Lower levels of household transmission of SARS-CoV-2 VOC Omicron compared to Wild-type: an interplay between transmissibility and immune status

Background Knowledge of SARS-CoV-2 household transmission dynamics guides infection control and vaccination measures. This household cohort study prospectively assessed the impact of both the Omicron BA.2 variant and immunity on household transmission using dense saliva sampling and sequence analysis. Methods Households consisting of a PCR-confirmed index and at least two household members were enrolled in March and April 2022 during the Omicron BA.2 wave in the Netherlands. SARS-CoV-2 PCR was performed on ten consecutive saliva samples. Serum-antibodies were measured at baseline and day 42. Household and per-person Secondary Attack Rate (SAR) 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, pre-immunisation 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%, per-person SAR 41.5%. Paediatric index cases were more likely to transmit. Transmission was negatively affected by household members’ immunity. Phylogenetic analysis showed multiple introductions in four 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 compared to 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 behavioural restrictions.