Social distancing and lockdowns reduce community transmission of SARS-CoV-2, but do not directly address household transmission. We study the effect and preventability of household transmission through the lens of secondary attack rate (SAR) and intra-household reproduction number (Rh). For the former, we use a Bayesian random-effects model to pool previous measurements, correcting for false negatives and asymptomatic cases: our corrected central estimate is a 30% (uncorrected, it is 20%). We identify significant heterogeneity across studies (95% CI: 0-67%), with the lowest SAR coming from places with household-focused precautions in place. We estimate Rh using testing data from Vo’, Italy and contact tracing data from Singapore (scraped from an online dashboard and open-sourced as part of this work): our central estimates are 0.37 (0.34, 0.40) and 0.32 (0.22, 0.42) respectively. We thus estimate that during lockdown, household transmission accounts for 30-55% of transmission and may be an effective target for interventions.