By Rachael Rettner, Senior Writer

Published: 11/04/2013 07:32 AM EST on LiveScience


A single dose of the human papillomavirus (HPV) vaccine may be enough to protect women against infection with the virus over the long-term, a new study from Costa Rica suggests.


In the study, women who received one, two, or the standard three doses of the HPV vaccine[1] all produced antibodies against the virus that remained at stable levels in their bodies for four years after vaccination.


In addition, women who received one dose of the vaccine had an immune response that was five to nine times stronger than that seen in women who were infected with HPV naturally. [5 Dangerous Vaccination Myths[2] ]


However, women who received only one vaccine dose produced antibodies at levels lower than those of women given two or three doses. Still, researchers don't yet know what level would be "good enough" for protection. It could be that the immune response generated after two or three doses is much more than a person needs to be protected.


In fact, a previous study by the same researchers suggested just that: women in that study who received all three HPV doses were protected against infection[3] no better than those who received one or two.


"We don't know what the minimum required [antibody level] might be for protection," said study researcher Mahboobeh Safaeian, of the National Cancer Institute. But in the new study, "the fact that [antibody levels] remained higher than natural infection, and remained stable, is a promising finding," Safaeian said.



More research is needed to validate the findings in other populations, and over longer periods. Until then, people in the United States should get the recommended three doses[4] , Safaeian said.


The HPV vaccine is currently licensed for people between ages 9 and 26, and it works best if given before people become sexually active. It is used to protect against cervical cancer, which is often caused by HPV infections.


If the number of recommended doses could be lowered from three to just two or one, it would simplify the vaccination schedule, Safaeian said. Studies show that in the United States, only about a third of teen girls get all three doses.


Fewer doses would also reduce the cost of vaccination, which has been a barrier to administering the vaccine to people in developing countries, Safaeian said.


Fewer doses?


The new study is part of a larger trial in which more than 3,500 women ages 18 to 25 were assigned to receive three doses of the Cervarix HPV vaccine -- a vaccine that protects against the cancer-causing strains HPV 16 and HPV 18. However, about 20 percent of participants did not end up receiving all three doses.


The researchers analyzed blood samples taken from 78 women who received one dose, 192 women who received two doses, and 120 who received three doses of the HPV vaccine, as well as 113 women who did not receive the HPV vaccine, but had previously been infected with HPV[5] .


Four years after vaccination, all of these women had antibodies against HPV 16 and HPV 18 in their blood.


Those who received two doses six months apart had about the same levels of antibodies as those who received all three doses.


Those who received one or two doses had antibody levels that were five to 24 times higher than the women who had been naturally infected.


Future research


"Because of the challenges associated with giving three doses [of HPV vaccine] I think it's very welcomed to see that there's a possibility that the vaccine may not need to be given in a three dose schedule," said Dr. Mike Brady, a pediatric infectious disease specialist at Nationwide Children's Hospital in Columbus, Ohio.


But before doctors would feel comfortable giving fewer than three doses, researchers will need to show that vaccination with one or two doses also reduces the risk of developing the early signs of cervical cancer[6] , Brady said. Currently, there's lots of evidence that three doses protects against cervical cancer, but the same cannot be said for fewer doses.


When researchers initially developed the HPV vaccine, which is a synthetic vaccine, they didn't know exactly how many doses would be needed for protection, Brady said. So they modeled the dosing schedule after that of other synthetic vaccines such as Hepatitis B, which require more than one dose.


However, the HPV vaccine is not like other synthetic vaccines -- it is made from so-called virus-like particles -- and it may turn out that fewer doses than initially thought are needed, Brady said.


Because the study evaluated only the Cervarix vaccine, it's not clear whether the findings apply to the Gardasil vaccine[7] , another HPV vaccine that protects against four strains of HPV, and is more widely used in the United States.


The study is published in the November issue of the journal Cancer Prevention Research.


Follow Rachael Rettner @RachaelRettner . FollowLiveScience @livescience , Facebook[10] & Google+ . Original article on LiveScience[12] .[8] [9] [11]



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  • An Abnormal Pap Test Means You Have High-Risk HPV


    Pap tests are the commonly accepted screening to prevent cervical cancer. A doctor scrapes a cell culture from a woman's cervix and then examines the cells for signs of abnormality. But just because a few of those cells appear abnormal, requiring further screening, doesn't necessarily mean that you've got a cancer-causing strain of HPV -- that's only one potential cause. "The difference could be due to local irritation, a non-HPV infection, a low-risk HPV type, or even a mistake in the preparation of the cell sample," writes the American Sexual Health Association.




  • Condom Use Prevents HPV


    HPV is passed via skin contact, rather than bodily fluid. For that reason, condoms can <em>lower</em> the risk of the disease, but they are not a sufficiently preventive measure, as they are for viruses like HIV and bacteria like gonorrhea.




  • Oral Sex Is Safe From Cancer Risk


    While the HPV-cancer connection most often relates to cervical health, a 2011 <em>Journal of Clinical Oncology</em> study found what doctors have long observed: There has been a surge in HPV-associated oral cancers. In fact, between 1988 and 2004, <a href="http://nyp.org/enews/oral-sex-hpv.html">HPV-associated oropharyngeal cancers rose 225 percent</a>. Oral sex is the primary culprit, making cancer screening of the mouth and esophagus another important test while visiting the doctor.




  • HPV Vaccine Means I Don't Have To Worry About Cervical Cancer


    The HPV vaccine protects against four strains of the virus that are most often associated with cancer and genital warts, but that doesn't mean it prevents cancer entirely. One concern within the medical community is that the vaccination will <a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3104818/">provide a false sense of security</a> and prevent innoculated men and women from receiving regular cancer screening. "Clearer information is needed concerning the incomplete protection offered by the vaccine, and that cervical screening will still be required," wrote a group of British public health researchers in the <em>Journal of Medical Screening</em>.




  • HPV Is A Serious, Life-Long Condition


    About 90 percent of HPV infections <a href="http://www.cdc.gov/vaccines/pubs/surv-manual/chpt05-hpv.pdf">are resolved by the body's immune system</a>.




  • Genital Warts Can Be Pre-Cancerous


    Some strains of HPV (<a href="http://www.ashastd.org/std-sti/hpv/myths-and-misconceptions.html">"low risk" types 6, 11, 42, 43 and 44</a>) cause benign growths known as genital warts and other strains (types 16, 18, 31 and 45) cause cancer of the cervix, vulva, vagina, penis, anus, or an area called the oropharynx, which includes the back of the throat, the base of the tongue and the tonsils. But that doesn't mean that one leads to the other -- genital warts, <a href="http://www.cdc.gov/std/hpv/stdfact-hpv.htm">which affect about one percent of the sexually active U.S. adult population</a> -- do not lead to cancer.




  • The HPV Vaccine Is For Girls


    The first HPV-preventive vaccine on the market, Gardasil, was approved by the FDA for use in girls in 2006 and <a href="http://www.fda.gov/newsevents/newsroom/pressannouncements/ucm187003.htm">in boys three years later</a>. What's more, there are <em>two</em> FDA-approved vaccines for girls and women: Gardasil and Cervarix; while only Gardasil is available to boys and men. Still, HPV vaccination is the responsibility of all. Both the Centers for Disease Control and Prevention and the American Academy of Pediatrics both recommend the vaccine for boys for two primary reasons. First, inoculated boys will not be vectors for the disease, which can contribute to herd immunity and prevent dangerous infection in women. But more, the incidence of HPV-associated cancers that affect men is also growing, including anal and penile cancer and cancers of the mouth and throat.




  • Girls Who Receive An HPV Vaccine Will Be More Sexually Active


    <a href="http://www.time.com/time/nation/article/0,8599,1206813,00.html">Despite initial hand-wringing</a> that the vaccine could lead to promiscuity, a study of adolescent girls -- both those who had been inoculated and those who hadn't -- found no evidence that those who received the vaccination engaged in riskier behavior: they were no more likely to contract an STI, become pregnant or even seek out contraception -- a measure of intended sexual activity. "HPV vaccination in the recommended ages was not associated with increased sexual activity–related outcome rates," <a href="http://pediatrics.aappublications.org/content/early/2012/10/10/peds.2012-1516.abstract%7CSexual">the researchers wrote</a>.