How do you stop the spread of a disease most effectively? By vaccinating all children, or only half? A new Dutch study suggests that the counterintuitive solution for halting HPV is to vaccinate only girls. How can protecting fewer children actually protect more?

Human papillomavirus, or HPV, can and does infect both men and women, and can cause genital warts and certain cancers. However, it is more prevalent among women (three fourths of all women will have HPV at some point, as opposed to only half of all men), and one of its most serious effects, cervical cancer, only targets females. Thus women have been the targets of most HPV educational efforts and vaccination campaigns. Recently, some have suggested that children of both sexes get vaccinations to help control HPV and prevent cervical cancer.

It makes intuitive sense that expanding vaccinations from girls to all children would increase the community, or "herd," immunity. (Herd immunity refers to the fact that when a critical percentage of the population has been immunized against a transmissible disease, then most members of that community, even those without vaccinations, will be protected.) But the authors of the new study think that extra vaccinations wouldn't make enough of a difference to be worth it. They write in PLoS Medicine:

"Despite limited data, HPV vaccination for boys is already licensed in several countries, and it is expected that other countries will consider licensure once more data become available. But the question of whether or not HPV vaccination should be recommended for boys depends only in part on vaccine efficacy, since a program directed at girls already confers health benefits for boys via a reduced transmission of HPV. In Australia, where coverage rates for ongoing vaccination of 12- to 13-y-old girls approach 80%, a modeling study estimated that the current female-only vaccination program will achieve 73% of the maximum possible vaccine-conferred benefit to males."

In other words, a program directed at girls only will still benefit males. If 8 tenths of females are vaccinated, then males will be 73 percent as protected from HPV as they would have been if they had gotten vaccinated as well. The authors add that this benefit holds true only if there is a high rate of female HPV vaccination, in the 70 to 80 percent range. Unfortunately, this saturation level is far from within reach: in the United States, less than 50 percent of girls between the ages of 13 and 17 had received a full HPV vaccination.

The authors go farther than suggesting that female-only vaccination would be effective - they think it would be more effective than vaccinating all children.

To come to this conclusion, the study asks what would be the most effective method for improving herd immunity to HPV-increasing the rate of female vaccination, or introducing vaccination to both sexes? Using mathematical models, the researchers tested the different methods in a situation where an only partially effective single-sex vaccination program was already in place. They found that below a threshold level of immunization coverage, a two-sex vaccination strategy would be best, but when vaccination levels were reaching "critical immunization coverage," keeping the single-sex program in place would be the most cost-effective solution.

In the case of HPV, according to the researchers, the goal should not be the elimination of the virus, only its reduction in the population. Bearing in mind that there is already a single-sex HPV vaccination program in place, and that HPV rates are higher in the female population, the researchers found that vaccinating only females would actually be the most effective way to reduce HPV in the overall population.

These results should probably be taken with a grain of salt. The researchers not only set the goal of reducing HPV rather than eliminating it, but they also had an eye on the cost-effectiveness of the program. The fact that single-sex vaccination would be cheaper than a full immunization may have had some influence.

In addition, the study makes the idealized assumption that a single-sex vaccination program would reach sufficient immunization coverage. The fact is that today, rates of HPV vaccinations are not high enough to ensure herd immunity. If we could vaccinate all girls against HPV, then perhaps the single-sex program would be as effective as the researchers say. But in the real world, if we are trying to boost herd immunity, practical thinking would indicate that vaccinating children of both sexes is the quickest way to do it.

Via PLoS Medicine.