A main finding of this characterization and exploratory analysis of the first 72,314 cases of COVID-19 found in China in the 40 days between first recognition of the outbreak of pneumonia with unknown etiology on December 31, 2019 to the end of the study period on February 11, 2020 is that this novel coronavirus is highly contagious. It has spread extremely rapidly from a single city to the entire country within only about 30 days. Moreover, it has achieved such far-reaching effects even in the face of extreme response measures including the complete shutdown and isolation of whole cities, cancellation of Chinese New Year celebrations, prohibition of attendance at school and work, massive mobilization of health and public health personnel as well as military medical units, and rapid construction of entire hospitals.

In light of this rapid spread, it is fortunate that COVID-19 has been mild for 81% of patients and has a very low overall case fatality rate of 2.3%. Among the 1,023 deaths, a majority have been ≥60 years of age and/or have had pre-existing, comorbid conditions such as hypertension, cardiovascular disease, and diabetes. Moreover, the case fatality rate is unsurprisingly highest among critical cases at 49%, and no deaths have occurred among those with mild or even severe symptoms (Table 1).

A major contribution of our study is a first description of the COVID-19 epidemic curves. We interpret the overall curve (Figure 3A) as having a mixed outbreak pattern—the data appear to indicate a continuous common source pattern of spread in December and then from early January through February 11, 2020, the data appear to have a propagated source pattern. This mixed outbreak time trend is consistent with the working theory that perhaps several zoonotic events occurred at Huanan Seafood Wholesale Market in Wuhan allowed 2019-nCoV to be transmitted from a still-unknown animal into humans and, due to its high mutation and recombination rates, it adapted to become capable of and then increasingly efficient at human-to-human transmission (3,8).

The early days of the outbreak have been reminiscent of SARS and MERS, and indeed, the discovery that the causative agent was a closely-related, never-before-described coronavirus predicted potential for nosocomial transmission and so-called “super-spreader” events (8). Unfortunately, 2019-nCoV did indeed infect health workers in China via nosocomial transmission. Here we offer a first description of the 1,716 confirmed cases among health workers. Overall, they also display a likely mixed outbreak pattern—perhaps the data are characterized by a point source curve beginning in late December 2019, which was eclipsed by a higher magnitude continuous source curve beginning on January 20, 2020. To date, there is no evidence of a super-spreader event occurring in any of the Chinese health facilities serving COVID-19 patients. However, we do not know whether this is due to the nature of the virus itself or whether these events have been successfully prevented.

It is these authors’ sincere hope and intent that this new analysis, on what has become a “public health emergency of international concern,” (12) helps to inform health and public health workers preparing for or perhaps already experiencing COVID-19 in their populations. This study provides important insight into several crucial open questions on this epidemic and how to design strategies to effectively control it (3). For instance, the downward trend in the overall epidemic curve suggests that perhaps isolation of whole cities, broadcast of critical information (e.g., promoting hand washing, mask wearing, and care seeking) with high frequency through multiple channels, and mobilization of a multi-sector rapid response teams is helping to curb the epidemic.

China’s response is certainly an echo of lessons learned during SARS and is a tribute to the work China and other low- and middle-income countries have been doing, with the much-needed help of international partners, over the past few decades to build infectious disease surveillance systems and public health infrastructure capable of catching outbreaks early and responding swiftly using evidence-based best practices. The 2019-nCoV and other coronaviruses may continue to adapt over time to become more virulent (3), and zoonosis is not going to stop. We must remain vigilant, hone our skills, fund our defenses, and practice our responses, and we must help our neighbors to do the same.

The very large number of cases included in our study was a major strength. Nevertheless, our study did have some important limitations. Firstly, a large proportion of cases included in our analysis (37%) were not confirmed by nucleic acid testing since this process is slow, labor intensive, and requires specialized equipment and skilled technicians. Yet all 72,314 cases were at least diagnosed clinically and investigated by trained epidemiologists. Secondly, some records did have missing data for a few important variables of interest—Wuhan-related exposure, comorbid conditions, and case severity—which limits our ability to draw conclusions from the data.

In conclusion, the present descriptive, exploratory analysis of the first 72,314 cases of COVID-19 reported through February 11, 2020 offers important new information to the international community on the epidemic in China. In particular, this analysis chronicles the extremely rapid spread of the novel coronavirus despite extreme efforts to contain it. However, important questions remain including identification of the animal reservoir, determination of infectiousness period, identification of transmission routes, and effective treatment and prevention methods including further test development, drug development, and vaccine development (3–4,8–9). As an international community, we must all be responsible partners in surveillance, communication, response, research, and implementation of evidence-based public health and clinical practice. The massive vigorous actions taken by the Chinese government have slowed down the epidemic in China and curbed spread to the rest of the world. Although the epidemic appears to be in decline in the lead up to February 11, 2020, we may yet face more challenges. Huge numbers of people will soon be returning to work and school after the extended New Year holiday. We need to prepare for a possible rebound of the COVID-19 epidemic in the coming weeks and months.