Introduction
Human papillomavirus (HPV) is the leading cause of cervical cancer, with over 300,000 related deaths in 2020. In the US, about 37,300 people are diagnosed with HPV-related cancers annually. Starting in 2006, the first-generation HPV vaccine, 4vHPV, was approved for administration to prevent infection and disease associated with HPV.
It has been studied that the vaccine effectiveness is associated with the timing of HPV vaccination, and that administering the vaccine prior to initiation of sexual activity and potential exposure to HPV offers the greatest protection. Thus, leading health experts, including those from the World Health Organization (WHO), recommend starting HPV vaccination in early adolescence (9-14 years old).
To verify this cause, this review tries to evaluate the effectiveness of HPV vaccination against infection, anogenital warts, cervical abnormalities, and cervical cancer by age at vaccination.
Methods
Databases, including Medline and EMBASE, were searched for suitable studies that evaluated HPV vaccine effectiveness by age at vaccination. To be eligible, studies needed to conduct an analysis of HPV vaccine effectiveness by age at series initiation or completion. The effectiveness was defined as a comparison of the likelihood of the disease outcome between vaccinated and unvaccinated participants.
Key study information was extracted using a standardized form, then discrepancies were resolved by a different author. Next, using an adapted version of the Risk of Bias in Non-Randomized Studies - of Interventions (ROBINS-I), the selection bias and information bias were both evaluated to ensure the objectivity of this study.
Studies were examined by outcome, then by age groups and methods of analysis utilized. The results are presented as adjusted vaccine effectiveness (VE), including risk ratios, incidence rate ratios, hazard ratios, prevalence ratios, or odds ratios.
Results and Limitations
Results:
Four cross-sectional studies reported HPV vaccine effectiveness against vaccine-type HPV infection. Across them, a gradient effect was observed with higher vaccine effectiveness among those who received the vaccine at younger ages. 89.1% effectiveness was observed between 12 and 13 years of age, and decreased slightly for each year later the vaccine was administered, to 28.9% effective among those receiving the vaccine after 18 years of age.
For genital warts, the vaccine seemed to be statistically significantly effective in those who initiated vaccination before 18 years old. However, regardless of sexual activity, the vaccine was not effective in reducing the incidence of genital warts among those who initiated vaccination after 18.
Ten studies were included to evaluate the effectiveness against cervical abnormalities, and all these studies found a general pattern of decreasing vaccine effectiveness as age at initiation or completion increased, especially when initiated after 18 years old. The same trend is seen with cervical cancer, except that the breaking point was closer to 17 years of age.
Limitations:
All of the studies were conducted in high-income countries and were primarily conducted in North America and Europe, which may result in a lack of reflection of the HPV vaccine impact and effectiveness in low- and middle-income countries. Additional studies may be needed in countries with lower vaccine coverage to understand vaccine effectiveness.
Conclusion
Overall, vaccine effectiveness studies are important for understanding how impactful a vaccine is to the world and the people. Understanding the trend of HPV vaccination and its correlation with vaccine effectiveness influences our choices in disease prevention and health. To better protect one's body from HPV, it is recommended that one start HPV vaccination between the ages of 10 and 18, as it will lower the chances of HPV related illnesses.
