Predictive performance of multi-model ensemble forecasts of COVID-19 across European nations

Sherratt, Katharine (author), Žibert, Janez (author)

Background: Short-term forecasts of infectious disease burden can contribute to situational awareness and aid capacity planning. Based on best practice in other fields and recent insights in infectious disease epidemiology, one can maximise the predictive performance of such forecasts if multiple models are combined into an ensemble. Here, we report on the performance of ensembles in predicting COVID-19 cases and deaths across Europe between 08 March 2021 and 07 March 2022. Methods: We used open-source tools to develop a public European COVID-19 Forecast Hub. We invited groups globally to contribute weekly forecasts for COVID-19 cases and deaths reported by a standardised source for 32 countries over the next 1–4 weeks. Teams submitted forecasts from March 2021 using standardised quantiles of the predictive distribution. Each week we created an ensemble forecast, where each predictive quantile was calculated as the equally-weighted average (initially the mean and then from 26th July the median) of all individual models’ predictive quantiles. We measured the performance of each model using the relative Weighted Interval Score (WIS), comparing models’ forecast accuracy relative to all other models. We retrospectively explored alternative methods for ensemble forecasts, including weighted averages based on models’ past predictive performance. Results: Over 52 weeks, we collected forecasts from 48 unique models. We evaluated 29 models’ forecast scores in comparison to the ensemble model. We found a weekly ensemble had a consistently strong performance across countries over time. Across all horizons and locations, the ensemble performed better on relative WIS than 83% of participating models’ forecasts of incident cases (with a total N=886 predictions from 23 unique models), and 91% of participating models’ forecasts of deaths (N=763 predictions from 20 models). Across a 1–4 week time horizon, ensemble performance declined with longer forecast periods when forecasting cases, but remained stable over 4 weeks for incident death forecasts. In every forecast across 32 countries, the ensemble outperformed most contributing models when forecasting either cases or deaths, frequently outperforming all of its individual component models. Among several choices of ensemble methods we found that the most influential and best choice was to use a median average of models instead of using the mean, regardless of methods of weighting component forecast models. Conclusions: Our results support the use of combining forecasts from individual models into an ensemble in order to improve predictive performance across epidemiological targets and populations during infectious disease epidemics. Our findings further suggest that median ensemble methods yield better predictive performance more than ones based on means. Our findings also highlight that forecast consumers should place more weight on incident death forecasts than incident case forecasts at forecast horizons greater than 2 weeks.

• Language: English
• Keywords: infectious diseases forecatsting, epidemiology, mathematical modeling, capacity planning, COVID-19, combining independent models, ensemble forecast
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: ZF - Faculty of Health Sciences
• Publication status: Published
• Publication version: Version of Record
• Publication date: 01.01.2023
• Year: 2023
• Number of pages: 19 str.
• Numbering: Vol. 12, art. e81916
• PID: 20.500.12556/RUL-146651
• UDC: 616-036.22:519.876.5
• ISSN on article: 2050-084X
• DOI: 10.7554/eLife.81916
• COBISS.SI-ID: 150833411
• Publication date in RUL: 05.06.2023

Record is a part of a journal

• Title: eLife
• Shortened title: eLife
• Publisher: eLife Sciences Publications
• ISSN: 2050-084X
• COBISS.SI-ID: 523069721

Licences

• License: CC0 1.0, Creative Commons CC0 1.0 Universal
• Link: https://creativecommons.org/publicdomain/zero/1.0/deed.en
• Description: CC Zero enables scientists, educators, artists and other creators and owners of copyright- or database-protected content to waive those interests in their works and thereby place them as completely as possible in the public domain, so that others may freely build upon, enhance and reuse the works for any purposes without restriction under copyright or database law.
• Licensing start date: 02.06.2023

Projects

• Funder: EC - European Commission
• Funding programme: H2020
• Project number: 101016233
• Name: Pan-European Response to the ImpactS of COVID-19 and future Pandemics and Epidemics
• Acronym: PERISCOPE

• Funder: Other - Other funder or multiple funders