BACKGROUND: Knowledge of SARS-CoV-2 household transmission dynamics guides infection control and vaccination measures. This household cohort study prospectively assessed the impact of the Omicron BA.2 variant and immunity on household transmission using dense saliva sampling and sequence analysis. METHOD: Households consisting of a polymerase chain reaction-confirmed index and at least 2 household members were enrolled in March and April 2022 during the Omicron BA.2 wave in the Netherlands. SARS-CoV-2 polymerase chain reaction was performed on 10 consecutive saliva samples. Serum antibodies were measured at baseline and day 42. Household and per-person secondary attack rates (SARs) were calculated to measure transmission. Whole genome sequencing was performed for phylogenetic analysis, followed by sensitivity analysis, to correct for multiple household introductions and index definition. Results were compared with the identical, early-pandemic, preimmunization predecessor study. RESULTS: Sixty-seven households were included, consisting of 241 individuals (median age, 33.0 years
IQR, 12.0-46.0). Maximum household SAR was 59.7% and per-person SAR 41.5%. Pediatric index cases were more likely to transmit. Transmission was negatively affected by household members' immunity. Phylogenetic analysis showed multiple introductions in 4 households. Sensitivity analysis resulted in a minimal household SAR of 51.0% and per-person SAR of 28.5%. CONCLUSIONS: The Omicron BA.2 variant is highly transmissible within households. However, the transmission rate is lower when compared with previous studies with other SARS-CoV-2 variants, highlighting the effect of immunity. Regardless of immune status, children have a crucial role in Omicron household transmission. Intensive sampling and phylogenetic analysis are beneficial for correctly calculating transmission rates, especially during periods of minimal behavioral restrictions.