Residential clustering of COVID-19 cases and efficiency of building-wide compulsory testing notices as a transmission control measure in Hong Kong (preprint)

Background: Despite relatively few reports of residential case clusters of COVID-19, building-wide compulsory testing notices on residential apartment blocks are frequently applied in Hong Kong with the aim of identifying cases and reducing transmission. Methods: We aimed to describe the frequency of residential case clusters and the efficiency of compulsory testing notices in identifying cases. The residences of locally infected COVID-19 cases in Hong Kong were grouped to quantify the number of cases per residence. Buildings targeted in compulsory testing notices were matched with the residence of cases to estimate the number of cases identified. Results: We found that most of the residential buildings (4246/7688, 55.2%) with a confirmed COVID-19 case had only one reported case. In the fourth and the fifth epidemic wave in Hong Kong, we estimated that compulsory testing notices detected 29 cases (95% confidence interval: 26, 32) and 46 cases (44, 48) from every 100 buildings tested (each with hundreds of residents), respectively. Approximately 13% of the daily reported cases were identified through compulsory testing notices. Conclusions: Compulsory testing notices can be an essential method when attempting to maintain local elimination (zero covid) and most impactful early in an epidemic when the benefit remains of stemming a new wave. Compulsory testing therefore appears to be a relatively inefficient control measure in response to sustained community transmission in the community.

An observational study on imported COVID-19 cases in Hong Kong during mandatory on-arrival hotel quarantine (preprint)

Background: Hong Kong has enforced stringent travel restrictions particularly for inbound travellers since the emergence of SARS-CoV-2. Understanding the characteristics of imported COVID-19 cases is important for establishing evidence-based control measures. Methods: We conducted a retrospective cohort study to summarise the characteristics of cases classified as imported cases that were detected on or soon after arrival into Hong Kong from 13 November 2020 through to 31 January 2022, when all arriving persons were required to quarantine in a hotel or a designated quarantine facility. We analysed individual demographics, and clinical information including symptoms and disease severity, virus variants, and Ct values. Results: There were 2269 imported COVID-19 cases aged 0-85 years identified in Hong Kong. Almost half (48.6%) of the imported cases were detected on arrival. A shorter median delay from arrival to isolation was observed in Delta and Omicron cases (3 days) than cases infected with the ancestral strain and other variants (12 days; p<0.001) while lower Ct values at isolation were observed in cases infected with Omicron than the ancestral strain or other variants. No Omicron cases were detected beyond 14 days after arrival, and the cases (n=58, 2.6%) detected after 14 days of quarantine more frequently presented without symptoms at isolation and had a higher RT-PCR Ct-value during isolation. At least some of these cases were post-arrival infections. Conclusions: Testing inbound travellers at arrival and during on-arrival quarantine can detect imported cases early although it may not be sufficient to prevent all introductions of COVID-19 into the community. Public health measures should be adjusted in responses to the emergence of new variants of SARS-CoV-2 based on the epidemiologic evidence from continuous surveillance.

Time-varying transmission heterogeneity of SARS and COVID-19 in Hong Kong (preprint)

Transmission heterogeneity is a notable feature of the severe acute respiratory syndrome (SARS) and coronavirus disease 2019 (COVID-19) epidemics, though previous efforts to estimate how heterogeneity changes over time are limited. Using contact tracing data, we compared the epidemiology of SARS and COVID-19 infection in Hong Kong in 2003 and 2020-21 and estimated time-varying transmission heterogeneity (kt) by fitting negative binomial models to offspring distributions generated across variable observation windows. kt fluctuated over time for both COVID-19 and SARS on a continuous scale though SARS exhibited significantly greater (p < 0.001) heterogeneity compared to COVID-19 overall and in-time. For COVID-19, kt declined over time and was significantly associated with increasingly stringent non-pharmaceutical interventions though similar evidence for SARS was inconclusive. Underdetection of sporadic COVID-19 cases led to a moderate overestimation of kt, indicating COVID-19 heterogeneity of could be greater than observed. Time-varying or real-time estimates of transmission heterogeneity could become a critical indicator for epidemic intelligence in the future.

Superspreading of SARS-CoV-2 infections: A Systematic Review and Meta-analysis (preprint)

Superspreading in transmission is a feature of SARS-CoV-2 transmission. We conducted a systematic review and meta-analysis on globally reported dispersion parameters of SARS-CoV-2. The pooled estimate was 0.55 (95% CI: 0.30, 0.79). The study location and method were found to be important drivers for its diversity.

Incorporating temporal distribution of population-level viral load enables real-time estimation of COVID-19 transmissibility (preprint)

Many locations around the world have used real-time estimates of the time-varying effective reproductive number (\({R}_{t}\)) of COVID-19 to provide evidence of transmission intensity to inform control strategies. Estimates of \({R}_{t}\) are typically based on statistical models applied to case counts and typically suffer lags of more than a week because of the incubation period and reporting delays. Noting that viral loads tend to decline over time since illness onset, analysis of the distribution of viral loads among confirmed cases can provide insights into epidemic trajectory. Here, we analyzed viral load data on confirmed cases during two local epidemics in Hong Kong, identifying a strong correlation between temporal changes in the distribution of viral loads (measured by cycle threshold values) and estimates of \({R}_{t}\) based on case counts. We demonstrate that cycle threshold values could be used to improve real-time \({R}_{t}\) estimation, enabling more timely tracking of epidemic dynamics.

Universal Community Nucleic Acid Testing for COVID-19 in Hong Kong Reveals Insights into Transmission Dynamics (preprint)

Background: Testing of an entire community has been used as an approach to control COVID-19. In Hong Kong, a universal community testing programme (UCTP) was implemented at the fadeout phase of a community epidemic in July to September 2020, to determine the prevalence of unrecognised cases and limit any remaining transmission chains. We described the utility of the UCTP in finding unrecognised cases, and analysed data from the UCTP and other sources to characterise transmission dynamics.Methods: We described the characteristics of people participating in the UCTP, and compared the clinical and epidemiological characteristics of COVID-19 cases detected by the UCTP versus those detected by clinical diagnosis and public health surveillance. We developed a Bayesian model to estimate the age-specific incidence of infection and the proportion of cases detected by clinical diagnosis and public health surveillance.Findings: 1.77 million people, 24% of the Hong Kong population, participated in the UCTP from 1 to 14 September 2020. The UCTP identified 32 new infections (1.8 per 100,000 samples tested), consisting of 29% of all local cases reported during the two-week UCTP period. Compared with the existing clinical diagnosis and public health surveillance, the UCTP detected a higher proportion of sporadic cases (62% versus 27%, p <0.01) and identified 6 (out of 18) additional transmission chains during that period. We estimated that 27% (95% credible interval: 22%, 34%) of all infections were detected by the existing clinical diagnosis and public health surveillance in the third wave.Interpretation: We reported empirical evidence of the utility of population-wide COVID-19 testing in detecting unrecognised infections and transmission chains. Around three quarters of infections have not been identified through existing surveillance approaches including contact tracing.Funding Statement: This project was supported by the Health and Medical Research Fund, Food and Health Bureau, Government of the Hong Kong Special Administrative Region (grant no. COVID190118).Declaration of Interests: BJC consults for Roche, Sanofi Pasteur, GSK and Moderna. The authors report no other potential conflicts of interest.Ethics Approval Statement: Our project was approved by the Institutional Review Board of the University of Hong Kong/Hospital Authority Hong Kong West Cluster (HKU/HA HKW IRB).

Clustering and superspreading potential of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections in Hong Kong (preprint)

Superspreading events have characterised previous epidemics of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV) infections. Using contact tracing data, we identified and characterized SARS-CoV-2 clusters in Hong Kong. Given a superspreading threshold of 6-8 secondary cases, we identified 5-7 probable superspreading events and evidence of substantial overdispersion in transmissibility, and estimated that 20% of cases were responsible for 80% of local transmission. Among terminal cluster cases, 27% (45/167) ended in quarantine. Social exposures produced a greater number of secondary cases compared to family or work exposures (p<0.001) while delays between symptom onset and isolation did not reliably predict the number of individual secondary cases or resulting cluster sizes. Public health authorities should focus on rapid tracing and quarantine of contacts, along with physical distancing to prevent superspreading events in high-risk social environments.