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

Asymptomatic influenza virus infection occurs but may vary by factors such as age, influenza vaccination status, or influenza season. We examined the frequency of influenza virus infection and associated symptoms using data from two case-ascertained household transmission studies (conducted from 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 the effectiveness of symptom-based surveillance systems.
Competing Interests: Competing Interest Statement: EAB receives research support from CSL Seqirus unrelated to the current work. HQN receives research support unrelated to this work from CSL Seqirus, GSK, ModernaTX Inc, and honorarium for participating in a consultancy group for Moderna outside the submitted work. Dr. Grijalva has received consulting fees from Merck, and research support from SyneosHealth, NIH, FDA and AHRQ.