The impact of public health and social measures on social contact patterns and transmission of COVID-19 in Hong Kong

BACKGROUND: Public health and social measures (PHSMs) are key interventions during pandemics. This study quantifies temporal changes in daily interpersonal contacts associated with PHSM implementation and their potential impact on disease transmission across different phases of the COVID-19 pandemic in Hong Kong from 2021 to 2023. METHODS: We recruited 7,053 individuals from the general population and 3,462 participants from selected high-contact groups between September 2021 and December 2023. The study period spanned the infection-naïve phase before 2022 through the city's fifth wave of COVID-19 (January-April 2022) driven by the Omicron BA.2 variant. Reported cumulative incidence was < 0.2% of the population before the fifth wave and rose to 16% afterward. The onset of the fifth wave prompted the government to impose unprecedentedly stringent PHSMs. Participants reported their daily number of contacts via contact surveys, from which we constructed social contact matrices. We then compared the dominant eigenvalues of these contact matrices to estimate the potential association of PHSMs with changes in the basic reproduction number ([Formula: see text]). RESULTS: Among the general population, the average number of daily contacts dropped significantly from 6.2 (mean; 95% confidence interval [CI]: 5.9-6.5) during pre-fifth wave to 5.1 (5.0-5.3) during the fifth wave. After the fifth wave, the average number of contacts remained low at 5.1 (5.0-5.3) when some PHSMs such as mask-wearing policies were still in place. Once all PHSMs were lifted after March 2023, the average number of contacts increased to 8.4 (8.3-8.5). Compared to the general population, participants from the selected high-contact groups reported more contacts by 23% (95% CI: 21%-26%) on average. Compared to pre-fifth wave levels, PHSMs imposed during the fifth wave were associated with a 20% reduction (median; 95% percentile interval: 1%-40%) in [Formula: see text]. Relaxation of PHSMs in the post-pandemic period suggested an increase in [Formula: see text] by 109% (61%-151%) relative to pre-fifth wave levels. CONCLUSIONS: During the COVID-19 pandemic in Hong Kong, PHSMs were temporally associated with reduced social contacts and, potentially, disease transmission. These quantitative estimates may indicate the potential value of PHSMs for informing future pandemic preparedness and response.