The HPV vaccine (Gardasil) may also protect older women against the human papillomavirus, researchers say.
In an off-label trial sponsored by drugmaker Merck, the vaccine - which is already approved for women ages 9 to 26 - was effective in those aged 24 to 45 who were not already infected with HPV, according to Nubia Munoz, M.D., of the National Institute of Cancer of Colombia in Bogota, and colleagues.
Although the vaccine is approved only for younger women, a rising proportion of older women are at risk of HPV infection because of rising age at first marriage and increasing divorce rates, the researchers wrote online in The Lancet.
So they tested the safety, immunogenicity, and efficacy of the quadrivalent HPV vaccine (types 6, 11, 16, and 18) in women ages 24 to 45.
A total of 3,819 women with no history of genital warts or cervical disease were enrolled in the ongoing multicenter, parallel, randomized, controlled, double-blind study.
About half received the vaccine and half received placebo.
The researchers found that among the per-protocol population (3,222 women), the vaccine’s efficacy against all strains of the virus in this age group was 90.5% (95% CI 73.7% to 97.5%).
There were four cases of infection in the vaccine group compared with 41 in the placebo group.
The vaccine’s efficacy against the two most common HPV strains - 16 and 18 - was 83.1% (95% CI 50.6% to 95.8%). Four cases occurred in the vaccine group compared with 23 in the placebo group.
The researchers also evaluated the intention-to-treat population, which included women who had not been completely vaccinated or had pre-existing HPV infection.
When these women were included in the analysis, vaccine efficacy against all four types of HPV was 31%, and against strains 16 and 18 was 24%.
The lower efficacy when this population was included suggests that the “public health effect of vaccinating women ages 25 to 45 will be smaller than that recorded after vaccinating susceptible adolescents,” the researchers said.
So the maximum effect of vaccinating older women “will be achieved in those who are susceptible to infection and . . . not previously exposed.”
But they noted that about two-thirds of women in their study were susceptible to all four vaccine HPV types.
“Most women who were HPV-positive were positive to only one HPV type,” they said, “meaning that the quadrivalent HPV vaccine could still potentially benefit these women via protection against vaccine HPV types they are not infected with.”
The study was limited in that the extent of disease that will be prevented by vaccinating women between ages 25 and 45 outside of a clinical trial is unknown.
It was also limited by a mean follow-up of only two years.
The study was supported by Merck.
Dr. Munoz reported receiving honoraria from Merck and Sanofi-Pasteur MSD, and is a member of the Merck global advisory board for HPV vaccine as well as a member of Sanofi-Pasteur MSD HPV steering committee.
Dr. Luna has received travel, speaker, and investigator grants from Sanofi-Pasteur MSD.
Dr. Monsonego reported working on previous HPV vaccine studies for Merck and GlaxoSmithKline, and is on the medical advisory board for GlaxoSmithKline, Geneprobe, Sanofi-Pasteur MSD, Roche, and Abbott.
Dr. Ault reported working on vaccine studies for Merck and GlaxoSmithKline and has been a consultant to Merck.
Some authors are members of the Merck HPV steering committee and some are employees of Merck and potentially own stock and/or stock options in the company.
Primary source: The Lancet
Munoz N, et al “Safety, immunogenicity, and efficacy of quadrivalent human papillomavirus (types 6, 11, 16, 18) recombinant vaccine in women aged 24-45 years: a randomized, double-blind trial” Lancet 2009; DOI:10.1016/S0140-6736(09)60691-7.