Factors Associated With the Transmission of the Delta Severe Acute Respiratory Syndrome Coronavirus 2 Variant in Households: The Israeli COVID-19 Family Study (ICoFS)

Understanding how interpersonal interactions and immunological factors shape severe acute respiratory syndrome coronavirus 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 1093 Israeli households (July-August 2021). Among the 2883 household contacts, 1096 (38%) were infected. Children were 38% (95% Credibleconfidence interval [CI], 7% -81%) more likely to be infected than adults. Isolation measures reduced transmission by 52% (95% CrI, 46% -57%). Transmission was 39% (95% CrI, 11% -76%) higher between children and female adults than between children and male adults. Vaccine effectiveness was 78% (95% CrI, 54%-90%), 85% (95% CrI, 70% – 94%), and 73% (95% CrI, 49% -88%), respectively, for 1, 2, and 3 recent vaccine doses (within <90 days) but dropped to 18% (95% 95% CrI, -6% to 36%) for 2 doses administered >90 days earlier. Household member interactions significantly shaped transmission, and isolation measures effectively reduced transmission.