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. 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 and pre-immunisation predecessor study. Results Sixty-seven households were included, consisting of 241 individuals (median age 33.0 years). Maximum household Secondary Attack Rate (SAR) was 59.7%, per-person SAR 41.5%. Underage indexes 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. Nevertheless, transmission rates are lower than our pre-immunisation reported ancestral rates: household SAR decreased from 88.2% to 59.7%, per-person SAR from 64.3% to 41.5%, implying immunity reduces transmission, even for a more transmittable variant. Regardless of immune status, children have a crucial role in Omicron household transmission. Intensive sampling and phylogenetic analysis are essential to correctly calculate transmission rate, especially in times of minimal behavioural restrictions.

The authors have declared no competing interest.

This study did not receive any funding

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All participants provided written informed consent. The Medical Ethical Committee of the Amsterdam University Medical Centre, The Netherlands, reviewed and approved this study (reference number 2022.0073).

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