Revisiting the initial COVID-19 pandemic projections
Authors: Adam T Biggs, Lanny F Littlejohn
Journal: The Lancet Microbe (2021)

Early projections of the COVID-19 pandemic prompted federal governments to action. One critical report, published on March 16, 2020, received international attention when it predicted 2 200 000 deaths in the USA and 510 000 deaths in the UK without some kind of coordinated pandemic response.1Ferguson NM Laydon D Nedjati-Gilani G et al.Impact of non-pharmaceutical interventions (NPIs) to reduce COVID-19 mortality and healthcare demand.https://www.imperial.ac.uk/media/imperial-college/medicine/sph/ide/gida-fellowships/Imperial-College-COVID19-NPI-modelling-16-03-2020.pdfDate: March 16, 2020Date accessed: December 1, 2020Google Scholar This information became foundational in decisions to implement physical distancing and adherence to other public health measures because it established the upper boundary for any worst-case scenarios. However, the authors derived these projections from best available estimates at the time. The evolving nature of empirical knowledge about COVID-19 provides current estimates with more accurate information than what would have been available merely weeks after first discovery of the virus—plus the benefit of hindsight. For example, asymptomatic transmission has been said to be the Achilles' heel of public health strategies to control the pandemic,2Gandhi M Yokoe DS Havlir DV Asymptomatic transmission, the Achilles' heel of current strategies to control Covid-19.N Engl J Med. 2020; 382: 2158-2160Crossref PubMed Scopus (746) Google Scholar and several factors about asymptomatic cases remained uncertain during the early days. The report assumed that asymptomatic individuals were 50% as infectious as symptomatic cases,1Ferguson NM Laydon D Nedjati-Gilani G et al.Impact of non-pharmaceutical interventions (NPIs) to reduce COVID-19 mortality and healthcare demand.https://www.imperial.ac.uk/media/imperial-college/medicine/sph/ide/gida-fellowships/Imperial-College-COVID19-NPI-modelling-16-03-2020.pdfDate: March 16, 2020Date accessed: December 1, 2020Google Scholar whereas the current US Centers for Disease Control and Prevention (CDC) estimates suggest a 75% infectiousness rate for asymptomatic individuals.3Centers for Disease ControlCOVID-19 pandemic planning scenarios.https://www.cdc.gov/coronavirus/2019-ncov/hcp/planning-scenarios.htmlDate: Sept 10, 2020Date accessed: December 1, 2020Google Scholar A more important difference is the infection fatality ratio as originally projected in the Imperial College London (London, UK) report1Ferguson NM Laydon D Nedjati-Gilani G et al.Impact of non-pharmaceutical interventions (NPIs) to reduce COVID-19 mortality and healthcare demand.https://www.imperial.ac.uk/media/imperial-college/medicine/sph/ide/gida-fellowships/Imperial-College-COVID19-NPI-modelling-16-03-2020.pdfDate: March 16, 2020Date accessed: December 1, 2020Google Scholar versus current estimations. A high ratio of asymptomatic individuals might have inflated the perceived mortality of the disease given the limited testing supplies and attention to symptomatic cases. These analyses explore the original projections in light of this new information, which might be especially important when comparing the efficacy of any public health measures undertaken to prevent loss of life. Three sources of information were referenced for these assumptions: the original Imperial College London report,1Ferguson NM Laydon D Nedjati-Gilani G et al.Impact of non-pharmaceutical interventions (NPIs) to reduce COVID-19 mortality and healthcare demand.https://www.imperial.ac.uk/media/imperial-college/medicine/sph/ide/gida-fellowships/Imperial-College-COVID19-NPI-modelling-16-03-2020.pdfDate: March 16, 2020Date accessed: December 1, 2020Google Scholar the best estimates of current CDC pandemic planning scenarios,3Centers for Disease ControlCOVID-19 pandemic planning scenarios.https://www.cdc.gov/coronavirus/2019-ncov/hcp/planning-scenarios.htmlDate: Sept 10, 2020Date accessed: December 1, 2020Google Scholar and a seroprevalence study covering 45 countries.4O'Driscoll M Dos Santos GR Wang L et al.Age-specific mortality and immunity patterns of SARS-CoV-2.Nature. 2020; (published online Nov 2.)https://doi.org/10.1038/s41586-020-2918-0PubMed Google Scholar The CDC estimates largely come from a separate study,5Hauser A Counotte MJ Margossian CC et al.Estimation of SARS-CoV-2 mortality during the early stages of an epidemic: a modelling study in Hubei, China and northern Italy.medRxiv. 2020; (published online July 12.) (preprint).https://doi.org/10.1101/2020.03.04.20031104Google Scholar but the values are used here as presented by available CDC information. Population information was collected from the 2010 US census and 2011 UK census as the most complete and recent government surveys of those populations. Actual calculations used the most refined infection fatality ratio and age groups available, although data presented in the table truncate these values to the CDC limits for ease of presentation after adjusting for age and population differences. The percentage of population infected, 81%, comes from the original estimation in the Imperial College London report.TableProjected deaths based on age-adjusted infection fatality ratios for the USA and UKAge 0–19 yearsAge 20–49 yearsAge 50–69 yearsAge >70 yearsTotalUSAPopulation83 267 556126 429 14471 216 11727 832 721308 745 538Projected deathsImperial College London report273389 358725 2321 532 0442 349 367CDC estimations202320 482288 4251 217 4031 528 333Seroprevalence135948 638259 2351 070 3811 379 612UKPopulation15 098 00026 193 00014 533 0007 359 00063 183 000Projected deathsImperial College London report49318 744159 069402 318580 624CDC estimations367424358 859321 883385 351Seroprevalence25010 18456 653279 548346 637Data are from the initial Imperial College London report1Ferguson NM Laydon D Nedjati-Gilani G et al.Impact of non-pharmaceutical interventions (NPIs) to reduce COVID-19 mortality and healthcare demand.https://www.imperial.ac.uk/media/imperial-college/medicine/sph/ide/gida-fellowships/Imperial-College-COVID19-NPI-modelling-16-03-2020.pdfDate: March 16, 2020Date accessed: December 1, 2020Google Scholar and two more recent parameter estimations from the CDC3Centers for Disease ControlCOVID-19 pandemic planning scenarios.https://www.cdc.gov/coronavirus/2019-ncov/hcp/planning-scenarios.htmlDate: Sept 10, 2020Date accessed: December 1, 2020Google Scholar and a retrospective study with data from 45 countries (seroprevalence).4O'Driscoll M Dos Santos GR Wang L et al.Age-specific mortality and immunity patterns of SARS-CoV-2.Nature. 2020; (published online Nov 2.)https://doi.org/10.1038/s41586-020-2918-0PubMed Google Scholar CDC=US Centers for Disease Control and Prevention. Open table in a new tab Data are from the initial Imperial College London report1Ferguson NM Laydon D Nedjati-Gilani G et al.Impact of non-pharmaceutical interventions (NPIs) to reduce COVID-19 mortality and healthcare demand.https://www.imperial.ac.uk/media/imperial-college/medicine/sph/ide/gida-fellowships/Imperial-College-COVID19-NPI-modelling-16-03-2020.pdfDate: March 16, 2020Date accessed: December 1, 2020Google Scholar and two more recent parameter estimations from the CDC3Centers for Disease ControlCOVID-19 pandemic planning scenarios.https://www.cdc.gov/coronavirus/2019-ncov/hcp/planning-scenarios.htmlDate: Sept 10, 2020Date accessed: December 1, 2020Google Scholar and a retrospective study with data from 45 countries (seroprevalence).4O'Driscoll M Dos Santos GR Wang L et al.Age-specific mortality and immunity patterns of SARS-CoV-2.Nature. 2020; (published online Nov 2.)https://doi.org/10.1038/s41586-020-2918-0PubMed Google Scholar CDC=US Centers for Disease Control and Prevention. This simplified assessment arrives at a comparable approximation of the original report—2 349 367 projected deaths in the USA and 580 624 deaths in the UK. Applying age-adjusted infection fatality ratio rates to the census population values reveals a striking difference from CDC estimates and seroprevalence reporting. CDC estimates place total deaths at 1 528 333 in the USA and 385 351 deaths in the UK, whereas seroprevalence estimates total deaths at 1 379 612 in the USA and 346 637 deaths in the UK. For the US estimates, the differences produce a 54–70% overestimation of approximately 1 million deaths. For the UK estimates, the differences produce a 51–68% overestimation of approximately 200 000 deaths. Such overestimations remind us of several lessons learned over the course of the pandemic. First, the initial projections were never going to be 100% accurate with a novel coronavirus. Initial projections built worst-case scenarios that would never happen as a means of spurring leadership into action. This upper boundary of possibility then demonstrates a functional value of modelling efforts for unmitigated pandemic progression. Second, asymptomatic cases inflated perceived mortality ratios in addition to complicating any containment challenges. Third, consensus predictions underscore the value of public health coordination—especially early in a novel outbreak. When information is scarce, information sharing from multiple sources becomes crucial to attaining the clearest prediction possible. Last, in democracies, these public health crises will be politicised, and it is incumbent upon guardians of the public trust in health-care institutions and services to remain apolitical—to remain focused on scientific knowledge and the needs of public health, just as the US Department of Defense remains apolitical and focused on the needs of national defence. Ultimately, the relative value of mask wearing and physical distancing, and the economic consequences of lockdowns will be analysed retrospectively. These evaluations will use worst-case scenarios of unmitigated progression as the measuring stick to describe the merit of different public health interventions. Still, initial projections were commendable efforts that brought about public action despite more than 2 million deaths in the USA and more than 500 000 deaths in the UK being a significant overestimation. We declare no competing interests. The views expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the US Department of the Navy, the US Department of Defense, or the US Government. The authors are military service members or employees of the US Government. This work was prepared as part of their official duties. Title 17 U.S.C. §105 provides that 'Copyright protection under this title is not available for any work of the United States Government.' Title 17 U.S.C. §101 defines a US Government work as a work prepared by a military service member or employee of the US Government as part of that person's official duties.