Study Compares 2 Doses of HPV Vaccine to 3May 2, 2013
Canadian researchers are studying whether 2 doses of the human papilloma virus (HPV) vaccine are as effective as the complete 3-dose series. Early results are promising, but the researchers themselves and other experts caution that more research needs to be done before fewer doses can be recommended.
Infection with HPV – a group of many different viruses – is the main risk factor for cervical cancer and pre-cancer and for genital warts. To be effective, the vaccine should be given before a person becomes exposed to HPV – usually through sexual activity. The American Cancer Society recommends the 3-dose vaccine for girls ages 11 to 18.
Reducing the number of doses needed would make the vaccine cheaper and easier to use, and could lead to more girls completing the series, especially in countries with limited financial resources.
The Canadian study included 830 girls, teens, and women. The researchers measured the body’s immune response to the vaccine by looking at the levels of antibodies to the virus in 3 groups: girls ages 9 – 13 who received 2 doses, girls ages 9 –13 who received 3 doses, and teens and women ages 16 – 26 who received 3 doses. All participants received the Gardasil vaccine, which protects against 4 strains of the virus.
The immune response of the girls who received 2 doses was just as good as that of the teens and women who received 3 doses. This was still true 3 years after vaccination.
The girls who received 2 doses also initially had about the same immune response as the girls who received 3 doses, but this did not last for all strains of the virus. By 2 to 3 years after vaccination, girls who received 2 doses had a reduced immune response to 2 HPV strains compared to girls who received 3 doses. It is not clear what this might mean in terms of long-term protection against HPV.
The study did not actually show whether the 2-dose series of the vaccine would protect against HPV infections, genital warts, or cervical cancers and pre-cancers. Longer term studies are needed to prove this, as well as to know if a 2-dose series would still offer the same level of protection beyond 3 years.
The study was published in the May 1, 2013 issue of the Journal of the American Medical Association.
An accompanying editorial calls the results “encouraging” but “preliminary.” Debbie Saslow, PhD, director of breast and gynecologic cancers at the American Cancer Society, said, “I know people hate to hear ‘promising but needs more research,’ but that's where we are with this. We should use this as an opportunity to encourage more girls to get vaccinated and to complete the 3-shot series.”
Immunogenicity of 2 Doses of HPV Vaccine in Younger Adolescents vs 3 Doses in Young Women. Published in the May 1, 2013 issue of the Journal of the American Medical Association (Vol. 309, No. 17). First author: Simon R. M. Dobson, MD, University of British Columbia, Vancouver, BC, Canada.