The epidemic curve based on symptom onset after January 4 showed a very different growth rate ( 6).

However, case count data can be confounded by reservoir spillover events, stochasticities in the initial phase of the outbreak, and low surveillance intensity. used very early case count data in Wuhan before January 4 ( 1).

These estimates were based on 2 broad strategies. Initial estimates of the exponential growth rate of the outbreak were 0.1–0.14/day (a doubling time of 6–7 days), and a basic reproductive number (R 0 defined as the average number of secondary cases attributable to infection by an index case after that case is introduced into a susceptible population) ranged from 2.2 to 2.7 ( 1, 3– 5). In addition, unavailability of diagnostic reagents early in the outbreak, changes in surveillance intensity and case definitions, and overwhelmed healthcare systems confound estimates of the growth of the outbreak based on data. Studying dynamics of a newly emerged and rapidly growing infectious disease outbreak, such as COVID-19, is important but challenging because of the limited amount of data available. In a period of 3 months, an outbreak of apparent idiopathic pneumonia had become the COVID-19 pandemic.

By March 16, the outbreak had led to >170,000 total confirmed cases and >6,500 deaths globally. By January 21, most provinces of China had reported COVID-19 cases. On January 8, 2020, the pathogen was identified ( 1), and human-to-human transmission was reported soon after. Initially, 41 cases of “pneumonia of unknown etiology” were reported to the World Health Organization by the Wuhan Municipal Health Committee at the end of December 2019 ( 2). Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the etiologic agent of the current rapidly growing outbreak of coronavirus disease (COVID-19), originating from the city of Wuhan, Hubei Province, China ( 1).

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