Correcting under-reported COVID-19 case numbers

Alexander Lachmann

Coronavirus Disease 19 (COVID-19) is a novel human illness caused by the Severe Acute Respiratory Syndrome Coron-aVirus 2 (SARS-CoV-2), a pathogen initially discovered in the Wuhan region of China at the end of 2019 (1) . It has reached pandemic status by the World Health Organization (WHO) within less than four months of initial reports of the disease. The origin of the virus can be traced back to related strains predominantly found in bats (2) . Individuals infected by the disease can experience a range of symptoms, including cough, chills, fever, and shortness of breath (1) . From data currently available, fatal disease progression is higher than that of the common influenza strains, and as such, more COVID-19-related deaths are occurring than had resulted from the recent Severe Acute Respiratory Syndrome (SARS) virus and the Middle East Respiratory Syndrome (MERS) combined (3) . The infection rate of COVID-19 has been estimated between an R 0 of 2 and up to 6.49 (4) compared with influenza, characterized by an R 0 of approximately 1.3 (5) . The severity of infection is highly correlated to the age of the infected individual. Younger segments of a population present a much lower risk than older populations. A current data release from the Center for Disease Control in South Korea shows that while there are few reported fatalities for individuals under 30 years of age, the case fatality rate (CFR) (percent of deaths per confirmed case) for individuals older than 80 is over 8% (6) . Figure 1 shows six countries with a significant number of reported COVID-19 cases. China, which has been the origin of the outbreak (7), initially registered the most cases early on with over 80,000, but has since been overtaken by the US. Through severe containment measures such as curfews and lockdowns of public life, new infections have slowed significantly (8) . Other countries that have been affected only recently are still in the exponential growth curve and have at this point overtaken China in the total number of confirmed COVID-19 cases. Countries like Italy and the US have only recently taken action to slow the spread of the virus. With a reported incubation time of about five days, there is a time-delay for the effects of a slowdown to becoe evident (9) . Another country that is currently experiencing high numbers of reported COVID-19 cases is Iran, with more than 12,000 confirmed cases. Similar to Italy the growth rates are slowing for more than two weeks as of writing. Due to the limited information available, most parameters describing the dynamics of the disease spread involve significant uncertainties. Healthcare systems in most countries are not capable of monitoring the exponential growth of a virus in this manner. South Korea, as of writing, has one of the most extensive capabilities of testing individuals per capita, with a capacity of more than 20,000 tests a day. Hence, South Korea represents the best benchmark country in order to predict the COVID-19 CFR. The proposed method uses demographic information to identify the fraction of the vulnerable population. Countries such as China have a generally younger population, reducing the overall risk of fatal outcomes, and this should result in a lower CFR compared with South Korea. Countries such as Italy, having an older population compared with South Korea, should have higher CFRs. Estimating the true case count is relevant to identifying the correct measures to stop the disease from sprea