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

Curmei, Mihaela; Ilyas, Andrew; Evans, Owain; Steinhardt, Jacob

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 (R(h)). 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 R(h) with respect to community transmission rates and mobility patterns. RESULTS: The corrected pooled estimates from prior studies for SAR and R(h) are 24% (20–28%) and 0.34 (0.30–0.38), respectively. Without corrections, the pooled estimates are: SAR = 18% (14–21%) and R(h) = 0.28 (0.25–0.32). The corrected estimates line up with direct estimates from contact-tracing data from Singapore [R(h) = 0.32 (0.22–0.42)] and population testing data from Vo’ [SAR = 31% (28–34%) and R(h) = 0.37 (0.34–0.40)]. The analysis of Singapore data further suggests that the value of R(h) (0.22–0.42) is robust to community-spread dynamics; our estimate of R(h) stays constant whereas the fraction of infections attributable to household transmission (R(h)/R(eff)) 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 R(h). 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.