Organised cervical screening seeks to identify human papilloma virus (HPV) infection and cervical abnormalities early enough for follow-up and treatment before invasive disease develops. Cervical cancer remains a major cause of mortality among women, with global estimates for 2020 indicating hundreds of thousands of diagnoses and deaths. In Ireland, around 290 women are diagnosed each year and about 90 die annually, with cervical cancer reported as a leading cause of cancer mortality among women aged 25–40. Ireland’s national programme, CervicalCheck, began in 2008 with a call-recall system and an integrated pathway for screening, diagnosis and treatment. Estimating prevention requires a counterfactual approach because screening changes what is detected and when.
From HPV Infection to Preventable Cancer
HPV infection is common and often clears without long-term consequences. Persistent infection with oncogenic strains can lead to cervical intra-epithelial neoplasia (CIN), ranging from low-grade to high-grade abnormalities. Lesions can regress, persist or progress over time. Screening aims to detect infection or abnormal cells among people without symptoms, enabling monitoring, colposcopy and treatment that can interrupt progression before cancer develops.
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Clinical and programme categories do not map perfectly onto histology. Cytology may be reported as low-grade or high-grade squamous intraepithelial lesions, while biopsy results are often expressed as CIN grades. High-grade reporting can combine CIN 2 and CIN 3, and this grouping matters because these grades do not carry the same likelihood of future invasive disease. Programme interpretation also depends on context, including demographic change and evidence of changing sexual behaviour during the period in which national screening was introduced.
Modelling a Counterfactual Without Screening
A Markov-chain model was used to estimate cancers prevented by comparing observed screening outcomes with a scenario without screening or intervention. The framework represented transitions across a small set of states spanning clearance, HPV infection, CIN grades and cancer. Clear and cancer were treated as absorbing states. CIN states could progress, persist or regress directly to clear, reflecting clearance linked to HPV resolution. This structure was treated as conservative, producing a lower-end estimate of cancers prevented.
Transition probabilities were informed by published evidence on regression, persistence and progression across HPV infection and CIN grades, with recognised variability due to classification differences, observation times and ethical constraints around untreated high-grade abnormalities. Treatment effects were incorporated by making cancer occurrence vanishingly unlikely for treated CIN 1 and CIN 2 and by applying high treatment efficacy for CIN 3 derived from reported long-term incidence after adequate treatment. Uncertainty was carried through the model to generate confidence bounds around key outputs.
Programme Findings and Service Implications
The model used CervicalCheck programme data and National Cancer Registry of Ireland (NCRI) figures from August 2008 to August 2022. Across the period analysed, 152,674 results were referred to colposcopy, representing approximately 55,902 unique women. The model estimated 5,557 cervical cancers prevented between 2008 and 2022, with a 95% confidence interval of 5,114–6,000. Using an NCRI mean treatment cost estimate per case, the prevented cancers corresponded to €102 million in future treatment costs avoided, excluding inflation.
Registry data showed a peak in diagnosed cancers soon after programme introduction, consistent with earlier detection. Over time, screening accounted for a substantial share of diagnoses, with an overall estimate of 48.8% of cervical cancers detected through screening since 2009, alongside variation across earlier and later periods. Screening activity fell in 2020 when services were reduced during the COVID-19 pandemic. Service design also evolved with the move to primary HPV screening in 2020, with HPV testing first and follow-up smears reserved for cases with high-risk HPV detected, reducing false positives by approximately a factor of 10 and lowering unnecessary colposcopy visits while maintaining benefits. HPV vaccination has been available in Ireland since 2010, with uptake returning to relatively high levels among girls after a confidence crisis linked to antivaccine disinformation, and vaccination later introduced for boys.
Modelling anchored in Irish programme and registry data estimated substantial prevention attributable to organised screening and treatment. The estimates included 5,557 cancers prevented between 2008 and 2022 and €102 million in future treatment costs avoided, excluding inflation. Detection patterns were consistent with earlier diagnosis soon after implementation and a sustained contribution of screening to case detection over time. The findings connect programme design, follow-up pathways and service continuity to long-term outcomes, while primary HPV screening and vaccination shape expectations for future screening volumes, colposcopy demand and cervical cancer burden in Ireland.
Source: European Journal of Public Health
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