Rates of the sexually transmitted infection dropped by 17 percent among women ages 15 to 25 from 2006 to 2010, although there was no change among men over the same period.

The findings are "in all likelihood" a sign the vaccine is working, said Amy Leval of Karolinska Institutet in Stockholm, who worked on the study. But they also suggest that not enough women have been vaccinated for men to be protected indirectly - a phenomenon known as herd immunity - she told Reuters Health by email.

Genital warts are caused by human papillomavirus, or HPV, which in rare cases can also lead to cervical, penile and anal cancers. Health authorities in the U.S. recommend that all preteen girls and boys get an HPV vaccine.

A U.S. expert without ties to the research welcomed the results from Sweden, but also sounded a cautious note.

"It's exactly what we would like to see," said Dr. Michael Brady, an expert in pediatric infectious diseases at Nationwide Children's Hospital in Columbus, Ohio. "But it is a little early to suggest that we know this is a vaccine response."

It's possible that something else, such as better education or an increased media focus, could also have influenced how the disease spread during the study, Brady explained.

"In order to really document this was related to the vaccine, you have to look at those who got the vaccine and those who didn't," he said.

Leval said her work, published in the Journal of Infectious Diseases, is the first to use data from the entire population of a country to estimate the rate of genital wart infections. Such a study would not be possible in the U.S., because there are no nationwide data on the condition.

The Swedish researchers linked two national registries - one on prescription drug use and one on hospital visits - to get estimates of how many people ages 10 to 44 were treated for genital warts.

Their results are likely to be lower than the real rates because the registries don't include people who got non-drug treatments at private practices, such as having their warts frozen off.

Women were most at risk of a new infection around age 20, whereas the rate peaked a few years later in men.

Initially, there was little difference in yearly infection rates between men and women - at 399 and 387 per 100,000 people, respectively. But that began to change after 2007.

While genital warts appeared to become more common among men, the rate dropped off among women, particularly teenagers and young adults.

Leval said HPV vaccinations were recommended for Swedish women up to age 26 during the study, and partially subsidized for girls between 13 and 17.

Twenty-seven percent of girls between 17 and 19 years old were fully vaccinated, she said, and the rate of new infections dropped by a quarter in this age group.

"We see no decline among men yet, indicating no herd immunity effect here when the portion of the population vaccinated is only roughly one-third in the recommended age-groups," Leval said

She added that studies have found signs of herd immunity in men from Australia, where vaccine uptake in girls is about 80 percent.

HPV is the most common sexually transmitted infection in the U.S. According to the Centers for Disease Control and Prevention, at least half of all sexually active people will catch genital HPV at some point, although the virus usually goes away on its own without causing any symptoms.

Researchers estimate that HPV types 16 and 18 are responsible for about 7,000 cases of cancer in men every year in the U.S. and 15,000 cases in women.

Clinical studies show HPV vaccines shield boys and girls against genital warts and cancers, although the protection isn't complete. U.S. health regulators have found no serious side effects apart from soreness at the injection site.

The new results suggest genital warts are the second-most common STI in Sweden after chlamydia, said Leval. This year, the Scandinavian country has replaced its earlier on-demand vaccination strategy with "a school-based and catch-up vaccination program," she added.

SOURCE: bit.ly/NU1BPU Journal of Infectious Diseases, online July 18, 2012.

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