Here we provide an initial assessment of the transmission dynamics and epidemiologic characteristics of NCIP. Although the majority of the earliest cases were linked to the Huanan Seafood Wholesale Market and the patients could have been infected through zoonotic or environmental exposures, it is now clear that human-to-human transmission has been occurring and that the epidemic has been gradually growing in recent weeks. Our findings provide important parameters for further analyses, including evaluations of the impact of control measures and predictions of the future spread of infection.

We estimated an R 0 of approximately 2.2, meaning that on average each patient has been spreading infection to 2.2 other people. In general, an epidemic will increase as long as R 0 is greater than 1, and control measures aim to reduce the reproductive number to less than 1. The R 0 of SARS was estimated to be around 3,12 and SARS outbreaks were successfully controlled by isolation of patients and careful infection control.13 In the case of NCIP, challenges to control include the apparent presence of many mild infections14 and limited resources for isolation of cases and quarantine of their close contacts. Our estimate of R 0 was limited to the period up to January 4 because increases in awareness of the outbreak and greater availability and use of tests in more recent weeks will have increased the proportions of infections ascertained. It is possible that subsequent control measures in Wuhan, and more recently elsewhere in the country as well as overseas, have reduced transmissibility, but the detection of an increasing number of cases in other domestic locations and around the world suggest that the epidemic has continued to increase in size. Although the population quarantine of Wuhan and neighboring cities since January 23 should reduce the exportation of cases to the rest of the country and overseas, it is now a priority to determine whether local transmission at a similar intensity is occurring in other locations.

It is notable that few of the early cases occurred in children, and almost half the 425 cases were in adults 60 years of age or older, although our case definition specified severe enough illness to require medical attention, which may vary according to the presence of coexisting conditions. Furthermore, children might be less likely to become infected or, if infected, may show milder symptoms, and either of these situations would account for underrepresentation in the confirmed case count. Serosurveys after the first wave of the epidemic would clarify this question. Although infections in health care workers have been detected, the proportion has not been as high as during the SARS and MERS outbreaks.15 One of the features of SARS and MERS outbreaks is heterogeneity in transmissibility, and in particular the occurrence of super-spreading events, particularly in hospitals.16 Super-spreading events have not yet been identified for NCIP, but they could become a feature as the epidemic progresses.

Although delays between the onset of illness and seeking medical attention were generally short, with 27% of patients seeking attention within 2 days after onset, delays to hospitalization were much longer, with 89% of patients not being hospitalized until at least day 5 of illness (Figure 2). This indicates the difficulty in identifying and isolating cases at an earlier stage of disease. It may be necessary to commit considerable resources to testing in outpatient clinics and emergency departments for proactive case finding, both as part of the containment strategy in locations without local spread yet as well as to permit earlier clinical management of cases. Such an approach would also provide important information on the subclinical infections for a better assessment of severity.

Our preliminary estimate of the incubation period distribution provides important evidence to support a 14-day medical observation period or quarantine for exposed persons. Our estimate was based on information from 10 cases and is somewhat imprecise; it would be important for further studies to provide more information on this distribution. When more data become available on epidemiologic characteristics of NCIP, a detailed comparison with the corresponding characteristics of SARS and MERS, as well as the four coronaviruses endemic in humans, would be informative.

Our study suffers from the usual limitations of initial investigations of infections with an emerging novel pathogen, particularly during the earliest phase, when little is known about any aspect of the outbreak and there is a lack of diagnostic reagents. To increase the sensitivity for early detection and diagnosis, epidemiology history was considered in the case identification and has been continually modified once more information has become available. Confirmed cases could more easily be identified after the PCR diagnostic reagents were made available to Wuhan on January 11, which helped us shorten the time for case confirmation. Furthermore, the initial focus of case detection was on patients with pneumonia, but we now understand that some patients can present with gastrointestinal symptoms, and an asymptomatic infection in a child has also been reported.17 Early infections with atypical presentations may have been missed, and it is likely that infections of mild clinical severity have been under-ascertained among the confirmed cases.18 We did not have detailed information on disease severity for inclusion in this analysis.

In conclusion, we found that cases of NCIP have been doubling in size approximately every 7.4 days in Wuhan at this stage. Human-to-human transmission among close contacts has occurred since the middle of December and spread out gradually within a month after that. Urgent next steps include identifying the most effective control measures to reduce transmission in the community. The working case definitions may need to be refined as more is learned about the epidemiologic characteristics and outbreak dynamics. The characteristics of cases should continue to be monitored to identify any changes in epidemiology — for example, increases in infections among persons in younger age groups or health care workers. Future studies could include forecasts of the epidemic dynamics and special studies of person-to-person transmission in households or other locations, and serosurveys to determine the incidence of the subclinical infections would be valuable.14 These initial inferences have been made on a “line list” that includes detailed individual information on each confirmed case, but there may soon be too many cases to sustain this approach to surveillance, and other approaches may be required.19