This paper aims to determine to what level of the two commonly used control measures, social distancing and facial mask usage, are necessary to prevent a resurgence of the epidemic due to either
residual active cases in Wuhan or imported cases after lifting the quarantine, examining in a model six proposed dates for quarantine lifting.

The outbreak of the novel coronavirus SARS-CoV-2 was first identified in the Chinese city of Wuhan in early December 2019, when a group of 27 patients with close contact with a seafood market were diagnosed with a pneumonia of unknown aetiology.

The number of infected cases increased rapidly in Wuhan during the first few weeks of the outbreak and then quickly spread to all 31 Chinese provinces and abroad.
As of 20th March 2020, China has reported 80,695 confirmed cases and
3,097 deaths, accounting for about one-third of all cases and deaths worldwide.

To curb the epidemic, the Chinese government introduced a ‘metropolitan-wide quarantine’ of the city of Wuhan from 23rd January 2020, by terminating all public transportation in the city and intercity links.
During the metropolitan-wide quarantine, the National Health Commission and the China Centre for Disease Control urged the use of facial masks in all public spaces, put in place strict home containment policies, postponed schools and industry reopening to reduce communal activities and person-to-person transmission. A massive screening program was implemented for individuals in close contact with the infected or high-risk individuals.

Lifting the quarantine restrictions in the city and reopening transport links with the rest of China has become the top priority for the policymakers.

This paper aims to determine to what level of the two commonly used control measures, social distancing and facial mask usage, are necessary to prevent a resurgence of the epidemic due to either
residual active cases in Wuhan or imported cases after lifting the quarantine.

The authors estimated that at the end of the epidemic, a total of 65,733 (45,722-99,015) individuals would be infected by the virus, among which 16,166 (11,238-24,603, 24.6%) would be infected through public contacts, 45,996 (31,892-69,565, 69.7%) through household contact, 3,571 (2,521-5,879, 5.5%) through hospital contacts (including 778 [553-1,154] non-COVID-19 patients and 2,786 [1,969- 4,791] medical staff members). The estimated number of infected individuals was higher than the officially reported number of confirmed cases because the model estimate also accounted for undocumented infections who had recovered before being diagnosed. A total of 2,821 (1,634-6,361) would die of COVID-19 related pneumonia in Wuhan.

In a model 6 proposed dates for quarantine lifting were examined.

When public contact recovered to 100% the pre-quarantine level and facial mask usage was high at 95%, the epidemic would follow a smooth decline to elimination regardless of which day for quarantine was lifted.

In contrast, when facial mask usage was reduced to 50%, any quarantine lifting date before 25th April would result in a second major outbreak (Figure 1b). Similarly, when facial mask usage was sustained at 80%, and public contact rate was recovered to 100% of the pre-quarantine level, an earlier lifting on 21st March may lead to a second minor outbreak (Figure 1c). But, if the public contact rate was 50% more than the pre-quarantine level, a second major outbreak would occur in all quarantine lifting dates except 25th April.

Combinations of high facial mask usage and reduced public contacts may lead to a smooth decline of the epidemic on various quarantine lifting dates.

For an early lifting date (21st March), facial mask needed to be sustained at a relatively high rate (≥85%) if public contacts were to recover to 100% of the pre-quarantine level.

In contrast, lifting the quarantine on 18th April allowed public person-to-person contact adjusted back to the pre-quarantine level with a substantially lower level of facial mask usage (75%).

However, a low facial mask usage (<50%) combined with an increased public contact (>100%) would always lead a significant second outbreak in most quarantine lifting scenarios.

Lifting the quarantine on 25th April would ensure a smooth decline of the epidemics regardless of the combinations of public contact rates and facial mask usage.