different strains of HPV. Some types of HPV cause warts on the hands or feet; others cause genital warts; and some have been linked with cancer, most notably cervical cancer. In fact, HPV is believed to be the leading cause of cervical cancer. Other HPV-related cancers include vulvar cancer, anal cancer, penile cancer, and certain types of head and neck cancer.
Gardasil® (quadrivalent human papillomavirus [types 6, 11, 16, 18] recombinant vaccine) is one of two FDA-approved vaccines that protect against the types of HPV associated with cervical cancer. The vaccine protects against four types of HPV—types 6, 11, 16, and 18. HPV types 16 and 18 cause roughly 70% of all cases of cervical cancer, and HPV types 6 and 11 account for roughly 90% of genital warts.
Although HPV vaccines are considered highly effective, less than one-third of U.S. teens receive all three doses, according to the U.S. Centers for Disease Control and Prevention’s National Immunization Survey of Teens. Some doctors have speculated that use of the vaccine may be impeded by cost. To determine whether two doses provide the same protection as three doses—thereby saving money—researchers conducted a randomized phase 3 study of 830 Canadian females between the ages of 9 and 26. The girls aged 9 to 13 were randomized to receive 3 doses of Gardasil at 0, 2, and 6 months or 2 doses at 0 and 6 months. Young women aged 16 to 26 received 3 doses of Gardasil at 0, 2, and 6 months. Antibody levels were measured at 0, 7, 18, 24, and 36 months.
The results indicated that the younger girls who got two doses of Gardasil appeared to make at least as many antibodies against the HPV strains as the teens and young women who received three doses. What’s more, their protection appeared to last just as long, up to three years after they started their shots.
The researchers cautioned that although the antibody response to two doses of the vaccine looks promising, it doesn’t prove that this approach protects against infections or cancers. A longer study is currently underway to determine whether two doses can offer the same level of protection as three. Thus far, though, the results are promising—and could prove to be cost-saving, which might lead to more girls being vaccinated. Until there is hard data, the researchers urge girls to continue to get all three doses.
Dobson SRM, McNeil S, Dionne M, et al. Immunogenicity of 2 doses of HPV vaccine in younger adolescents vs 3 doses in young women: A randomized clinical trial. JAMA. 2013; 309(17):1793-1802.
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