Asymptomatic and mildly symptomatic influenza virus infections by season --Case-ascertained household transmission studies, United States, 2017-2023

Abstract Asymptomatic influenza virus infection occurs but may vary by factors such as age, vaccination status, or season. We examined the frequency of influenza virus infection and symptoms using data from two case-ascertained household transmission studies (2017—2023) with prospective, systematic collection of respiratory specimens and symptoms. From the 426 influenza virus infected household contacts that met our inclusion criteria, 8% were asymptomatic, 6% had non-respiratory symptoms, 23% had acute respiratory symptoms, and 62% had influenza-like illness symptoms. Understanding the prevalence of asymptomatic and mildly symptomatic influenza cases is important for implementing effective influenza prevention strategies and enhancing symptom-based surveillance systems.