HERD PROTECTION WITH HPV VACCINATION

Despite recommendations that all kids beginning at the age of 11-12 years be vaccinated with the human papillomavirus (HPV) vaccine, pickup has been dismal, with only half of American teenagers fully vaccinated, far below the Healthy People 2020 goal of 80% penetration. But would reaching 80% penetration provide adequate protection on a population level? A new study suggests that the answer is yes.

A long-running surveillance study^[1] involving almost 1600 young women (aged 18-26 years) living in Cincinnati, Ohio, examined the question of herd immunity. The women entered the study in four sequential waves from 2006 through 2017; none of the women enrolled in the first wave (2006-2007) had received the HPV vaccine, which was introduced in 2006. Encouragingly, the proportion of vaccinated women rose in each subsequent wave, and by wave 4 (2016-2017), 84.3% of the women were vaccinated. Of note, almost all participants in waves 1-3 received the quadrivalent vaccine. By wave 4, 12% of the women had received the newer, 9-valent preparation, which provides protection against five additional strains.

All of the young women in the study were sexually active at the time of enrollment, and most-more than 80%—had had two or more sexual partners. Just more than half had experienced a sexually transmitted infection before enrollment.

In general, the prevalence of HPV infection with both 4-valent and 9-valent vaccine strains decreased over time in vaccinated women, regardless of which vaccine version they received. Infections with the five additional strains covered by the 9-valent vaccine decreased not just in those women who received the newer preparation but also in women who had only received the older quadrivalent vaccine; that decrease continued with each subsequent wave of enrollees.

In the nonvaccinated group, the percentage of infections with the virus strains included in the quadrivalent vaccine fell a statistically significant 40%, decreasing from about a third of all infections (32.4%) to just under a fifth (19.4%). However, HPV infections resulting from the five additional strains in the 9-valent vaccine almost doubled (adjusted odds ratio, 1.90).

The reasons for this are unclear, and an important caveat is that it is also not known whether this trend may reverse once a higher proportion of young women have received the 9-valent vaccine.

The results demonstrate the high clinical effectiveness of both the quadrivalent and 9-valent vaccines.

Viewpoint

An accompanying editorial^[2] notes that the definition of "vaccinated" used for the study included "one or more" HPV vaccines. Therefore, as it is unlikely that every woman in the study completed the series by receiving two or more doses, it is remarkable that 80%-90% vaccine effectiveness was achieved with what we would still consider suboptimal vaccination.

The other finding emphasized by the editorialist is the cross-protection experienced by the women who had received a quadrivalent vaccine. These women had reduced odds of contracting the additional five strains included in the 9-valent vaccine, even though they were not in the original quadrivalent vaccine.

Of course, the findings in regard to herd immunity are remarkable. At a minimum, these data should reignite everyone's interest in improving delivery of HPV vaccine. We can only hope to see the herd effect spill over into preventing HPV-related cancers as well.