The heat map in Fig 1 visualizes the epidemic transmission in its early stage, based on daily diagnosed patient numbers across China. It's clear that the epidemic spread from Wuhan (Left: the hotspot in the middle) to its surrounding areas, and further to the rest of the country, especially the large metropolitan areas, such as Beijing, Shanghai, and Guangzhou (Right: hotspots from the North to South along the coast), which became secondary epidemic centers to its vicinities. Such transmission pattern reflects the large population movement prior to the Chinese New Year (January 25), when people usually return home to celebrate a week-long Spring Festival. On January 23, the Wuhan city exercised an extreme wholecity quarantine measure by shutting down all transportations going in and out of the Wuhan city. 
We take the period January 31, approximately a week after the city closure, as the first stage of the epidemic development, after that, the second stage. The one-week duration is the averaged incubation time that it takes an infected person to show symptoms. At the first stage, the new Coronavirus infection cases diagnosed outside Wuhan are mainly imported from Wuhan through population movement. We collect data from several public sources [1] [2] and visualize population volume on map.
From the visualization (Fig. 2) , we can see strong correlation between the population of diagnosed infection and migration from Wuhan (Left-Middle) in the first stage, however, the epidemic development start to take different paces at different provinces. This reflects new cases of human-to-human transmission, with varying rates in different provinces. Next, we simulate epidemic dynamics to numerically analyze and predict the transmission procedures, and provide evaluation on different control measures. 
On January 31, The Lancet published an article [3] on nowcasting and forecasting of COVID-19 disease (then called 2019-nCoV) spreading in China. In this work, the authors used the classic SEIR model, which divides the population into four categories of individuals: susceptible (S), exposed (E),infected (I), and removed (R). The model assumes that individuals transfer between categories with a certain probability. The regenerative number R0 is estimated to be 2.68. From this model, they estimated that the number of infected people in Wuhan had reached about 75,815 by January 25, and a daily infection number to be hundreds of thousands. They estimated several other populous cities (Beijing, Shanghai, Chongqing, Guangzhou) to have somewhat identical trend, except a couple of days shift on the peaking dates. The article basically dismissed the effect of the city-wide quarantine of Wuhan and other aggressive control measures around the country. The forecasting depicted a grave scenario both within China and globally, was helpful in alerting the public, but seemed to be quite off from the reality even at the time of its publishing.
One major problem of the above simulation is that it does not consider the aggressive control measures conducted around China, where (1) increasingly efficient testing procedures are put in place to timely diagnose infected individuals, who are then immediately quarantined, if not already; (2) individuals who had contact with the infected people are also quarantined; and (3) a further group of individuals who are suspected to be in the above two categories are also quarantined. The individuals in the latter two categories may be either released or moved to the first category after further observation and testing. In addition to these, there are more measures on reducing mobility within the city.
We aim to employ a new epidemic dynamics model to better consider the above measures in China. To this end, we employ the C-SEIR model [4] , which has added two new quarantined groups in SEIR (Fig. 3.1 ): the quarantined suspected infection group (P), and the quarantined diagnosed infection group (Q). Individuals in groups P and Q do not have the ability to transmit the virus onward. The virus infectious rate function β(t) is defined as the daily number of newly infected people ΔE divided by the number of untreated patients I.
Considering that the new virus may also have a certain infectivity during the incubation period, it can be written as:
with k as 0.1, an empirical parameter meaning that the infectious capacity during the incubation period is 0.1 times that when symptoms are expressed. In order to determine the specific form of β(t), we first calculate the estimated value of β(t) using the daily number of confirmed patients and the estimated incubation period published by the Health and Medical Commission of China to infer the daily number of incubation periods and the number of infections, and then fit the data with certain chosen functions.
In order to compute the infectious rate β(t), it is necessary to calculate the number of exposed individuals (E) and the number of infected patients (I) for each day. Since only data of newly diagnosed patients are available, we need to estimate E and I based on it. Like in the article [4] , we assume that the time between two generations of infection, and the time between infection and treatment of 2019-nCoV, are similar to SARS, which are 9 days and 3 days, respectively. That is to say, we can roughly assume that a patient exposed at day t will be infected at day t+6, and be treated at day t+9. Thus, it can be estimated that the total number of patients admitted during t ~ t + 9 is equal to the number of exposed individuals on day t, and the total number of patients admitted during t ~ t + 3 is equal to the number of infected patients on day t. From this, we can estimate E and I of each day, then β (t).
During the spread of most infectious diseases, the infectious rate β(t) decreases exponentially with time. We then use an exponential function to fit β(t) for each day. Taking Beijing as an example, the fitted result is shown in Figure 3 .2. By applying similar calculation to the data from other provinces and cities, we have obtained largely similar infection rate curves. This means that the virus infection rates have dropped very quickly, a testimony to the effectiveness of the control measures. 
We estimate model parameters and run the simulation, aiming to fit the output with the daily confirmed diagnose data reported from Hubei Province, Wuhan being its capital city. Through this, we obtain prediction on the future epidemic development, shown in Figure 3 Here, we generate two different prediction curves (blue / green), where the solid line is the number of daily confirmed patients and the dashed line the cumulative number of confirmed patients. In either case, the curve fits with the actual number of people diagnosed (beige point) in the early stage. It can be seen from the figure that slight deviation on this curve fitting in the early stage may lead to significant departure going forward. Nevertheless, the 'pessimistic' prediction (blue curve) gives a peak date around Feb 10, and the cumulatively confirmed patients number to be 50K on March 1 st , which is quite close to the official numbers publicized. In stark contrast, the prediction of the Lancet article [2] is an order of magnitude that of the reality.
We like to conduct simulation by varying a few parameters to evaluate the effectiveness of different control measures.
First of all, proper quarantine is the most important, especially at the early stage of the epidemic development. It is believed that the outbreak of COVID-19 started as early as mid-November, but Wuhan city did not alert the public until Jan 20, and exercise effective quarantine measure until January 23 rd . As can be seen from the simulation (Fig 3.4) , shifting the quarantine date either earlier or later by 2 days could result in almost double amount of decrease or increase of the infected people cumulatively.
It is important to note that too large a number of infected people may exhaust or even paralyze available medical resource. This unfortunately became the case for the Wuhan city, which was then rescued by pouring medical resources (medical personnel and supplies) from the rest of the country. The city also exercised an extremely aggressive whole-city quarantine on Jan 23, and a reduction of inter-city mobility to almost zero, with the aim of halting any disease transmission. As of today (March 2, 2020),the epidemic in China is well under control. Outside Wuhan has seen zero to single digit daily confirmed infection for a week or so. This is achieved through the aforementioned aggressive quarantine measure across the nation, as described in the report by the WHO-China Joint Mission [6] .
Next, we examine the impact of relaxing quarantine measures on epidemic development. Figure 3 .5 shows predictions on different quarantine measures: if the quarantine is cancelled completely after reaching the daily peak, the rate of epidemic mitigation will be greatly reduced, resulting in a possible second peak of infection (blue), while a partially relaxed quarantine leads to only slight increase of infection (beige), comparing with the normal quarantine measure (green).This illustrates both the importance of quarantine to avoid a second strike, and the necessity of exercising a balanced measure to maintain life and work normalcy as much as possible. 
Our simulation based on the C-SEIR model has shown significant effectiveness of quarantine measures, which has been thoroughly exercised in China. While we applaud the success of these efforts, we must point out that this comes at a price much too high. In the hindsight, this could have been avoided by exercising a more proper (better balance between effectiveness and inconvenience) quarantine, but much earlier.
The COVID-19 virus seems to be spreading globally, every country has a different scenarios and government reactions. For individuals, there are a few points to consider:
(1) Be educated about the virus, e.g., its signature symptoms being fever and dry cough, unlike cold (running nose), and its prevention (not much different than preventing flu).
(2) Be vigilant and take extra precaution. Avoid big gatherings, stay low in social activities, and keep social distances in daily lives. Cancel or postpone non-essential travels. (3) Quarantine if needed. Individuals with light symptoms, or having identified close contacts with those infected, should quarantine themselves in their own living places, or anywhere conditioned for the purpose. Individuals with strong symptoms should check in hospital.
An alert and corroborating public is probably the most important for containing the epidemic.
A recent article by the frontline health experts in China reported a death rate of 1.4% [7] . But outside Hubei province, the death rate seems to be much lower than this, down to 0.85%. The reported death rates outside China (e.g., U.S.) seem to be even lower. It is worth investigating whether the transmissibility of the virus would also reduce over time. Nevertheless, the COVID-19 virus is definitely more than a strong flu; therefore, it deserves extra precaution from individuals and well prepared medical resource from governments.