Constructing and adjusting estimates for household transmission of SARS-CoV-2 from prior studies, widespread-testing and contact-tracing data

Abstract

Background: With reduced community mobility, household infections may become increasingly important in SARS-CoV-2 transmission dynamics. Methods: We investigate the intra-household transmission of COVID-19 through the secondary-attack rate (SAR) and household reproduction number (Rh). We estimate these using (i) data from 29 prior studies (February-August 2020), (ii) epidemiologically linked confirmed cases from Singapore (January-April 2020) and (iii) widespread-testing data from Vo' (February-March 2020). For (i), we use a Bayesian random-effects model that corrects for reverse transcription-polymerase chain reaction (RT-PCR) test sensitivity and asymptomatic cases. We investigate the robustness of Rh with respect to community transmission rates and mobility patterns. Results: The corrected pooled estimates from prior studies for SAR and Rh are 24% (20-28%) and 0.34 (0.30-0.38), respectively. Without corrections, the pooled estimates are: SAR = 18% (14-21%) and Rh = 0.28 (0.25-0.32). The corrected estimates line up with direct estimates from contact-tracing data from Singapore [Rh = 0.32 (0.22-0.42)] and population testing data from Vo' [SAR = 31% (28-34%) and Rh = 0.37 (0.34-0.40)]. The analysis of Singapore data further suggests that the value of Rh (0.22-0.42) is robust to community-spread dynamics; our estimate of Rh stays constant whereas the fraction of infections attributable to household transmission (Rh/Reff) is lowest during outbreaks (5-7%) and highest during lockdowns and periods of low community spread (25-30%). Conclusions: The three data-source types yield broadly consistent estimates for SAR and Rh. Our study suggests that household infections are responsible for a large fraction of infections and so household transmission may be an effective target for intervention.