Explosive household spread of the SARS-CoV-2 Omicron variant in China in late 2022

Abstract

Immediately after easing the 3-year zero-COVID policy in mainland China, an explosive spread of the Omicron variant of SARS-CoV-2 occurred in late 2022, despite most people staying at home. We characterise the virus transmission within households and the associated risk factors. We conducted an online survey, which was done by professors and postgraduate students in the discipline of building environment and energy engineering, and all questionnaires were completed on December 31, 2022 and collected over a period of 11 h. The self-reported infection status of 12,037 people in 4421 households was collected in mainland China. Data on vaccination status, mask wearing, hand hygiene, ventilation, and bathroom air odours were analysed. Of the surveyed population, 79.2% (9533/12,037) reported infection based on symptoms or rapid test results by 31 December. The daily number of cases peaked on 20 December nationally and between 16 and 26 December in different provinces. Better ventilation in shared corridors and living rooms was associated with a lower household daily incidence rate and a lower fever occurrence. Residing on upper floors was associated with a higher daily incidence rate. Wearing an N95 respirator when leaving home could effectively reduce household infection risk. The observed similarity in the peak infection rate and its date of occurrence in different provincial regions suggest the spontaneous and simultaneous spread of the Omicron variant across the country. Crowded households, poor ventilation in shared common corridors, and residing on upper floors were associated with a higher incidence and more symptoms of disease.