School immunization coverage in adolescents during the COVID-19 pandemic: A retrospective cohort study

INTRODUCTION: Few studies have assessed the impact of the coronavirus disease 2019 (COVID-19) pandemic on immunization coverage for adolescents, and little is known about how coverage has changed throughout the pandemic. We aimed to: (1) assess the change in coverage for school-based vaccines in Alberta, Canada resulting from the pandemic; (2) determine whether coverage differed by geographic health zone and school type; and (3) ascertain whether coverage has returned to pre-pandemic levels. METHODS: Using a retrospective cohort design, we used administrative health data to compare coverage for human papillomavirus (HPV) and meningococcal conjugate A, C, Y, W-135 (MenC-ACYW) vaccines between pre-pandemic (2017–2018 school year) and pandemic (2019–2020 and 2020–2021 school years) cohorts (N = 289,420). Coverage was also compared by health zone and authority type. The 2019–2020 cohort was followed over one year to assess catch-up. RESULTS: Compared to 2017–2018, immunization coverage for HPV was significantly lower in the 2019–2020 (absolute difference: 60.8%; 95% CI: 60.4–61.3%) and 2020–2021 cohorts (absolute difference: 59.9%; 95% CI: 59.4–60.3%). There was a smaller, significant decline in MenC-ACYW coverage comparing 2017–2018 to 2019–2020 (absolute difference: 6.1%; 95% CI: 5.6–6.5%) and 2020–2021 (absolute difference: 32.2%; 95% CI: 31.6–32.7%). Private schools had low coverage overall, while coverage fluctuated by zone. During follow-up of the 2019–2020 cohort, coverage for HPV and MenC-ACYW increased from 5.6% to 50.2%, and 80.7% to 83.0%, respectively. CONCLUSION: There was a substantial decrease in school-based immunization coverage during the COVID-19 pandemic, and coverage has not returned to pre-pandemic levels, suggesting further catch-up is needed.