Single Dose of HPV Vaccine May Be Enough to Guard Against Cervical Cancer
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The HPV vaccine is currently recommended as a three-dose series, but doctors have found it difficult to finish out the series for many girls.
However, the researchers discovered active human papillomavirus antibodies in Costa Rican women four years after they had received only one dose of Cervarix, a vaccine that protects against two HPV strains.
The study also found that women who received two doses six months apart appeared to have just as much antibody protection against HPV as those who received three doses.
Funded by the U.S. National Cancer Institute (NCI), the study was published in the November issue of Cancer Prevention Research.
HPV vaccination rates in the United States are low. In 2012, only about 54 percent of girls even received one dose, and just 33 percent went on to receive all three shots, according to a recent report from the U.S. Centers for Disease Control and Prevention.
But the impact of only having to get one or two doses of the vaccine would be felt most in developing countries, where cervical cancer death rates are higher but the high cost of the vaccine makes it difficult to protect all women.
"Vaccination with two or even one vaccine dose could simplify the logistics and reduce the cost of vaccination in the developing world, where more than 85 percent of cervical cancer occurs and it is the most common cause of cancer death in women," said study author Mahboobeh Safaeian, an investigator in the NCI's division of cancer epidemiology and genetics.
More than 275,000 women die from cervical cancer each year worldwide, making it the sixth-leading cause of cancer death, according to the World Health Organization. In the United States, active screening programs have reduced the cervical cancer death rate to about 3,900 women a year, according to the CDC.
HPV vaccine doses run about $130 apiece, according to the American Cancer Society, and cost is a big consideration, one expert noted.
"If you reduce the cost by a third, you can give it to three times as many people," said Dr. Shashikant Lele, clinical chief of gynecologic oncology at Roswell Park Cancer Institute, in Buffalo, N.Y. "If one dose is adequate to protect, we can vaccinate three times the number of women with the same amount of money."
Researchers made these findings during a clinical trial to test the effectiveness of Cervarix in women from Costa Rica.
They measured immune response to the vaccine in blood samples drawn from 78, 192 and 120 women who received one, two and three doses of the vaccine, respectively.
The doctors then compared those results to data from 113 women who were not vaccinated but had antibodies against the viruses in their blood because they had been infected with HPV in the past.
Up to four years later, all of the women in all three vaccination groups had antibodies for HPV 16 and 18, the two strains Cervarix guards against.
Although antibody levels among women who received one dose were lower than among those who received three doses, the levels appeared stable, according to the researchers, which suggests that these are lasting responses.
Moreover, the levels of antibodies in women from the one- and two-dose groups were between five and 24 times higher than the levels of antibodies in women who were not vaccinated but were infected with HPV.
The contents of the HPV vaccine might provide another explanation, Safaeian said.
The vaccine is created using genetically engineered versions of the virus that prompt an immune response but do not have the ability to multiply and cause illness, she explained.
This type of vaccine might cause a stronger immune response than vaccines made from parts of live viruses (such as the tetanus vaccine) which often require periodic boosting to maintain immunity, Safaeian said.
Debbie Saslow, director of breast and gynecologic cancer for the American Cancer Society, called the findings "very exciting."
"This would be absolutely amazing if we could only give one dose," she said.
However, Saslow noted a few concerns. For one thing, the study did not involve the HPV vaccine used 99 percent of the time in the United States -- Gardasil, which guards against four strains of the virus, she noted.
Also, researchers could only verify protection for four years after receiving the dose. "If we are vaccinating girls at age 12, we need to make sure that immunity lasts," Saslow said. "We don't know, and they don't know, if having three doses will last any longer than one dose."
Finally, she pointed out that a very small number of women were involved in the Costa Rican study, and recommended that these results be verified by tracking girls in the United States who are receiving the vaccine.
"We need to set up some sort of surveillance system for those who only got one or two doses," Saslow said. "If we're identifying those girls, let's see if we can follow them. We don't need to start a big randomized trial. We can look back using medical records."
SOURCES: Mahboobeh Safaeian, Ph.D., investigator, division of cancer epidemiology and genetics; U.S. National Cancer Institute; Shashikant Lele, M.D., clinical chief, gynecologic oncology, Roswell Park Cancer Institute, Buffalo, N.Y.; Debbie Saslow, director, breast and gynecologic cancer, American Cancer Society; November 2013 Cancer Prevention Research