In a world obsessed with curing cancer, prevention is less headline-grabbing, but it's also generally less painful and cheaper. Vaccines for the human papillomavirus (HPV) are a formidable weapon in the arsenal against multiple kinds of cancer, yet the uptake of the vaccine in the US is low, especially when compared to other high-income countries.

According to the President’s Cancer Panel Annual Report 2012-2013, only 33.4 percent of girls in the US complete the course of three HPV vaccines, compared to 60.4 percent in the UK and 71.2 percent in Australia. The vaccination rate for boys is even lower, at less than seven percent—unsurprising given that many public health campaigns specifically target girls.

This is a problem given how common HPV is, and how dangerous it can be. At any given time, one in four Americans is infected with at least one strain of the virus. Almost all sexually active people will be infected with it at some point in their lives. Most infections will be cleared by the immune system, but the remaining cases may lead to cervical, anal, or oral cancer, with approximately 26,000 cases in the US every year.

Two vaccines against HPV have been available for some time. Cervarix is "bivalent", protecting against the two most common cancer-causing strains, HPV-16 and 18. Quadrivalent Gardasil protects against four strains: 6, 11, 16 and 18. However, these aren’t the only HPV strains associated with cancer. A recent paper in the New England Journal of Medicine reports that a new vaccine, Gardasil 9, offers protection against the original four strains, as well as against HPV-31, 33, 45, 52 and 58.

The randomized, double-blind clinical trial, conducted in 14,215 women, showed that the new 9-valent Gardasil offered increased protection against genital cancers in women between the ages of 16 and 26.

For diseases where medical treatment is already available and recommended, like the quadrivalent Gardasil vaccine, it's considered unethical to compare a new treatment to placebo. This is because the control group receiving the placebo would be missing out on recommended treatment, putting them at risk. So, in this trial, Gardasil-9 was compared to Gardasil.

The results showed that Gardasil-9 provided the same protection against HPV-6, 11, 16 and 18 as Gardasil, and increased protection against the five additional strains. Overall, the researchers report, the 9-valent vaccine could prevent approximately 90 percent of cervical cancers, compared to Gardasil's 70 percent protection.

There is some evidence that the bivalent and quadrivalent vaccines already provide some protection against additional strains of HPV. A recent paper in Clinical and Vaccine Immunology reporting on the clinical trial for Cervarix reported that the vaccine is more than 96 percent effective against disease associated with strains 16 and 18, and more than 50 percent effective against diseases associated with any HPV strain.

The Cervarix trial found that the vaccine is most effective in the youngest age group of women tested, between 15 and 17 years old, says Dan Apter, Director of The Sexual Health Clinic, Family Federation of Finland, Helsinki. He explains that older women in the trial were more likely to already have been infected. The vaccine doesn't work for people who already have HPV, so there were lower efficacy rates for older groups. This highlights the importance of vaccinating young teenagers before they become sexually active, he says.

In the Gardasil-9 trial, the 9-valent vaccine was associated with more side effects, but the effects were not comparably dangerous to the kinds of cancers prevented by the vaccine. The slightly higher rate was to be expected, the researchers note, because the new vaccine has more viruslike antigens.The most common effects included swelling and pain at the injection site, and some patients experienced headaches, nausea, dizziness, and fatigue.

HPV vaccines and public health campaigns shouldn’t be limited to girls, says Apter, and Prof. Elmar Joura, lead author on the Gardasil-9 paper, agrees. “The female-only campaigns leave men who have sex with men unprotected,” says Joura. In societies where vaccination rates among girls are low, men who have sex with women are also at risk from HPV-related disease. Being vaccinated not only protects men individually, but also contributes to herd immunity, says Apter.

There’s still work to be done. More studies are needed to explore the effects of the vaccine in boys, says Apter. And it’s necessary to continuously evaluate results in countries with good HPV vaccine coverage, like Australia and the UK. “Performance in real life situations is key,” says Joura. “It’s there that you see if the vaccines do what they are expected to do.”

New England Journal of Medicine, 2015. DOI: 10.1056/NEJMoa1405044 (About DOIs).