The epidemiological features of the coronavirus disease 2019 (Covid-19) outbreak are described and evaluated the impact of non-pharmaceutical interventions on the epidemic in Wuhan, China.

Individual-level data on 25,961 laboratory-confirmed Covid-19 cases reported through February 18, 2020 were extracted from the municipal Notifiable Disease Report System. Based on key events and interventions, the epidemic was divided into four periods: before January 11, January 11-22, January 23 – February 1, and February 2-18.

Epidemiological characteristics were compared across periods and different demographic groups and a susceptible-exposed-infectious-recovered model was developed to study the epidemic and evaluate the impact of interventions.

The median age of the cases was 57 years and 50.3% were women. The attack rate peaked in the third period and substantially declined afterwards across geographic regions, sex and age groups, except for children (age <20) whose attack rate continued to increase.

Healthcare workers and elderly people had higher attack rates and severity risk increased with age. The effective reproductive number dropped from 3.86 (95% credible interval 3.74 to 3.97) before interventions to 0.32 (0.28 to 0.37) post interventions.

The interventions were estimated to prevent 94.5% (93.7 to 95.2%) infections till February 18.

At least 59% of infected cases were unascertained in Wuhan, potentially including asymptomatic and mild-symptomatic cases.

Considerable countermeasures have effectively controlled the Covid-19 outbreak in Wuhan. Special efforts are needed to protect vulnerable populations, including healthcare workers, elderly and children. Estimation of unascertained cases has important implications on continuing surveillance and interventions.