September 9th, 2011
06:28 PM ET
A new study suggests that girls and women may not need all three recommended doses of HPV vaccine to get the necessary protection to prevent cervical cancer.
The human papillomavirus is the most common sexually transmitted disease according to the Centers for Disease control and Prevention and the second leading cause of female cancer mortality worldwide, according to the World Health Organization.
There are currently two FDA-approved vaccines to protect against the types of HPV that cause most cervical cancers, Gardasil and Cervarix. While each vaccine uses different substances to rev up the immune system, both are given as shots and must be received in three doses over a six-month period, according to the manufacturers.
The cost of the vaccine has been an issue for poorer countries. But even in wealthier countries like the United States, getting girls and young women to come back for all three shots has proven to be quite a hurdle. Only 32% of 13- to 17-year-old girls get all three doses according to the Centers for Disease Control and Prevention.
So researchers wanted to find out if young women can get the same protection with fewer doses of the vaccine. Scientists studied more than 7,000 18-25 year-old women in Costa Rica who were already sexually active and were scheduled to get doses of the vaccine Cervarix or another (non-HPV) vaccine.After following the women for four years they found "two doses working really well in the short term," says lead study author Aimee Kreimer of the National Cancer Institute. She says even a single dose showed the same protection from cancer as three doses, but Kreimer says she's more cautious about the one-dose vaccine because usually to get adequate protection from a vaccine, people are given one dose to prime the body and then a booster to reinforce the immunity.
The study was published in the Journal of the National Cancer Institute, which is a publication of Oxford Press, not the NCI.
Gardasil was first approved in 2006 for females age 9 to 26. Two years ago the FDA also approved the use of this vaccine in 9- to 26-year-old males, for the prevention of warts caused by HPV.
In 2009, the FDA approved Cervarix. It is licensed for use in females aged 10 to 25. The idea is to vaccinate young people before they become sexually active and could be infected.
The study in Costa Rica was started five years before Cervarix was approved and data from this clinical trial contributed to the approval of this vaccine, says Kreimer. She says researchers studied only this vaccine because clinical trials are very expensive and if they had included Gardasil the trial would have been too expensive overall.
However, the study authors do cite a study that found girls who received two and three doses of Gardasil has similar antibody titers (a measure of antibodies) several years after receiving the vaccinations. Merck, the manufacturer of Gardasil, tells CNN in a statement that based on the studies it has conducted, Merck continues to recommend the three-dose regimen for Gardasil.
Cosette Marie Wheeler, a professor in the University of New Mexico's Department of Pathology and Obstetrics and Gynecology, has studied HPV for more than 20 years. She writes in an accompanying editorial in JNCI that the results of the Costa Rican study must be viewed with caution in part because the study was composed largely of sexually active women aged 18 to 25 years.
Kreimer agrees, particularly because the women in this trial were healthy. "This should also be replicated in other populations," she says.
Perhaps the best way to confirm that fewer doses can provide very good protection against cervical cancer would be to conduct larger studies, as Wheeler puts it in her editorial, "to evaluate the efficacy of one-, two-, and three-dose regimens in young adolescent girls." But she recognizes that this is expensive, so she suggests girls and women who have already received some of the vaccine should be followed and studied to determine how effective the vaccine is for them.
More than 80% of cervical cancers occur in developing countries according to the WHO, because most women in those areas do not have the opportunity to be screened for early signs of cancer using a pap smear. Vaccines could prevent girls from being infected with the virus in the first place.
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