A novel coronavirus (SARS-CoV-2) that emerged out of the city of Wuhan, China in December 2019 has already demonstrated its potential to generate explosive outbreaks in confined settings and cross borders following human mobility patterns (1) . While COVID-19 frequently induces mild symptoms common to other respiratory infections, it has also exhibited an ability to generate severe disease among certain groups including older populations and individuals with underlying health issues such as cardiovascular disease and diabetes (2) . Nevertheless, a clear picture of the epidemiology of this novel coronavirus is still being elucidated.
The number of cases of COVID-19 in the province of Hubei, the disease epicenter, quickly climbed following an exponential growth trend. The total number of COVID-19 cases is at 81,109 including 2718 deaths in China as of February 26, 2020 (3) . Fortunately, by February 15, 2020 the daily number of new reported cases in China started to decline across the country although Hubei Province is still reporting hundreds of cases per day as of February 26 th , 2020 (3) . While the epidemic continues to decline in China, more than 2,900 COVID-19 cases have been reported in 37 countries outside of China including South Korea, Japan, Italy, Iran, Singapore, U.S., and Thailand (3) . In particular, South Korea has quickly become the second hardest hit country with COVID-19, with steadily increasing number of cases over the last few days. Hence, it is crucial to monitor the progression of these outbreaks and assess the effects of various public health measures including the social distancing measures in real time.
The first case in South Korea was identified on January 20, 2020 followed by the detection of one or two cases on average in the subsequent days. However, the number of confirmed cases of SARS-CoV-2 infection started to increase rapidly on February 19, 2020 with a total of 1,261 confirmed COVID-19 cases including twelve deaths reported as of February 26, 2020 according to the Korea Centers for Disease Control and Prevention (KCDC) (4). The epicenter of the South Korean COVID-19 outbreak has been identified in Daegu, a city of 2.5 million people, approximately 150 miles South East of Seoul.
The rapid spread of COVID-19 in South Korea has been attributed one case linked to a superspreading event that has led to at least 40 secondary cases stemming from church services in the city of Daegu (5, 6 ). This has led to sustained transmission chains of COVID-19, with about 52% of the cases associated with the church cluster in Daegu (7) . Moreover, three other clusters have been reported including one set in Chundo Daenam hospital in Chungdo-gun, Gyeongsanggbuk-do (114 cases), one Pilgrimage to Israel cluster in Gyeongsanggbuk-do (31 cases) and a cluster A composed of one imported and 11 local cases in Seoul. These few clusters have become the major driving force of the infection. A total of 33 cases were imported while the three major clusters are composed of local cases as described in Supplementary Table S1 .
Transmission of SARS-CoV-2 in Korea has been exacerbated by amplified transmission in confined settings including a hospital and a church in the city of Daegu. The hospital-based outbreak alone involves 114 individuals including 9 hospital staff (8) , which is reminiscent of past outbreaks of SARS . CC-BY-NC 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. 
We obtained the daily series of confirmed cases of COVID-19 in South Korea from January 20, 2020
to February, 26, 2020 that are publicly available from the Korea Centers for Disease Control and Prevention (KCDC) (4). Our data includes the dates of reporting for all confirmed cases, the dates of symptom onsets for the first 28 reported cases, and whether the case is autochthonous (local transmission) or imported. We also summarize the case clusters comprising one or more cases according to the source of infection according to the field investigations conducted by the KCDC (4). Accordingly, four major clusters were identified.
To estimate the growth rate of the epidemic, it is ideal to characterize the epidemic curve according to dates of symptoms onset rather than according to dates of reporting. For the COVID-19 data in Korea, the symptom onset dates are available for only the first 28 reported cases. Moreover, all of the dates of symptoms onset are available for the imported cases. Therefore, we utilize this empirical distribution of reporting delays from the onset to diagnosis to impute the missing dates of onset for the remainder of the cases with missing data. For this purpose, we reconstruct 300 epidemic curves by dates of symptoms onset from which we derived a mean incidence curve of local case incidence and drop the last three data points from the analysis to adjust for reporting delays in our real-time analysis (11) .
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We assess the effective reproduction number, , which quantifies the time dependent variations in the average number of secondary cases generated per case during the course of an outbreak due to intrinsic factors (decline in susceptible individuals) and extrinsic factors (behavior changes, cultural factors, and the implementation of public health measures) (9, 12, 13) . Using the Korean incidence curves for imported and local cases, we estimate the evolution of Rt for COVID-19 in Korea. First, we characterize daily local case incidence using the generalized growth model (GGM) (14) . This model characterizes the growth profile via two parameters: the growth rate parameter ( ) and the scaling of the growth rate parameter ( ). The model captures diverse epidemic profiles ranging from constant incidence ( = 0), sub-exponential or polynomial growth ( 0 < < 1 ), and exponential growth ( = 1 ) (14) . The generation interval is assumed to follow a gamma distribution with a mean of 4.41 days and a standard deviation of 3.17 days (15, 16) .
Next, to estimate the most recent estimate of Rt, we simulate the progression of incident cases from GGM, and apply the discretized probability distribution ( ) of the generation interval using the renewal equation (17-19) given by
In the renewal equation we denote the local incidence at calendar time by , and the raw incidence of imported cases at calendar time by . The parameter 0 ≤ ≤ 1 quantifies the relative contribution of imported cases to the secondary disease transmission (20) . The denominator represents the total number of cases that contribute to the incidence cases at time . Next, we estimate for 300 simulated curves assuming a Poisson error structure to derive the uncertainty bounds around the curve of (21).
The reconstructed daily incidence curve of COVID-19 after the imputing the onset dates for the Korean cases is shown in Figure 1 . Between January 20 and February 18, 2020 an average of 2 new cases were reported each day, whereas between February 19-26, 2020, 153 new cases were reported each day on average.
Under the empirical reporting delay distribution from Korean cases with available dates of onset, the intrinsic growth rate (r) was estimated at 0.6 (95% CI: 0.5, 0.7) and the scaling of growth parameter (p)
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Spatial distribution of the Korean clusters is shown in Figure 3 and the characteristics of each cluster are presented in Table 1 . is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. 
This cluster is composed of one imported case and 12 local cases, and was identified with its first case (the 3rd patient in the country) confirmed on Jan 26, 2020. Its first case is suspected to be an imported case from Wuhan, China, transmitting SARS-CoV-2 to the 6 th and 28 th confirmed cases in Korea, which resulted in secondary cases. No further cases have been added in this cluster since February 21, 2020.
This is the first study to report estimates of the transmission potential of COVID-19 in Korea based on the trajectory of the epidemic, which was reconstructed by using the dates of onset of the first reported cases in Korea. The estimates of R clearly indicate sustained transmission of the novel coronavirus in Korea. Moreover, the imported cases contribute little to the secondary disease transmission in Korea, as majority of these cases occurred in the early phase of the epidemic, with the most recent imported case reported on February 9, 2020. These findings support the range of social distancing interventions that the Korean government put in place in order to bring the outbreak under control as soon as possible.
Our estimates of the reproduction number can be compared with earlier estimates reported for the epidemic in China where the estimates of R lie in the range 2-7.1 (1, (23) (24) (25) (26) (27) (28) (29) . Moreover, the mean R reached values as high as ~11 for the outbreak that unfolded aboard the Princess Cruises Ship during January-February 2020 (30) . In contrast, a recent study on Singapore's COVID-19 transmission Singapore and all Chinese provinces except Hubei (31, 32) .
. CC-BY-NC 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (34) . Amplification of MERS in the hospital setting has been associated with diagnostic delays, which increase the window of opportunity for the generation of secondary cases (9) . This underscores the need for rapid testing, case detection and active contact tracing to isolate infectious individuals.
Beyond Korea, substantial COVID-19 transmission has been reported in Singapore, Iran, Italy, Germany and aboard the Diamond cruise ship (35, 36) . While the cluster A and the Chungdo Daenam hospital cluster in Korea seem to have stabilized, the other two clusters are likely to keep growing in the short term. Public health authorities are currently focused on containing the outbreak in the city of Daegu, the epicenter of the outbreak and North Gyeongsang Province where active contact tracing is being conducted. Nation-wide preventative measures are expected to reduce the community transmission and ultimately bring below one.
This is the first study to estimate the transmission potential of COVID-19 in Korea. Our current findings suggest sustained disease transmission in the region. Our estimates support the implementation of a wide array of social distancing measures to rapidly contain the outbreak in Korea. . CC-BY-NC 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
(which was not peer-reviewed) The copyright holder for this preprint . https://doi.org/10.1101/2020.02. 27.20028829 doi: medRxiv preprint . CC-BY-NC 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
(which was not peer-reviewed) The copyright holder for this preprint . https://doi.org/10.1101/2020.02. 27.20028829 doi: medRxiv preprint 