Disparities in Human Papillomavirus vaccination coverage among adolescents in Australia: A geospatial analysis
Introduction
Australia has been a pioneering country in establishing innovative public health strategies to reduce the burden associated with human papillomavirus (HPV) infections [1]. Launched in 2007, the Australian HPV Vaccination Programme initially provided immunisation for girls through a school-based approach and for young women aged up to 26 years. More recently, in 2013, Australia became the first country to expand the eligibility for HPV vaccination by implementing a strategy of gender-neutral HPV immunisation for adolescents [1]. This step towards providing universal HPV vaccination for boys and girls represented a major advance in terms of health equity and program acceptability in Australia [2]. While an increasing number of countries have introduced HPV vaccination (approximately 55% of the World Health Organisation Member States), coverage rates remain extremely low, reaching only 15% of girls in the target age for HPV vaccination worldwide [3]. Only one-third of the HPV vaccination programs worldwide are gender-neutral, mostly in high-income countries [4].
Positive health and economic effects of HPV vaccination have been widely documented [5], [6], [7]. The introduction of HPV vaccines and the implementation of high-coverage HPV immunisation programs have created a promising opportunity to eliminate the burden of HPV-related diseases within a few decades [8]. It is estimated that 630,000 new cases of cancer per year are attributable to HPV worldwide, accounting for 4.5% of all cancers [9]. Although the vast majority of HPV-related malignant lesions are cervical cancer (84%), there has been an increase in the incidence rates of HPV-attributable cancers involving other sites such as oropharynx, anus, and penis [10], [11], [12]. While HPV-attributable oropharyngeal cancer is particularly predominant in high-income countries, approximately two-thirds of cervical cancer cases are concentrated in low- and middle-income countries [9].
The recent development of cutting-edge geospatial technologies has allowed social epidemiologists to examine the effects of area-level factors on health outcomes, contributing to better understanding of mechanisms that lead to health disparities within and across populations [13]. Geographic heterogeneity in factors related to population characteristics (such as ethnicity, age, and educational status) and the context in which people live (such as neighbourhood-level deprivation and proximity to health services) can help explain disparities in vaccination coverage, in addition to identifying at-risk populations in need of specific interventions [14]. Trogdon and Ahn used geospatial methods to investigate the geographic distribution of HPV vaccination uptake among Children in North Carolina, showing a clustering effect of neighboring areas for boys but not for girls [15]. A strong spatial dependence of HPV vaccination rates across ZIP codes in Minnesota was reported by Nelson et al, demonstrating the need to employ spatial models to accurately estimate associations with HPV vaccination [16]. Staples et al. revealed contrasting effects of area-level socioeconomic factors for HPV vaccine initiation and completion among adolescents [17]. Other studies have shown that area-level characteristics such as ethnic composition, population density, gender distribution, area-level socioeconomic status, geographic location, and local health services are associated with the geospatial patterns of HPV vaccination coverage across different populations [18], [19], [20].
In Australia, the school-based HPV vaccination program has achieved meaningful outcomes, reaching 80.5% and 77.6% of all boys and girls aged 15 years in 2020. However, inequalities in vaccine course completion remain substantial across states and between Indigenous and non-Indigenous groups (for instance, the HPV vaccine course completion rate for Indigenous boys in South Australia is as low as 47.5%) [21]. Regional disparities in HPV vaccination coverage are likely to further expand the gaps related to the incidence of HPV infections and HPV-attributable cancers between groups. In this ecological study, we adopted a machine learning approach and a sophisticated geospatial technique to model the geographic distribution of HPV vaccination rates among Australian adolescents. This study aimed to 1) examine the geographic patterns in HPV vaccination rates among boys and girls aged 15 years across locations in Australia; and 2) assess contextual area-level factors that may explain the variations in HPV vaccination coverage.
Section snippets
Methods
This study adopted the 2011 Australian Statistical Areas 4 (SA4) as the geographic scale for the analyses. SA4 are the largest regions within states in the Main Structure of the Australian Statistical Geography Standard (ASGS) and include a minimum population of 100,000 people [22]. Non-spatial SA4s (Migratory - Offshore - Shipping and No Usual Address) were not considered.
Main outcomes
Area-level data for this secondary analysis were obtained from official Australian Government agencies. Publicly available data on 2015–2016 HPV vaccination rates for boys and girls aged 15 years were obtained from the Australian Institute of Health and Welfare (AIHW). Records are managed by the National HPV Vaccination Program Register (HPV Register) and include data on boys and girls who had completed the three-dose vaccination scheme for HPV by 30 June 2016. Vaccination rates were computed
Data processing
A correlation matrix was estimated to assess the multicollinearity among the explanatory variables. Correlation coefficients greater than 0.75 were used as the criterion to drop redundant predictors [26].
Gradient Boosting Machine model
The Gradient Boosting Machine (GBM) model is an ensemble machine learning algorithm that trains decision trees in a gradual, progressive, and additive, approach. By iteratively training new trees based on the revised residuals of previous steps, GBM can successfully identify areas where data is
Results
Overall, 216,575 (76.9%) Australian adolescents aged 15 years had completed the recommended HPV immunisation scheme in 2015/2016. Girls (79.9%) had a slightly higher vaccination rate compared to boys (73.9%). A different pattern was observed in the remote areas of South Australia, where Boys presented higher coverage (65.7% against 62.2% for girls). HPV vaccination rates across the 88 SA4 regions varied from 57.6% and 90.6% among girls, and from 53.6% and 85.5% among boys. Areas with higher
Discussion
This study examined the geospatial patterns of HPV vaccination rates among Australian girls and boys aged 15 years. We selected a range of area-level factors to help elucidate regional disparities in HPV immunisation and applied machine learning algorithms to identify the most relevant predictors. Geographically weighted regression models were employed to explore variations in the importance of each predictor across different SA4 regions. Findings demonstrate that there is an unequal geospatial
Conclusion
Area-level socioeconomic and educational factors were the most important predictors of regional HPV vaccination rates among Australian boys and girls. No factor presented a uniform effect on HPV vaccination across SA4 regions. Instead, important spatial heterogeneity in the effect of predictors was identified across the study area. A higher proportion of adults who never attended school was strongly associated with lower vaccination rates of boys and girls living in Western Australia. The SEIFA
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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