Human Papillomavirus, more commonly known as HPV, has been the topic of discussion most recently in reference to the ability to vaccinate against it and prevent its association with cervical cancer. There are certain strains of HPV that that are classified as high-oncogenic-risk (likely to cause cancer). Risk of cervical cancer is the main force behind the push for vaccination against HPV. HPV infection types such as HPV-16 and -18 are a necessary cause of cervical and other types of cancers (1). Because it is so dangerously associated with cervical cancer, HPV infection presents a serious burden for females. Others like HPV-6 and -11 are low-oncogenic-risk but cause other symptoms like genital warts, and some cancers found in men (penile cancer).
Young girls around age 12 have begun receiving the vaccine against HPV in order to combat the associated diseases like cervical cancer. Results suggest that so far it is effective. There are two prophylactic vaccines currently used in females: the bivalent and quadrivalent vaccines. The bivalent protects against HPV -16/18 and the quadrivalent protects against HPV – 16/18/6/11. Previous research has determined that these vaccines are efficacious and cost-effective in preadolescent girls. Currently, there is debate over whether or not young boys should also be vaccinated against HPV. Recent studies have also found the HPV quadrivalent vaccine to be 86% effective in young males, which presents another option for vaccination against HPV infection (2). Therefore, Brisson, et al. (2011) wanted to investigate the potential incremental impact of vaccinating boys against HPV.
A very important factor that must be considered in this instance is herd immunity and its effect on conferring immunity to males from female HPV vaccinations. Because of vaccination against females, there is a natural reduction in the risk of infection in males solely from reduced exposure. If herd immunity has an effect in the community, then vaccinating boys could potentially have limited further reductions in HPV infection. If that is the case, then vaccinating boys would not be considered cost effective. Therefore, Brisson, et al. (2011) are not only investigating the potential incremental impact of vaccinating boys against HPV, but are taking into account the effects of herd immunity as well.
The authors started by making some base assumptions about the current vaccination program for girls: 99% vaccine efficacy, 20 years of protection, and 70% coverage. These assumptions make good sense considering the data available to the authors. They used an individual-based transmission-dynamic model of partnership formation and dissolution, which may sound a bit confusing. Essentially what the authors did was assume individuals enter the population prior to sexual maturity/activity and therefore have two different risks for HPV infection – Gender and sexual activity. HPV is dependent “on sexual behavior, per-partnership risk of transmission, duration of infectiousness, and natural immunity” (1). Further, the model fit data for age-specific sexual activity and epidemiological data related to type-specific incidences of HPV infection. Population-level vaccine effectiveness and vaccine program efficiency were subsequently measured.
The model predicted that vaccinating 70% of 12-year-old boys in addition to girls produced a slightly faster decline in HPV-16/18 prevalence and a lower stable equilibrium prevalence. Further, the incremental reduction in HPV-16/18 over 70 years from vaccinating boys was 16% in females and 23% in males. The vaccine against HPV-6/11 was more significantly more effective in girls alone compared to HPV-16/18, and therefore it was assumed that the impact of adding boys to that vaccine was negligible. In other words, the incremental impact of vaccinating boys decreased with improved vaccination characteristics in girls.
Even though vaccinating boys in addition to girls produced initial incremental benefits, the return on investments may not be as large as expected. By vaccinating 70% of 12-year-old girls, overall HPV-16/18 prevalence is reduced by 64% long term, which produces an effectiveness-coverage ratio of 1.8. That ratio of over one indicates to us that herd immunity is occurring in the population because more individuals are protected than vaccinated. However, if the number of vaccines is doubled by vaccinating 70% of boys as well, the incremental reduction is 24% and yields an effectiveness-coverage ratio of only 0.7. This should indicate to us that the effectiveness-coverage ratio of vaccinating boys becomes less with increased vaccine coverage.
Brisson et al. (2011) were able to conclude that the predicted herd immunity from vaccinating girls under moderate to high vaccine coverage decreases the potential incremental gains of vaccinating boys to limited. Males will be protected against HPV infection and disease due to herd immunity from vaccinating girls, especially with improved vaccine efficacy, duration, and coverage. Vaccinating males in addition to herd immunity would result in redundancy in vaccine delivery. Therefore, vaccinating boys is not cost effective, when >50% of girls are receiving coverage. The logic behind encouraging more females to get vaccinated, over adding boys to the vaccine regime, is that when 100% of girls are vaccinated and 0% of boys are vaccinated, 100% of heterosexual partnerships have at least one person protected. But, if 50% of girls and boys are vaccinated, only 75% of heterosexual partnerships are protected.
Heterosexual males will benefit almost to the same extent as females from girls-only vaccination because of herd immunity. So although it wouldn’t hurt to vaccinate boys to further reduce HPV related disease in males and females, it would not be a good investment of the limited health care resources currently available to us. Ultimately, the authors recommend increased coverage in girls because it is more cost-effective.
(1)Brisson, M., et al. (2011) Incremental Impact of Adding Boys to Current Human Papillomavirus Vaccination Programs: Role of Herd Immunity. JID (204) 372-376.
(2) Giuliano, A. R., et al. (2011) Efficacy of quadrivalent HPV vaccine against HPV infection and disease in males. N Engl JMed (364) 401–11.