The number of confirmed cases for COVID-19 is dramatically understated due to insufficient testing


The number of confirmed cases for the novel coronavirus disease COVID-19 officially issued by countries and widely commented on by national and international media outlets dramatically understates the true number of infections, a recent report from the University of Göttingen suggests.

Dr Christian Bommer and Professor Sebastian Vollmer from Göttingen University have used estimates of COVID-19 mortality and time until death from a recent study published in The Lancet Infectious Diseases to test the quality of official case records.

Their data shows that countries have only discovered on average about 6% of coronavirus infections and the true number of infected people worldwide may already have reached several tens of millions.

Insufficient and delayed testing may explain why some European countries, such as Italy and Spain, are experiencing much higher casualty numbers (relative to reported confirmed cases) than Germany, which has detected an estimated 15.6% of infections compared to only 3.5% in Italy or 1.7% in Spain.

Detection rates are even lower in the United States (1.6%) and the United Kingdom (1.2%) – two countries that have received widespread criticism from public health experts for their delayed response to the pandemic.

In sharp contrast to this, South Korea appears to have discovered almost half of all its SARS-CoV-2 infections.

By early spring, Europe had become the worst-affected region, with Italy and Spain particularly hard hit. However, differences in testing mean that the number of cases may be understated for some countries.

Confirmed cases of Covid-19 for selected countriesShowing the number of cases since the day of the 100th case, using a log scale. Data correct at 23.59 UTC 6 April

The number of deaths is a more dependable indicator. The disease is hitting Italy and Spain with particular cruelty. But the trajectory in many countries is the same; the UK and US are a couple of weeks behind Italy in the progress of the epidemic.

Confirmed deaths of Covid-19 for selected countriesShowing the number of deaths since the day of the first one, using a log scale. Data correct at 23.59 UTC 6 April

Meanwhile in Asia, where the disease began, the spread continues, although in China it seems for now to have passed its peak.

The authors estimate that on 31 March 2020, Germany had 460,000 infections. Based on the same method, they calculate that the United States had more than ten million, Spain more than five million, Italy around three million and the United Kingdom around two million infections.

On the same day the Johns Hopkins University reported that globally there were less than 900,000 confirmed cases, meaning that the vast majority of infections were undetected.

Sebastian Vollmer, Professor of Development Economics at the University of Göttingen, says, “These results mean that governments and policy-makers need to exercise extreme caution when interpreting case numbers for planning purposes.

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.”

Christian Bommer adds: “Major improvements in the ability of countries to detect new infections and contain the virus are urgently needed.”

I would like to analyze a very interesting case, in which the danger of the asymptomaticity of COVID-19 is evident!

An outbreak of coronavirus disease 2019 (COVID-19) unfolded on board a Princess Cruises’ ship called the Diamond Princess.

Shortly after arriving in Yokohama, Japan, this ship had been placed under quarantine orders from 5 February 2020, after a former passenger had tested positive for the virus responsible for the disease (i.e. severe acute respiratory syndrome coronavirus 2; SARS-CoV-2), subsequent to disembarking in Hong Kong.

In this study, we conducted a statistical modelling analysis to estimate the proportion of asymptomatic individuals among those who tested positive for SARS-CoV-2 on board the ship until 20 February 2020 included, along with their times of infections.

The model accounted for the delay in symptom onset and also for right censoring, which can occur due to the time lag between a patient’s examination and sample collection and the development of illness.

Epidemiological description and data

By 21 February 2020, 2 days after the scheduled 2-week quarantine came to an end, a total of 634 people including one quarantine officer, one nurse and one administrative officer tested positive for SARS-CoV-2.

These individuals were among a total of 3,711 passengers and crew members on board the vessel.

Laboratory testing by PCR had been conducted, prioritising symptomatic or high-risk groups.

Daily time series of laboratory test results for SARS-CoV-2 (both positive and negative), including information on presence or absence of symptoms from 5 February 2020 to 20 February 2020 were extracted from secondary sources [1].

The reporting date, number of tests, number of persons testing positive by PCR (i.e. cases) and number of symptomatic and asymptomatic cases at the time of sample collection are provided, while the time of infection and true asymptomatic proportion are not available.

A total of 634 people tested positive among 3,063 tests as at 20 February 2020.

Of 634 cases, a total of 313 cases were female and six were aged 0–19 years, 152 were aged 20–59 years and 476 were 60 years and older (Figure).

Cases were from a total of 28 countries, with most being nationals of six countries, namely Japan (n = 270 cases), the United States (n = 88 cases), China (n = 58 cases; including 30 from Hong Kong), the Philippines (n = 54 cases), Canada (n = 51 cases) and Australia (n = 49 cases).

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Age distribution of reported coronavirus disease 2019 cases on board the Diamond Princess cruise ship stratified by sex, Yokohama, Japan, 20 February 2020 (n = 634 cases)

Of the 634 confirmed cases, a total of 306 and 328 were reported to be symptomatic and asymptomatic, respectively.

The proportion of asymptomatic individuals appears to be :

  • 16.1% (35/218) before 13 February,
  • 25.6% (73/285) on 15 February,
  • 31.2% (111/355) on 16 February,
  • 39.9% (181/454) on 17 February,
  • 45.4% (246/542) on 18 February,
  • 50.6% (314/621) on 19 February and
  • 50.5% (320/634) on 20 February (Table).

Soon after identification of the first infections, both symptomatic and asymptomatic cases were transported to designated medical facilities specialised in infectious diseases in Japan.

However, these patients were treated as external (imported) cases, and a detailed description of their clinical progression is not publicly available.


Data on characteristics, test results for severe acute respiratory syndrome coronavirus 2, and disembarking of passengers and crew of the Diamond Princess cruise ship, Yokohama, Japan, February 2020 (n = 3,711)

Date (2020)aNumber of passengers and crew members on boardNumber of disembarked passengers and crew members (cumulative)Number of testsNumber of tests (cumulative)Number of individuals testing positiveNumber of individuals testing positive (cumulative)Number of symptomatic casesNumber of asymptomatic casesNumber of asymptomatic cases (cumulative)
5 Feb3,711NA31311010NANANA
6 FebNANA711021020NANANA
7 FebNANA1712734161NANANA
8 FebNANA6279364NANANA
9 FebNANA57336670NANANA
10 FebNANA10343965135NANANA
12 FebNANA5349239174NANANA
13 FebNANA22171344218NANANA
15 FebNANA21793067285293873c
16 FebNANA2891,219703553238111
17 Feb3,183528b5041,723994542970181
18 FebNANA6812,404885422365246
19 FebNANA6073,011796211168314
20 FebNANA523,0631363476320
NA: data not available.
Reported date.
As this is a cumulative number, the exact date of disembarking is unavailable.
As this is a cumulative number, the reported date for 35 asymptomatic cases are unavailable.

The asymptomatic proportion was defined as the proportion of asymptomatically infected individuals among the total number of infected individuals.

University of Gottingen



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