Women who receive just one dose of the human papillomavirus (HPV) vaccine may get long-lasting protection from contracting the sexually transmitted disease.
A new study published in the Nov. 2013 issue of Cancer Prevention Research found that women who received one vaccine instead of the recommended three shots had stable levels of antibodies that help prevent the most virulent strains of HPV linked to cervical and other cancers, four years after receiving the jab.
"Our findings suggest promise for simplified vaccine administration schedules that might be cheaper, simpler, and more likely to be implemented around the world," study author Mahboobeh Safaeian, an investigator in the Division of Cancer Epidemiology and Genetics at the National Cancer Institute (NCI) in Bethesda, Md., said in a press release. "Vaccination with two doses, or even one dose, could simplify the logistics and reduce the cost of vaccination, which could be especially important in the developing world, where more than 85 percent of cervical cancers occur, and where cervical cancer is one of the most common causes of cancer-related deaths."
HPV are a group of 100 common viruses, 40 of which can be transmitted sexually. The Centers for Disease Control and Prevention believe that about 79 million Americans are currently infected with an HPV strain, and nearly all sexually active men and women will get at least one strain of sexually transmitted HPV during their lives.
When a person is infected with HPV, their body typically builds up natural immunities called antibodies which fight the virus off. However, for some unknown reason, others who contract the virus will go on to cause certain cancers including oropharynx (back of the throat, including the base of the tongue and tonsils), cervix, vulva, vagina, penis and anus, while others will develop genital warts.
Two strains of HPV, strains 16 and 18, are connected to 70 percent of cervical cancer cases, while two others (strains 6 and 11) cause 90 percent of genital warts, according to the U.S. Department of Health and Human Services.
There are two, three-dose HPV vaccines currently approved for use in the U.S. market. Gardasil, which the Food and Drug Administration okayed for anyone between the ages of 9 to 26, provides protection for the strains that are the leading causes of cervical and other cancers and genital warts. Cervarix, which is only approved for women between 10 and 25, only provides protection against the strains closely linked to the cancers.
The doses are given over a six-month period, with the CDC recommending the second dose be given one to two months after the first, and the third dose be given six months after the first.
The vaccines work by introducing antibodies before a person is sexually active, so when they contract the virus for the first time, they already have protection against the infection. It is recommended that boys and girls aged 11 to 12 begin getting the HPV vaccine.
For the new study sponsored by the National Institutes of Health, researchers looked at Ceravix's long-term ability to provide protection for HPV strains 16 and 18 in women from Costa Rica. Seventy-eight women received one shot, 192 received two doses and 120 completed the three-dose regimen. They were compared with 113 women who were not vaccinated because they had contracted HPV sometime in the past.
All women who received any of the vaccine doses had antibodies to protect against both HPV strains for up to four years. The women who received two doses six months apart had about the same levels of antibodies as those who received all three full doses.
While the women who received only one dose had lower levels of antibodies compared to their more vaccinated counterparts, their levels remained about the same throughout the four year follow-up period. This suggested that receiving one shot provided lasting protection.
The women in groups that received one or two shots had HPV antibody levels five to 24 times higher than those who had not been vaccinated at all, but had been exposed to the virus.
Dr. A. Bennett Jenson, a senior scientist at the University of Louisville who helped develop the HPV vaccine but was not involved in the study, said that if the results were true, it can help provide HPV protection for women in countries that might not be able to afford the full three-dose series.
"In places like India, if it works, it would really be great," he said to USA Today.
The CDC believes that because of the HPV vaccine, strains related to genital warts and some cancers have decreased 56 percent among teen girls. However, the agency also discovered in a July study that vaccination rates for girls between 13 to 17 did not increase -- and the number of girls who received all three doses decreased -- between 2011 and 2012. About 54 percent of girls got one shot, and about one-third received the full vaccination course.
Other countries, including Chile and British Columbia, only recommend two doses of the HPV vaccine, the new study's authors noted.
CBS News contributor Dr. David Agus, an oncologist who heads the USC Norris Westside Cancer Center in Los Angeles, told CBS This Morning Monday that study intended for the subjects to complete the entire vaccination schedule, but many of them simply didn't do it -- similar to what is seen in the real world. If it is true that just one dose might help, this could be a game changer.
"It really calls into question why instead of talking about Affordable Care Act and websites, the leadership isn't talking, 'How can we save more lives here? Why are vaccines optional?'" he pointed out.
However, Safaeian said the study results are too preliminary to recommend that the U.S. switch to a fewer-dose model.
"For instance, it is important to note that persistence of antibody responses after a single dose has not been evaluated for Gardasil, the quadrivalent HPV vaccine that is more widely used in the United States and many other countries," she added.