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PAS 2017: Vaccine Reduces Human Papillomavirus Prevalence Among Young Women

The prevalence of human papillomavirus (HPV) types included in the most widely used vaccine has decreased among adolescent and young women in the U.S., with "herd immunity" extending to those who were not vaccinated themselves, according to study results presented at the Pediatric Academic Societies Meeting last week in San Francisco.

Sara Oliver from the U.S. Centers for Disease Control and Prevention (CDC) and colleagues compared HPV prevalence before and after the introduction of HPV vaccines. While the impact of vaccination on cancer rates will not be apparent for decades, changes in HPV infection rates can be assessed sooner.

HPV is the most widespread sexually transmitted infection, with more than 150 known types, Oliver noted as background. "High-risk" types can cause cervical, anal, and other types of cancer, while "low-risk" types can cause non-malignant tissue changes such as genital warts.

A quadrivalent vaccine (Gardasil) that protects against 2 high-risk HPV types (16 and 18) and 2 low-risk types (6 and 11) was approved in 2006. That year the CDC's Advisory Committee on Immunization Practices (ACIP) recommended routine vaccination for girls at age 11 or 12; this recommendation was extended to boys in 2011. Catch-up vaccination is recommended for young women (up to age 26) and young men (up to age 21) who were not vaccinated earlier.

Last year the CDC recommended that 11- and 12-year-olds should receive 2 doses of the quadrivalent vaccine instead of the previously recommended 3 doses. In 2015 ACIP included in its recommendations a new "9-valent" vaccine (Gardasil-9) that covers 9 different HPV types.

Oliver's team analyzed data from the National Health and Nutrition Examination Survey (NHANES), an ongoing, nationally representative U.S. household survey. Participants complete interviews about demographic and health-related factors and receive physical examinations. Female participants over age 14 are asked to submit a self-collected cervico-vaginal swab sample to be tested for HPV.

The researchers separated the total population of 6686 young women by age group (14-19, 20-24, 25-29, and 30-34 years). Data was divided into 3 time periods: pre-vaccine era (2003-2006), early vaccine era (2007-2010), and later vaccine era (2011-2014). HPV types were classified as those covered by the quadrivalent vaccine and non-covered types.

Results

• During the early vaccine era, 34% of young women in the 14-19 age group, 18% in the 20-24 group, 8% in the 25-29 group, and 4% in the 30-34 group had received at least 1 vaccine dose.
• By the later vaccine era, vaccination rates had risen to 55%, 43%, 25%, and 7%, respectively.
• The prevalence of vaccine-covered HPV types decreased by 56% between the pre-vaccine era and the early vaccine era in the 14-19 age group (from 11.5% to 5.0%).
• During this period, HPV rates remained about the same for women in the older age groups, who were less likely to have been vaccinated.
• By the later vaccine era, the prevalence of covered HPV types had fallen by 71% (to 3.3%) in the youngest age group, and by 61% (to 7.2%) in the 20-24 year group.
• Among vaccinated women, the prevalence of vaccine-covered HPV types among sexually active women in all age groups fell from 18.5% in the pre-vaccine era to 2.0% in the later vaccine era.
• A smaller decline in prevalence, from 18.5% to 12.2%, was seen among unvaccinated women.
• Unlike the vaccine-covered HPV types, no significant changes over time were observed for non-covered types.
• Overall vaccine effectiveness was 83%.

While the decline in prevalence was most evident for girls and women who were vaccinated, it also fell among those who had not been vaccinated, indicating the development of herd immunity, Oliver said.

"Declined in vaccine types, with no declines in non-vaccine types, support the conclusion that the vaccination programs is primarily responsible for the declines," according to the researchers.

Oliver noted that future research will look at changes in HPV prevalence among young men, HPV types covered by the 9-valent vaccine, and the effect of the new 2-dose vaccine schedule.

5/16/17

Sources

S Oliver, ER Unger, R Lewis, et al. Prevalence of Human Papillomavirus (HPV) Before and After HPV Vaccine Introduction -- National Health and Nutrition Examination Surveys, United States, 2003-2014. Pediatric Academic Societies Meeting. San Francisco, May 6-9, 2017. Abstract 3560.3.