Household transmission of SARS-CoV-2 during the Omicron wave in
Shanghai, China:a case-ascertained study
Abstract
Since late 2021, the highly transmissible SARS-CoV-2 Omicron variant has
driven a new surge of infections across the world. We used a
case-ascertained study to determine the features of household
transmission of SARS-CoV-2 Omicron variant in Shanghai, China. We
collected detailed information on 323 pediatric cases and their 951
household members, all received consecutively intensive RT-PCR testing.
We estimated the transmission parameters. Both secondary infection
attack rates (SARI) and secondary clinical attack rates (SARC) among
adult household contacts were computed, through which the transmission
heterogeneities in infectivity and susceptibility were characterized and
the vaccine effectiveness were estimated. The mean incubation period and
serial interval of Omicron variant were estimated to be 4.6±2.1 days and
3.9±3.7 days. The overall SARI and SARC 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, while infectivity was not significantly different in primary
cases by age, sex, vaccination status and clinical severity. Full
vaccination and booster vaccination of inactivated vaccines 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 symptomatic disease. Overall, we found high household
transmission during the Omicron wave in Shanghai due to asymptomatic and
pre-symptomatic transmission in the context of city-wide lockdown,
indicating the importance of early detection and timely isolation of
SARS-CoV-2 infections and quarantine of close contacts. Marginal
effectiveness of inactivated vaccines against Omicron infection poses
great challenge for prevention and control of the SARS-CoV-2 Omicron
variant.