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.

Potential Conflicts of Interest: 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.

Financial Support: These research studies were supported by CDC (grant 5U01IP001078 awarded to the Marshfield Clinic Research Institute, grant 5U01IP001083 awarded to Vanderbilt University Medical Center) and contract 75D30121C11656 jointly funded by the CDC and the US Food and Drug Administration and awarded to Vanderbilt University Medical Center). The Vanderbilt Institute for Clinical and Translational Research (VICTR) is funded by the National Center for Advancing Translational Sciences (NCATS) Clinical Translational Science Award (CTSA) Program, Award Number 5UL1TR002243-03. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.

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Study protocols and procedures were reviewed by the Institutional Review Boards at the Marshfield Clinic Research Institute and Vanderbilt University. CDC determined these activities were conducted consistent with applicable federal law and CDC policy (see 45 C.F.R. part 46; 21 C.F.R. part 56).

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