Household transmission of SARS‐CoV‐2 during the Omicron wave in Shanghai, China: A case‐ascertained study

OBJECTIVES: We used a case‐ascertained study to determine the features of household transmission of SARS‐CoV‐2 Omicron variant in Shanghai, China. METHODS: In April 2022, we carried out a household transmission study from 309 households of 335 SARS‐CoV‐2 pediatric cases referred to a designated tertiary Children's Hospital. The detailed information can be collected from the 297 households for estimating the transmission parameters. The 236 households were qualified for estimating the secondary infection attack rates (SAR(I)) and secondary clinical attack rates (SAR(C)) among adult household contacts, characterizing the transmission heterogeneities in infectivity and susceptibility, and assessing the vaccine effectiveness. RESULTS: We estimated the mean incubation period and serial interval of Omicron variant to be 4.6 ± 2.1 and 3.9 ± 3.7 days, respectively, with 57.2% of the transmission events occurring at the presymptomatic phase. The overall SAR(I) and SAR(C) among adult household contacts were 77.11% (95% confidence interval [CI]: 73.58%–80.63%) and 67.03% (63.09%–70.98%). We found higher household susceptibility in females. Infectivity was not significantly different between children and adults and symptomatic and asymptomatic cases. Two‐dose and booster‐dose of inactivated COVID‐19 vaccination were 14.8% (5.8%–22.9%) and 18.9% (9.0%–27.7%) effective against Omicron infection and 21.5% (10.4%–31.2%) and 24.3% (12.3%–34.7%) effective against the symptomatic disease. CONCLUSIONS: We found high household transmission during the Omicron wave in Shanghai due to presymptomatic and asymptomatic transmission despite implementation of strict interventions, indicating the importance of early detection and timely isolation of SARS‐CoV‐2 infections. Marginal effectiveness of inactivated vaccines against Omicron infection poses a great challenge for outbreak containment.