Genomic Epidemiology of Hospital‐Associated SARS‐CoV‐2 Clusters in Hong Kong During a Period of Relaxed Visitation (May–August 2022)

Background Between May and August 2022, as Hong Kong shifted from a “zero‐COVID” strategy towards relaxation, including relaxed hospital visitation, understanding SARS‐CoV‐2 nosocomial transmission dynamics was critical. Methods We used genomic epidemiology to investigate hospital‐associated infections between May 28 and August 18, 2022. A total of 162 viral genomes from 29 suspected clusters across 17 hospitals were sequenced and analyzed, covering more than half of all officially reported nosocomial infections during the period. Bayesian phylogeographic analysis for the dominant BA.2.2 lineage was used to infer the rate of community‐to‐hospital viral introductions. Generalized additive models (GAMs) then assessed the association between these inferred introduction rates and concurrently measured community and cross‐border mobility indices. Results From the initial 29 suspected clusters, 18 nosocomial clusters involving 126 genomes across 13 hospitals were confirmed, representing 51.9% of the total reported nosocomial cases in that period. The dominant lineage was BA.2.2. Most clusters were consistent with single intraward transmission chains, and no evidence of interward spread was found. Bayesian analysis revealed ongoing community‐to‐hospital introductions with a rate that varied considerably over time. However, after accounting for this dominant temporal trend, GAMs found no statistically significant association between the daily rate of these introductions and any of the tested mobility indices. Conclusions During periods of sustained community transmission, broad community mobility metrics may not directly predict the risk of SARS‐CoV‐2 introduction into hospitals. Robust hospital‐level infection prevention and control measures, effective visitor screening, and integrated genomic surveillance remain paramount for mitigating nosocomial transmission, irrespective of general population movement.