Table 1.

The five stages with different virus-spread status and mobility intervention policies in Shenzhen and Hong Kong.

Stages	Time windows	Major mobility intervention policies and their mobility effects
Stage #0: Before COVID-19	2019/12/01–2020/01/14	Although Hong Kong initiated a severe response level to the COVID-19 pandemic on January 4, 2020, which included enhanced health measures for individuals arriving from Wuhan at the port, no policy was formulated at this stage to interfere with cross-border mobility between Shenzhen and Hong Kong.
Stage #1: Warning Stage	2020/01/15–2020/01/23	This stage commenced on January 15 when China’s National Health Commission issued the first edition of the COVID-19 diagnosis and treatment plan. Concurrently, the Hong Kong Food and Health Bureau declared that the virus’s limited human-to-human transmission could not be disregarded. Following the eruption of the pandemic in mainland China, on January 20 it was confirmed that the COVID-19 virus could be spread through human contact. This resulted in the implementation of more stringent policies in Shenzhen and Hong Kong to curb propagation of the virus, leading to a significant decrease in population mobility compared with that in the previous stage. In summary, the early measures for pandemic prevention and control centered on publicizing information and directing public opinion.
Stage #2: Freezing Stage	2020/01/24–2020/02/23	Since Guangdong’s first-level response to a major public health emergency on the evening of January 23, Shenzhen and Hong Kong raised pandemic prevention and control to the highest level, suspending the operation of 10 port control stations and keeping only three open. On February 8, Hong Kong mandated a 14-day quarantine for most mainland visitors. Through mandatory measures, Shenzhen and Hong Kong controlled population mobility, reduced contact, lowered the risk of close contact, and shortened the effective transmission period of the infected. As a result, there was minimal daily mobility between Shenzhen and Hong Kong. On February 9, Shenzhen required a health code for entry and exit, which further restricted senior from traveling due to digital pandemic prevention measures.
Stage #3: Adjustment Stage	2020/02/24–2020/04/29	Guangdong Province made the decision on February 24 to change from a first-level to a second-level response to a major public health emergency. Shenzhen’s pandemic prevention and control measures had been effective during this period, but Hong Kong had seen a surge in confirmed cases. As of midnight on March 19, all overseas visitors to Hong Kong, regardless of their residency status, were required to undergo 14 days of mandatory quarantine or medical surveillance.
Stage #4: Recovery Stage	2020/04/30–2020/05/31	Public services were restored by the Immigration Department of Hong Kong on April 30, and mandatory quarantine was lifted for cross-border students, businesspeople, and professionals. Shenzhen and Hong Kong efficiently managed the COVID-19 outbreak, and due to their close economic and social ties, they eased mandatory quarantine requirements for specific groups through a series of exemption policies. However, the travel aspirations of seniors were not fulfilled during this period.