Evolution and Control of COVID-19 Epidemic in Hong Kong

Abstract

Hong Kong SAR has adopted universal masking, social distancing, testing of all symptomatic and high-risk groups for isolation of confirmed cases in healthcare facilities, and quarantine of contacts as epidemiological control measures without city lockdown or border closure. These measures successfully suppressed the community transmission of pre-Omicron SARS-CoV-2 variants or lineages during the first to the fourth wave. No nosocomial SARS-CoV-2 infection was documented among healthcare workers in the first 300 days. The strategy of COVID-19 containment was adopted to provide additional time to achieve population immunity by vaccination. The near-zero COVID-19 situation for about 8 months in 2021 did not enable adequate immunization of the eligible population. A combination of factors was identified, especially population complacency associated with the low local COVID-19 activity, together with vaccine hesitancy. The importation of the highly transmissible Omicron variant kickstarted the fifth wave of COVID-19, which could no longer be controlled by our initial measures. The explosive fifth wave, which was partially contributed by vertical airborne transmission in high-rise residential buildings, resulted in over one million cases of infection. In this review, we summarize the epidemiology of COVID-19 and the infection control and public health measures against the importation and dissemination of SARS-CoV-2 until day 1000.

Keywords: COVID-19; SARS-CoV-2; infection control; variants.

Figure 1

Cumulative number of COVID-19 cases in 14 countries reporting more than 10 million cases as of 25 September 2022 (day 1000).

Figure 2

Epidemic curve of the first to the fourth wave of COVID-19 in Hong Kong. An imported case is defined as a case having traveled to a country or area with community transmission of COVID-19 during the incubation period (IP) (i.e., operationally defined as 7 days before symptom onset or a positive test for an asymptomatic case) or tested positive during compulsory quarantine without having any activity in the local community. A local case is defined as a case without travel history during the IP. A possibly local case is defined as those having traveled to a country or area not known to have community transmission within the IP (this category was historically used in the first two waves in early 2020 only). Imported cases, local cases, and cases epidemiologically linked with local cases constituted the main burden of COVID-19 cases.

Figure 3

Epidemic curve of the fifth wave of COVID-19 in Hong Kong. RT-PCR, reverse-transcription–real-time polymerase chain reaction; RAT, rapid antigen test.

Figure 4

Time-resolved phylogenetic tree of 1168 viral genomes from January 2020 to March 2022 in Hong Kong. The tree was constructed using the Nextstrain command-line interface. The color of the dots represents the outbreak wave of a particular strain. The branch length was determined by the collection date of a sample. Wuhan-Hu-1 was used as the reference sequence during the tree construction.

Figure 5

The evolution of the COVID-19 Stringency Index in the first 1000 days. The official announcement of the outbreak of community-acquired pneumonia in Wuhan, Hubei Province, China, was defined as day 1. The figure was generated from the website of the COVID-19 Government Response Tracker [114] and formatted for presentation.

Figure 6

The number of inbound travelers, including Hong Kong residents and visitors from mainland China, arriving at Hong Kong International Airport. DQHs, designated quarantine hotels.

Figure 7

The number of COVID-19 tests under different categories of requests. Community testing centers provided self-paid services (from 15 November 2020, day 321) and free testing for persons under compulsory testing (from 22 November 2020, day 328). The Targeted Group Testing Scheme refers to essential staff providing services to critical infrastructure (from 14 July 2020, day 197). The Hospital Authority and Department of Health were responsible for testing hospitalized patients and out-patients during the COVID-19 pandemic.

Figure 8

COVID-19 testing in the community under restriction-testing declaration for residents in residential buildings during the fifth wave (primary and secondary Omicron waves) of COVID-19 in Hong Kong.

Figure 9

Daily doses of COVID-19 vaccine administrated in Hong Kong from 26 February 2021 (day 424) to 25 September 2022 (day 1000). The figure was generated from the website of “Our World in Data” [140] and formatted for presentation.

Figure 10

Monthly trend of COVID-19 vaccination rate among the general population until 25 September 2022 (day 1000) in Hong Kong. On 1 August 2022 (day 945), the Scientific Committee on Emerging and Zoonotic Diseases and the Scientific Committee on Vaccine Preventable Diseases of the Centre for Health Protection recommended using CoronaVac for those aged 6 months to under 3 years.