With many vaccines, it's easy to see a quick impact: the number of cases of a disease rapidly drops. But for other vaccines that target slower-moving ailments, it can take years or even decades until a population reaps the benefits. Such appears to be the case with human papillomavirus (HPV), a virus that can spread during sex and cause cervical cancer and genital warts. A new analysis of the first 3 years of an HPV vaccination program in Victoria, Australia, reveals a drop in the incidence of cervical lesions among young women . The study didn't find a similar effect in women over 18, but that's likely to come with time.
The first HPV vaccine was licensed in 2006. Although many countries now offer an HPV vaccine, Australia was the first to roll out a national program, in April 2007, that aimed to vaccinate as many girls and young women as possible. It focused on those between 12 and 26 years old. Because the vaccine only prevents infection from the virus and can't help once someone already has HPV, doctors favor vaccinating girls and women who have not yet started having sex.
Recently, public health physician Julia Brotherton and physician and epidemiologist Dorota Gertig, both of the Victorian Cytology Service in East Melbourne, Australia, along with their colleagues, speculated that 3 years of the HPV vaccine in Australia might be enough to see an effect. So they combed through Pap smear data from the 4 years before the vaccine program launched, and from April 2007 to December 2009, after the vaccine was being offered. The researchers were interested in whether the frequency of certain cervical abnormalities, called "high-grade" and "low-grade," had changed. Using the Victorian Cervical Cytology Registry, which is one of eight registries in Australia that gathers information on Pap smear results, Brotherton and her colleagues gathered data on about 337,000 girls and women under age 26 who had Pap smears before the vaccine and 208,000 after.
The team found that high-grade cervical abnormalities, which include precancerous changes to cells that can progress to cancer, were much less common in the under-18 set in the post-vaccine years. Before the vaccine was introduced, 0.8% of girls tested had a high-grade abnormality. Afterward, it was 0.42%. The total numbers were small—109 in the first group, 23 in the second—because few women this young get Pap smears at all. (Australia recommends that Pap smear screening not begin until age 18 or 2 years after the first time a girl has sex, whichever is later. However, not all doctors follow the guidelines, and some offer Pap smears to younger girls who are sexually active.) The researchers didn't see any change in high-grade abnormalities in those over 18. Nor were there post-vaccine changes in low-grade abnormalities in either age group, they report online today in The Lancet.
Nonetheless, seeing any effect this early is arguably impressive, given how long it takes for HPV infection to progress to precancerous changes and cervical cancer. "Most models have predicted that we would not see an impact from the vaccine until 7 to 10 years" after its introduction, says Mona Saraiya, a preventive medicine and public health physician at the U.S. Centers for Disease Control and Prevention in Atlanta. But Australia has "really high vaccine coverage," she notes, which may make it easier to see an effect quickly.
It's not clear why there was no sign yet of a benefit from the vaccine in women over 18. Brotherton and her colleagues note that one limitation of the study was that they don't know that all these women actually got the vaccine, just that they were likely offered it. Saraiya also wonders whether screening might have become less common in the under-18 crowd, suggesting that cervical abnormalities had dropped when they haven't—or at least, not yet.
"It's not like I have any doubts that the vaccine will have an impact," Saraiya says. "My opinion is, it will just take a little bit longer." And, she predicts, Australia will be one of the first countries with the evidence to show for it.