Scientists at the US Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, announced good news last month. The prevalence of key strains of disease-causing human papilloma­virus (HPV) fell by 56% in US girls aged 14–19 years in the years after 2006, when a vaccine was added to the routine US immunization schedule for girls (L. E. Markowitz et al. J. Infect. Dis. 208, 385–393; 2013).

This is a clear-cut vaccine success story. The decline represents a drop from more than 1 in 10 girls in this age range carrying the relevant strains of the virus to just 1 in 20. This is a significant result. About 14 million people in the United States, most in their late teens and early 20s, become infected with HPV each year. The two most dangerous strains — those targeted by vaccines made by both Merck and GlaxoSmithKline — cause cervical cancer, other anogenital cancers and throat cancer. Merck’s vaccine also protects against two further strains that cause genital warts.

The vaccine is given in three doses over six months. The CDC first recommended that all girls aged 11 and 12 be vaccinated. In 2011, it said the same for boys, for whom only the Merck vaccine is licensed. The idea is to immunize children before they become sexually active. Given that HPV is the most common sexually transmitted disease, waiting longer only increases the odds that the protection will be provided too late. The vaccine has been shown to be both safe and highly effective, and continuing experience — as of March, about 56 million doses had been distributed in the United States — reinforces that.

Yet proportional uptake in the United States has been poor. In 2010, a national survey found that only 49% of girls aged 13 to 17 had received at least one dose of vaccine, and only 32% had received all three doses. By comparison, Rwanda has achieved more than 80% vaccination coverage and several Canadian provinces have reached 85%.

“HPV is an equal-opportunity infectious agent.”

It is possible that unvaccinated girls in the United States are already benefiting from the compliance of the parents who have stepped up to have their children immunized. When the CDC scientists explored whether the decrease in HPV prevalence among 14–19-year-old girls might be due to herd immunity, they found that vaccinated sexually active girls showed a striking 88% decrease in prevalence of the relevant HPV strains, compared with the pre-vaccine era. But they also found a 28% decrease in prevalence among unvaccinated girls. The finding was not statistically significant, and was difficult to interpret owing to differences in the reported sexual behaviour of the two groups — for instance, the unvaccinated girls reported fewer sexual partners. Nevertheless, herd immunity is a possible explanation, and other studies have indicated that it is at play.

It is worth noting that cervical cancer, almost all of which is caused by HPV, disproportionately affects black and Hispanic women in the United States, possibly because they have reduced access to screening. And among women who do contract cervical cancer, black women have proportionally the highest death rate.

But neither white people nor parents of boys of any race or ethnicity should be complacent when considering whether to vaccinate. The vaccine is not only about preventing cervical cancer, nor even only about preventing anal cancer in males who have sex with males. Consider that the proportion of US throat cancers associated with HPV has exploded in recent decades among white men and women. (Similar increases have occurred in Canada and some European countries.) As the actor Michael Douglas was frank enough to acknowledge in an interview published last month, throat cancer of the type that he was treated for in 2010 is caused by HPV contracted through oral sex. (Douglas’s representatives later denied that he had intended the statement to refer to his own particular case.)

The take-home message is that HPV is an equal-opportunity infectious agent. As the CDC noted when it announced the findings last month, cervical cancer is simply the most common among about 19,000 cases of cancer caused by HPV in US women each year, and throat cancer is the most frequent among 8,000 cases of such cancers in men. The costs are sobering: the CDC calculates, for instance, that 50,000 girls alive today who will get cervical cancer during their lifetimes would not have done so had the country quickly reached 80% vaccination rates.

Squeamishness among parents being asked to vaccinate 11-year-olds against a sexually transmitted disease is understandable. But in the face of such a clearly effective means of protecting our young people, ducking the issue, hoping for the best or relying on the responsible actions of others is not.