SickMix: Temporal Changes in Social Contact Patterns among People with Acute Infection and Their Close Contacts

Infected individuals drive infectious disease transmission, yet empirical data on social interactions during acute infection remain limited. We conducted a prospective longitudinal study of 1,000 medically attended acute gastroenteritis and acute respiratory infection cases and 709 household members in the northwestern United States from 2024 to 2025 to quantify illness-associated changes in social contacts and their implications for transmission modeling. Cases substantially reduced social contacts during peak illness; weighted mean number of contacts increased 2.0-fold (95% confidence interval, 1.8–2.2) over the two-week follow-up. In transmission modeling, the simulated outbreak trajectory and estimated effectiveness of interventions differed substantially between models that did and did not account for temporal reductions in social contacts during acute infection. These findings show that behavioral responses during acute infection could substantially influence transmission dynamics and intervention evaluation, and provide an empirical framework for incorporating illness-associated behavioral change into infectious disease models.