Optimising health and economic impacts of COVID-19 vaccine prioritisation strategies in the WHO European Region

Abstract

Background: Countries in the World Health Organization (WHO) European Region differ in terms of the COVID-19 vaccine roll-out speed. We evaluated the health and economic impact of different age-based vaccine prioritisation strategies across this demographically and socio-economically diverse region.

Methods: We fitted country-specific age-stratified compartmental transmission models to reported COVID-19 mortality in the WHO European Region to inform the immunity level before vaccine roll-out. Building upon broad recommendations from the WHO Strategic Advisory Group of Experts on Immunisation (SAGE), we examined four strategies that prioritise: all adults (V+), younger (20-59 year-olds) followed by older adults (60+) (V20), older followed by younger adults (V60), and the oldest adults (75+) (V75) followed by incremental expansion to successively younger five-year age groups. We explored four roll-out scenarios based on projections or recent observations (R1-4) - the slowest scenario (R1) covers 30% of the total population by December 2022 and the fastest (R4) 80% by December 2021. Five decision-making metrics were summarised over 2021-22: mortality, morbidity, and losses in comorbidity-adjusted life expectancy (cLE), comorbidity- and quality-adjusted life years (cQALY), and the value of human capital (HC). Six sets of infection-blocking and disease-reducing vaccine efficacies were considered.

Findings: The optimal age-based vaccine prioritisation strategies were sensitive to country characteristics, decision-making metrics and roll-out speeds. Overall, V60 consistently performed better than or comparably to V75. There were greater benefits in prioritising older adults when roll-out is slow and when VE is low. Under faster roll-out, V+ was the most desirable option.

Interpretation: A prioritisation strategy involving more age-based stages (V75) does not necessarily lead to better health and economic outcomes than targeting broad age groups (V60). Countries expecting a slow vaccine roll-out may particularly benefit from prioritising older adults.

Funding: World Health Organization, Bill and Melinda Gates Foundation, the Medical Research Council (United Kingdom), the National Institute of Health Research (United Kingdom), the European Commission, the Foreign, Commonwealth and Development Office (United Kingdom), Wellcome Trust.

Research in context: Evidence before this study: We searched PubMed and medRxiv for articles published in English from inception to 9 Jun 2021, with the search terms: ("COVID-19" OR "SARS-CoV-2") AND ("priorit*) AND ("model*") AND ("vaccin*") and identified 66 studies on vaccine prioritization strategies. Of the 25 studies that compared two or more age-based prioritisation strategies, 12 found that targeting younger adults minimised infections while targeting older adults minimised mortality; an additional handful of studies found similar outcomes between different age-based prioritisation strategies where large outbreaks had already occurred. However, only two studies have explored age-based vaccine prioritisation using models calibrated to observed outbreaks in more than one country, and no study has explored the effectiveness of vaccine prioritisation strategies across settings with different population structures, contact patterns, and outbreak history.Added-value of this study: We evaluated various age-based vaccine prioritisation strategies for 38 countries in the WHO European Region using various health and economic outcomes for decision-making, by parameterising models using observed outbreak history, known epidemiologic and vaccine characteristics, and a range of realistic vaccine roll-out scenarios. We showed that while targeting older adults was generally advantageous, broadly targeting everyone above 60 years might perform better than or comparably to a more detailed strategy that targeted the oldest age group above 75 years followed by those in the next younger five-year age band. Rapid vaccine roll-out has only been observed in a small number of countries. If vaccine coverage can reach 80% by the end of 2021, prioritising older adults may not be optimal in terms of health and economic impact. Lower vaccine efficacy was associated with greater relative benefits only under relatively slow roll-out scenarios considered.Implication of all the available evidence: COVID-19 vaccine prioritization strategies that require more precise targeting of individuals of a specific and narrow age range may not necessarily lead to better outcomes compared to strategies that prioritise populations across broader age ranges. In the WHO European Region, prioritising all adults equally or younger adults first will only optimise health and economic impact when roll-out is rapid, which may raise between-country equity issues given the global demand for COVID-19 vaccines.

Figure 1.. Model Framework

This figure describes the overall model framework of the study, which consists of the fitting and projection stages. The “known parameters based on existing knowledge” were used in both fitting and projection stages. The remaining input was used in only one of the stages as specified. In the conceptual diagram of the model: S - Susceptible; E - Exposed; V - Vaccinated; E_v - Exposed among vaccinated; I_p - presymptomatic & infectious; I_s - symptomatic & infectious; I_a - asymptomatic & infectious; R - Removed.

Figure 2.. Key inputs and assumptions.

(A) Example population age structure (for the United Kingdom, unit = million). (B) Example age-specific within-population contact patterns (for the United Kingdom). (C) Vaccine roll-out scenarios and the respective proportions of populations expected to be covered at different time points. Note that under different vaccine roll-out scenarios, the starting time of vaccination programs may diff. Grey lines in the background represent observed country-level vaccine uptakes (of the first dose) over time reported in the WHO European Region. (D) Vaccine prioritisation strategies. Hatched areas indicate when no vaccine is allocated into the corresponding age groups. (E) Vaccine profiles consisting of vaccine efficacy against infection and disease.

Figure 3.. Results of the fitting stage

(A-C) Comparisons between observed COVID-19 (purple line), predicted COVID-19 deaths using a deterministic realisation using fitted parameters (black line), and 100 stochastic realisations using the same fitted parameters (grey lines) in Georgia, Hungary, and the United Kingdom. (D) The estimated proportions of individuals no longer susceptible (infectious or recovered) to SARS-CoV-2 infection on 01 Jan 2021 by deciles. Countries marked by crosshatch patterns are those that were not included in the fitting stage; countries marked by solid grey are outside the WHO European Region. Shapefiles were downloaded from Eurostat GISCO.

Figure 4.. Optimal vaccine prioritisation strategies under different roll-out scenarios and decision-making metrics

Main panel - Optimal strategies across the WHO European Region that minimise COVID-19 deaths, cases, losses in adjusted life expectancy (cLE), quality-adjusted life-years (cQALY), and human capital (HC) as decision-making metrics. Top right insert within each panel - y-axis: Difference in outcome (totalled over the region) when a given prioritisation strategy is used across the entire WHO European Region compared to if the optimal prioritisation strategy in each country is used (black) x-axis: ranking. Sidebars to the right of each panel - the proportion of total population for which each prioritisation strategy is optimal. Shapefiles are downloaded from Eurostat GISCO; countries marked by crosshatch patterns are those that were not included in the projection stage.

Figure 5.. Optimal vaccine prioritisation strategies given different vaccine profiles under R2 and R3.

Optimal strategy for each country and vaccine profile while minimising mortality, morbidity, and losses in adjusted life expectancy (cLE), quality-adjusted life-years (cQALY), and human capital (HC) for 38 countries in the WHO European Region. Countries are arranged in the order of the estimated proportions of the population no longer susceptible to SARS-CoV-2 on 01 Jan 2021 (descending). Countries are labelled by their World Bank country codes - a reference table can be found in Appendix 1.