Factors associated with the transmission of the delta SARS-CoV-2 variant in households: the Israeli COVID-19 Family Study (ICoFS)

Understanding how interpersonal interactions and immunological factors shape SARS-CoV-2 transmission in households is crucial for designing control measures. We developed a Bayesian data augmentation transmission model to evaluate the effects of isolation, parental care, and vaccine-induced immunity on Delta variant transmission from the follow-up of 1,093 Israeli households (July–August 2021). Among the 2883 household contacts, 1096 (38%) were infected. Children were 38% (CI: 7-81) more likely to be infected than adults. Isolation measures reduced transmission by 52% (CI: 46-57). Transmission was 39% (CI: 11-76) higher between children and adult females than males. Vaccine effectiveness was 78% (CI: 54-90), 85% (CI: 70-94), and 73% (CI: 49-88) for one, two, and three doses of recent vaccination (< 90 days), respectively but dropped to 18% (CI: (-6)-36) for two doses administered more than 90 days ago. Household member interactions significantly shaped transmission, and isolation measures effectively reduced transmission.