Young women who received the quadrivalent human papillomavirus (HPV) vaccine had fewer and fewer infections with high-risk HPV strains covered by the vaccine year after year, but the incidence of high-risk strains that were not covered by the vaccine increased over the same 12-year period, researchers report in a study published August 23 in JAMA Open Network.
“One of the unique contributions that this study provides is the evaluation of a real-world example of the HPV infection rates following immunization in a population of adolescent girls and young adult women at a single health center in a large U.S. city, reflecting strong evidence of vaccine effectiveness,” write Nicolas F. Schlecht, PhD, a professor of oncology at Roswell Park Comprehensive Cancer Center, Buffalo, and his colleagues. “Previous surveillance studies from the U.S. have involved older women and populations with relatively low vaccine coverage.”
In addition to supporting the value of continuing to vaccinate teens against HPV, the findings underscore the importance of continuing to screen women for cervical cancer, Dr. Schlecht said in an interview.
“HPV has not and is not going away,” he said. “We need to keep on our toes with screening and other measures to continue to prevent the development of cervix cancer,” including monitoring different high-risk HPV types and keeping a close eye on cervical precancer rates, particularly CIN3 and cervix cancer, he said. “The vaccines are definitely a good thing. Just getting rid of HPV16 is an amazing accomplishment.”
Kevin Ault, MD, a professor of ob/gyn and academic specialist director of clinical and translational research at the University of Kansas, Kansas City, told this news organization that other studies have had similar findings, but this one is larger with longer follow-up.
“The take-home message is that vaccines work, and this is especially true for the HPV vaccine,” said Dr. Ault, who was not involved in the research. “The vaccine prevents HPV infections and the consequences of these infections, such as cervical cancer. The results are consistent with other studies in different settings, so they are likely generalizable.”
The researchers collected data from October 2007, shortly after the vaccine was approved, through September 2019 on sexually active adolescent and young women aged 13 to 21 years who had received the HPV vaccine and had agreed to follow-up assessments every 6 months until they turned 26. Each follow-up included the collecting of samples of cervical and anal cells for polymerase chain reaction testing for the presence of HPV types.
More than half of the 1,453 participants were Hispanic (58.8%), and half were Black (50.4%), including 15% Hispanic and Black patients. The average age of the participants was 18 years. They were tracked for a median 2.4 years. Nearly half the participants (48%) received the HPV vaccine prior to sexual debut.
For the longitudinal study, the researchers adjusted for participants’ age, the year they received the vaccine, and the years since they were vaccinated. They also tracked breakthrough infections for the four types of HPV covered by the vaccine in participants who received the vaccine before sexual debut.
“We evaluated whether infection rates for HPV have changed since the administration of the vaccine by assessing longitudinally the probability of HPV detection over time among vaccinated participants while adjusting for changes in cohort characteristics over time,” the researchers write. In their statistical analysis, they made adjustments for the number of vaccine doses participants received before their first study visit, age at sexual debut, age at first vaccine dose, number of sexual partners in the preceding 6 months, consistency of condom use during sex, history of a positive chlamydia test, and, for anal HPV analyses, whether the participants had had anal sex in the previous 6 months.
The average age at first intercourse remained steady at 15 years throughout the study, but the average age of vaccination dropped from 18 years in 2008 to 12 years in 2019 (P < .001). More than half the participants (64%) had had at least three lifetime sexual partners at baseline.
After adjustment for age, the researchers found that the incidence of the four HPV types covered by the vaccine – HPV-6, HPV-11, HPV-16, and HPV-18 – dropped more each year, shifting from 9.1% from 2008-2010 to 4.7% from 2017-2019. The effect was even greater among those vaccinated prior to sexual debut; for those patients, the incidence of the four vaccine types dropped from 8.8% to 1.7% over the course of the study. Declines over time also occurred for anal types HPV-31 (adjusted odds ratio [aOR] = 0.76) and HPV-45 (aOR = 0.77). Those vaccinated prior to any sexual intercourse had 19% lower odds of infection per year with a vaccine-covered HPV type.
“We were really excited to see that the types targeted by the vaccines were considerably lower over time in our population,” Dr. Schlecht told this news organization. “This is an important observation, since most of these types are the most worrisome for cervical cancer.”
They were surprised, however, to see overall HPV prevalence increase over time, particularly with the high-risk HPV types that were not covered by the quadrivalent vaccine.
Prevalence of cervical high-risk types not in the vaccine increased from 25.1% from 2008-2010 to 30.5% from 2017-2019. Odds of detection of high-risk HPV types not covered by the vaccine increased 8% each year, particularly for HPV-56 and HPV-68; anal HPV types increased 11% each year. Neither age nor recent number of sexual partners affected the findings.
“The underlying mechanisms for the observed increased detection of specific non-vaccine HPV types over time are not yet clear.”
“We hope this doesn’t translate into some increase in cervical neoplasia that is unanticipated,” Dr. Schlecht said. He noted that the attributable risks for cancer associated with nonvaccine high-risk HPV types remain low. “Theoretical concerns are one thing; actual data is what drives the show,” he said.
The research was funded by the National Institutes of Health and the Icahn School of Medicine at Mount Sinai, New York. Dr. Schlecht has served on advisory boards for Merck, GlaxoSmithKline (GSK), and PDS Biotechnology. One author previously served on a GSK advisory board, and another worked with Merck on an early vaccine trial. Dr. Ault has disclosed no relevant financial relationships.
A version of this article first appeared on Medscape.com.