The American government's goal of vaccinating young girls against the human papillomavirus has been disappointing, with less than a third of teenagers having completed a full course of HPV vaccine. But now the United States can look to Australia, which six years into a successful nationwide HPV vaccination campaign has experienced a sharp decline in the number of new cases of genital warts among young men and women.
The country, one of the first to establish a nationally financed HPV vaccination program for girls and young women, has also seen a decrease in the number of cases of cervical abnormalities, a precursor to cervical cancer.
Australia's program, which started in 2007, offers free HPV vaccination to girls who are 12 and 13 years old, and catch-up programs for girls and women under 26. The vaccine protects against genital warts as well as cancers of the cervix, head and neck.
The vaccine is typically administered in three doses, beginning around age 12. In 2010, coverage rates for girls that age in Australia's school-based programs reached 83 percent for the first dose, 80 percent for the second dose and 73 percent for the third.
The findings suggest that Australia's program, which has experienced little of the resistance that has stymied vaccination efforts in the United States, has been an overwhelming success, said Basil Donovan, an author of the study and a professor at the University of New South Wales in Sydney.
As part of the new research, which was published in BMJ, a British medical journal, Dr. Donovan and colleagues compared rates of HPV-related diseases in the three years before the vaccine program began and in the four years afterward. The study included data on nearly 86,000 people who visited clinics in Australia between 2004 and 2011.
The researchers found that diagnoses of genital warts among young women ages 12 to 26 plummeted 59 percent in the two years after the program began. For men in the same age group, genital warts cases dropped 39 percent. During the same period, there was also a striking decline in the rate of high-grade cervical abnormalities in teenage girls, a sign that a decline in cervical cancer cases may be on the horizon.
Although a driving force behind the vaccine recommendations is the prevention of cancer, it is too soon to measure an impact there. The incubation period from HPV infection to the development of an HPV-related cancer is typically two to three decades. But for genital warts, the incubation period is about three months.
The findings showed a trend of sharply decreasing diagnoses among younger age groups. The rate of genital warts cases among girls younger than 21, for example, fell to less than 1 percent in 2011 from about 12 percent in 2007, a decline of nearly 93 percent. Among women ages 21 and 30, the decline in diagnoses was roughly 70 percent, to just 3 percent in that age group in 2011 from about 11 percent in 2007. At the same time, however, there was no significant decline in cases among women 30 and over.
A similar pattern was seen in every age group of men, with the infection rate falling more and more sharply with decreasing age. Dr. Donovan said it was clear that the high rate of immunization among young women was protecting young men who have not been vaccinated, a phenomenon known as herd immunity.
"The mathematical modelers told us that a large decline was to be expected, but we tend not to believe it until we see it," Dr. Donovan said. "We were particularly surprised to see a 93 percent drop in genital warts in young women when only 85 percent were vaccinated. This suggests that the herd immunity that is protecting men is, in turn, also protecting unvaccinated women."
Australia's vaccination campaign sharply contrasts with the program in the United States where, studies show, parents often opt out of HPV vaccination for their children, calling the vaccine unnecessary, citing concerns about its safety or saying they have difficulty explaining to their teenagers what the shots are for. Some parents have also hesitated over fears that HPV vaccination might give their teenagers license to have sex, even though studies have countered the notion that the vaccine alters sexual behavior.
"There was little resistance to the HPV vaccine in Australia, just the usual anti-vaccination people and a few religious groups," Dr. Donovan said. "But even the religious groups have gone quiet, and I suspect that many of them are quietly getting their children vaccinated."
An earlier photo caption incorrectly stated that the HPV vaccine protects against two strains of human papillomavirus. The Gardasil vaccine protects against four strains.
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