Robust estimates of the true (population) infection rate for COVID-19: a backcasting approach.

Steven J Phipps,R Quentin Grafton,Tom Kompas

doi:10.1098/rsos.200909

Abstract

Differences in COVID-19 testing and tracing across countries, as well as changes in testing within each country over time, make it difficult to estimate the true (population) infection rate based on the confirmed number of cases obtained through RNA viral testing. We applied a backcasting approach to estimate a distribution for the true (population) cumulative number of infections (infected and recovered) for 15 developed countries. Our sample comprised countries with similar levels of medical care and with populations that have similar age distributions. Monte Carlo methods were used to robustly sample parameter uncertainty. We found a strong and statistically significant negative relationship between the proportion of the population who test positive and the implied true detection rate. Despite an overall improvement in detection rates as the pandemic has progressed, our estimates showed that, as at 31 August 2020, the true number of people to have been infected across our sample of 15 countries was 6.2 (95% CI: 4.3–10.9) times greater than the reported number of cases. In individual countries, the true number of cases exceeded the reported figure by factors that range from 2.6 (95% CI: 1.8–4.5) for South Korea to 17.5 (95% CI: 12.2–30.7) for Italy.

Highlights

COVID-19, caused by SARS-CoV-2, was declared a pandemic by the World Health Organization on 11 March 2020, after first being identified in China in December 2019
If f (x; α, β) is the probability density function for the Gamma distribution with infection fatality rate defined by IFR, the number of new infections estimated to have occurred on day t0 and to have resulted in fatalities on day t is given by ni(t0, t) f(t À IFR
We used our backcasting approach to study the progression of the COVID-19 pandemic within 15 countries: the 11 European countries studied by Flaxman et al [5], as well as Australia, Canada, South Korea and the USA

Summary

Introduction

COVID-19, caused by SARS-CoV-2, was declared a pandemic by the World Health Organization on 11 March 2020, after first being identified in China in December 2019. Since January 2020, researchers have used the reported cases of COVID-19, detected using tests for 2 the presence of RNA material in nasal secretions or sputum in individuals, to estimate the rate of infection within a population. An initially inadequate number of both testing kits and testing facilities, coupled with restrictions on who could be tested, meant that the number of confirmed cases as a proportion of the total population underestimated the true (population) infection rate. Multiple lines of evidence suggest that COVID-19 may be much more widespread within the population than is suggested by the outcomes of the limited direct testing conducted to date [2,3,4,5,6,7,8,9,10,11,12]

Methods

Results

Conclusion