Study: Actual Number of COVID-19 Cases Vastly Underestimated

Apr 07, 2020

According to the findings, on average, only 6% of actual SARS-CoV-2 infections have been detected worldwide, meaning that the vast majority of infections have gone undetected.

Although the estimation of reported confirmed COVID-19 cases has hit almost 1.4 million at the time of this publication, the actual number of cases may have already reached several tens of millions, according to a new study published in The Lancet Infectious Diseases.1,2

Because of varying delays and insufficiencies in testing across different countries, reported numbers worldwide may not accurately reflect the amount of actual COVID-19 infections. Assessing the severity of the pandemic is crucial to identifying appropriate mitigation strategies.2

“However, crude case fatality ratios obtained by dividing the number of deaths by the number of cases can be misleading,” the authors wrote in the study.2 This is because it can take a couple of weeks between a person developing symptoms, the case subsequently being detected and reported, and observation of the final outcome.

For the study, the authors used estimates of COVID-19 mortality and time until death to examine the quality of official case records.2

According to the findings, on average, only 6% of actual SARS-CoV-2 infections have been detected worldwide, meaning that the vast majority of infections have gone undetected.2

Much of the data informing global estimates of case fatality ratio are from the early outbreak in Wuhan, China, the authors noted. Because the health system in this city was quickly overwhelmed, the authors suggested that there is substantial underestimation of cases in younger age groups compared with elsewhere in China. Reported cases in Wuhan were more frequent in older age groups, which may reflect higher severity, and cases outside of Wuhan may also show bias in terms of the relationship between age and travel.2

“The case fatality ratio is likely to be strongly influenced by the availability of health care facilities,” they added.2

Although the study’s estimates suggest lower case fatality ratio for COVID-19 than some of the crude estimates made to date, the authors indicated that they are still substantially higher than for recent influenza pandemics. Furthermore, the authors project that, with the continued spread of COVID-19 and proportion of individuals requiring hospitalization, “even the most advanced health care systems are likely to be overwhelmed.”2

“These results mean that governments and policy-makers need to exercise extreme caution when interpreting case numbers for planning purposes,” Sebastian Vollmer, PhD, professor of development economics at the University of Göttingen, said in a press release.3 “Such extreme differences in the amount and quality of testing carried out in different countries mean that official case records are largely uninformative and do not provide helpful information.”


1. Coronavirus COVID-19 Global Cases by the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University. Johns Hopkins University’s website. Accessed April 7, 2020.

2. Verity R, Okell LC, Dorigatti N, et al. Estimates of the severity of coronavirus disease 2019: a model-based analysis. The Lancet Infectious Diseases. March 30, 2020. S1473-3099(20)30243-7

3. Press release: COVID-19: on average only 6% of actual SARS-CoV-2 infections detected worldwide [news release]. University of Göttingen’s website. Accessed April 7, 2020.