Human Papillomavirus Vaccination Coverage Among Adolescent Girls Aged 13–17 Years — U.S.-Affiliated Pacific Islands, 2013–2023

Abstract

Worldwide, cervical cancer is the fourth most common cancer among women, and the World Health Organization (WHO) Western Pacific Region, where the U.S.-affiliated Pacific Islands (USAPI) are located, accounts for one quarter of all estimated cases. Human papillomavirus (HPV) vaccines are recommended at age 11–12 years to prevent most cervical cancers. HPV vaccines were introduced across USAPI during 2007–2016, predominantly provided through school-located vaccination programs. Retrospective analysis using data from jurisdictional immunization information systems was used to estimate vaccination coverage among adolescent girls as of the last day of each calendar year during 2013–2023. This analysis measured progress toward the WHO 2030 vaccination coverage goal of ≥90% completion of the HPV vaccination series among girls by age 15 years. As of December 2023, initiation of the HPV vaccination series among adolescent girls aged 13–17 years ranged from 58.0% in Palau to 97.2% in the Northern Mariana Islands, and HPV vaccination series completion coverage ranged from 43.4% in Palau to 91.8% in the Northern Mariana Islands. HPV vaccination series completion coverage is >90% in the Northern Mariana Islands and is on track to meet WHO goals by 2030 in American Samoa. Assessment of adolescent vaccination coverage can help immunization programs monitor progress toward regional goals and identify populations and areas with low coverage. Implementing evidence-based strategies to increase vaccine access and coverage would benefit jurisdictions with lagging coverage.

Introduction

Worldwide, cervical cancer is the fourth most common cancer among women, and the World Health Organization (WHO) Western Pacific region* accounts for one quarter of all estimated cases (1); the age-standardized rate of cervical cancer in the Marshall Islands (74 per 100,000 women) is the highest in the world (2). Nearly all cervical cancers are caused by human papillomaviruses (HPV). HPV vaccines, which have been licensed for use since 2006, are estimated to have the potential to prevent approximately 75% of all cervical cancers (3). CDC recommends HPV vaccination for both boys and girls at age 11–12 years.^† However, to assess progress toward reaching vaccination goals in the 2020 WHO Global Strategy to Accelerate the Elimination of Cervical Cancer as a Public Health Problem,^§ this report focuses on HPV vaccination coverage among adolescent girls. The WHO strategy recommends that HPV vaccines be included in all national immunization programs; the goal is for ≥90% of girls to complete the HPV vaccination series by age 15 years, by 2030 (4).

HPV vaccines were introduced across the U.S.-affiliated Pacific Islands (USAPI)^¶ during 2007–2016,** predominantly provided through school-located vaccination programs.^†† Assessment of vaccination coverage among adolescent girls can help immunization programs monitor progress toward regional goals and identify populations and areas with low coverage. These data can be used to guide evidence-based interventions, adapted to the local context, to improve vaccination coverage. This report describes annual HPV vaccination coverage among adolescent girls in five of the six USAPI^§§ jurisdictions during 2013–2023.

Methods

Data Sources and Inclusion and Exclusion Criteria

Patient-level data from jurisdictional immunization information systems (IISs) were aggregated at the jurisdiction level for this retrospective analysis. Persons were included in the denominator for annual analyses if they 1) were adolescent girls aged 13–17 years as of January 1 of the assessment year, 2) had an active patient status^¶¶ in the IIS through the end of the assessment year, and 3) had received any vaccine within the most recent 5 years. Exclusion criteria consistent with the Modeling of Immunization Registry Operations Work Group managing active patient status guidance was retrospectively applied to mitigate IIS denominator inflation (5). Patients were excluded from all analyses if they had zero vaccine doses recorded in the IIS or if the last vaccination date recorded in the IIS was before January 1, 2006.

Estimation of HPV Vaccination Coverage

Retrospective point-in-time analysis (i.e., coverage as of a specific date) was used to estimate vaccination coverage as of December 31 of each year during 2013–2023. All HPV vaccine doses received as of the end of the assessment year were included in coverage estimates. Vaccination coverage indicators included receipt of ≥1 HPV vaccine dose and HPV vaccination series completion status.*** Completion of the HPV vaccination series is defined as receipt of ≥3 HPV vaccine doses, or receipt of 2 doses if the series was initiated at age <15 years, and if ≥5 months minus 4 days have elapsed between receipt of the first and second dose.

HPV vaccination series dropout was measured as the proportion of adolescents who had not completed the HPV vaccination series by the end of the assessment year, among those who received the first dose. SAS software (version 9.4; SAS Institute) was used to conduct all analyses. This activity was reviewed by CDC, deemed not research, and was conducted consistent with applicable federal law and CDC policy.^†††

Results

Jurisdictional HPV Vaccination Coverage Among Adolescent Girls Aged 13–17 Years

Coverage with ≥1 HPV vaccine dose and HPV vaccination series completion status varied by jurisdiction (Table). As of December 2023, coverage with ≥1 HPV dose among adolescent girls aged 13–17 years ranged from 58.0% in Palau to 97.2% in the Northern Mariana Islands. HPV vaccination series completion coverage ranged from 43.4% in Palau to 91.8% in the Northern Mariana Islands. As of 2023, the Northern Mariana Islands is the only jurisdiction to have already met the WHO 2030 HPV 90% vaccination coverage goal.

Trends in HPV Vaccination Coverage Among Adolescent Girls, 2013–2023

During 2013–2023, coverage with ≥1 HPV vaccine dose increased by 35.2–72.8 percentage points across jurisdictions (Figure 1), and HPV vaccination series completion coverage increased by 35.3–72.9 percentage points (Figure 2). The percentage of adolescent girls who had received ≥1 HPV vaccine dose and who completed the vaccination series increased each year from 2013 to 2023 in all jurisdictions except Palau, where ≥1-dose coverage and HPV vaccination series completion coverage peaked in 2020 at 71.6% and 59.0%, respectively, and have since declined to 58.0% and 43.4%, respectively, in 2023 (Table). In American Samoa, HPV vaccination series completion coverage increased from 78.0% to 82.8% (4.8 percentage points) from 2022 to 2023. If coverage continues to increase at the same rate, American Samoa will meet the WHO 2030 ≥90% HPV vaccination series completion coverage goal by 2025.

HPV vaccination series dropout varied across jurisdictions and years; during 2013–2023, dropout decreased in all jurisdictions except in Palau, where it increased from a low of 17.2% in 2021 to a high of 25.2% in 2023 (Table). Dropout was lowest in the Northern Mariana Islands, where only 5.6% of adolescent girls aged 13–17 years who initiated the HPV vaccination series had not completed it in 2023.

Discussion

HPV vaccines are a critical public health tool to prevent most cervical cancers. HPV vaccination coverage has increased markedly in USAPI since the vaccination programs commenced, and HPV vaccination series completion coverage in the Northern Mariana Islands currently exceeds the ≥90% WHO 2030 goal. If the current coverage trends continue, American Samoa will also be on track to meet the WHO 2030 coverage goal. The 2023 rates of initiation of the HPV vaccination series among adolescent girls aged 13–17 years in American Samoa (95.7%) and the Northern Mariana Islands (97.2%) are higher than those in the three freely associated USAPI jurisdictions (Federated States of Micronesia, Marshall Islands, and Palau) (range = 58.0%–71.4%).

The differences in coverage among the USAPI jurisdictions might be attributed, at least in part, to differences in access to the vaccine. Both American Samoa and the Northern Mariana Islands have offered the vaccine through a mix of school-located vaccination programs as well as in public health clinics. These two jurisdictions receive vaccination program funding and vaccine supply through the Section 317 Immunization Program and the U.S. Vaccines for Children (VFC) program. The three freely associated jurisdictions are not eligible to receive VFC funding and thus have a more limited vaccine supply; therefore, these jurisdictions have not consistently been able to offer HPV vaccine in clinics or other locations outside the school setting.

The school-located HPV vaccination program is an evidence-based intervention to increase HPV vaccination coverage, particularly in low- and middle-income settings; however, jurisdiction-level coverage is constrained when the vaccine is only available in the school setting (6). For example, secondary school enrollment^§§§ among girls is approximately 66% in Federated States of Micronesia, 83% in Marshall Islands, and 80% in Palau, compared with approximately 97% in American Samoa and the Northern Mariana Islands (7–9). Strategies to reach out-of-school adolescent girls are needed to improve vaccination coverage in these settings. Providing vaccine access to girls who are not enrolled in school is also an important health equity consideration. Some research suggests that girls who drop out of school are more likely to contract sexually transmitted infections, such as HPV, than are those who remain in school (10).

In addition to challenges associated with accessing adolescent girls who are not in school, school-based HPV vaccination programs in some areas might have been suspended while schools were closed during the COVID-19 pandemic. The decline in coverage after 2020 in Palau might be evidence of the pandemic’s impact because coverage was trending up among girls who reached the target vaccination age of 11–12 years before 2020, compared with girls who reached age 11–12 years in 2020 and later. More research is needed to assess the underlying reasons for the lower coverage in the freely associated USAPI and to design and implement evidence-based interventions to improve vaccination outcomes adapted to the local context. Specific strategies might be needed to increase vaccination coverage among populations that have recently experienced larger declines in coverage, including those who would have been within the recommended age for vaccination during the pandemic.

Limitations

The findings in this report are subject to at least three limitations. First, accuracy of coverage estimates in this assessment is dependent upon completeness and accuracy of jurisdictional IIS data. Working with the jurisdictions, CDC has found high levels of completeness and accuracy of vaccination data (i.e., consistency in recorded dose dates and product types between paper and IIS records) across the five USAPI IISs included in this assessment through evaluations conducted since 2016. However, IIS data completeness before 2016 has not been evaluated. Second, the active patient population size could be inflated in IISs compared with census estimates because of difficulties tracking out-migration and deaths, which can lead to an underestimation of vaccination coverage. However, recent U.S. Census Bureau data were not available for denominator estimation for all jurisdictions included in this assessment. For this reason, exclusion criteria consistent with the Modeling of Immunization Registry Operations Work Group managing active patient status guidance were applied to retrospectively classify likely active patient status to patients in the IIS for each assessment year. Finally, vaccination coverage for Guam is assessed via the National Immunization Survey and was not included in this analysis. Differences in vaccination coverage estimation methods might mean that results are not directly comparable with IIS-based estimates for the other USAPI presented in this report.

Implications for Public Health Practice

Only two of the five USAPI have met or are on track to meet the WHO 2030 goal of ≥90% completion of the HPV vaccination series among girls by age 15 years. Identifying and implementing evidence-based strategies to increase vaccine access and coverage would benefit jurisdictions with lagging coverage. The USAPI immunization programs partner with various international governmental, nongovernmental, and academic organizations on immunization and comprehensive cancer control initiatives. Vaccination coverage data can support development of their activities by providing performance indicators and data for modeling health outcomes related to HPV vaccination, promoting health equity, and attaining the WHO 2030 goal of 90% HPV vaccination series completion coverage.