The success of Australia's human papillomavirus (HPV) vaccination catch-up program has given the country a world-leading trove of real-world data on the effectiveness of Merck's ($MRK) Gardasil. By mining this database, researchers have estimated the vaccine halves the risk of young women developing high-grade cervical abnormalities.
A team of Queensland-based researchers reported the figure in BMJ after looking at the health records of 100,000 young women. Pap smear results were compared to vaccination history to give an estimate of how effective Gardasil is at preventing precancerous lesions. Women who had received three doses were 46% less likely to develop high-grade cervical abnormalities. The vaccine also offered 34% protection against other cervical abnormalities.
"We would expect when we continue to monitor these women into the future that they're going to have lower rates of cancer. Cervical cancer usually takes 10 or 20 years to develop so we would expect once these women get into their 30s or 40s, we'll start to see really substantial declines of cancer," study co-author David Whiteman told Australian newspaper The Courier-Mail. Australia's HPV vaccine catch-up program gave three doses of Gardasil to 70% of school-age girls.
The study suggests these young women are better protected than their peers who received two or fewer doses, but this could be driven by confounding factors. Vaccination with two doses provided just 21% protection against high-grade and other cervical abnormalities. While the difference may be partly due to the two-dose regimen offering less protection, many of the partially vaccinated women were older and therefore more likely to have been infected with HPV before being immunized.
More research is needed to tease out what caused the difference between the protection provided by the two- and three-dose regimens. Such data will show whether countries are right to switch to a two-dose model. Parts of Canada give two doses and this week a European Medicines Agency (EMA) committee backed the model too. The EMA opinion means a two-dose regimen for Gardasil--which is sold in Europe by a Merck-Sanofi ($SNY) joint venture--could soon be approved in the region.