Approximately 20% of people had an increase in antibodies against influenza in their blood after an influenza “season”. However, about three-quarters of infections were symptom-free, or so mild they weren’t identified through weekly questioning about whether participants had a cough, cold, sore throat, or a “flu-like illness”.
https://www.nhs.uk/news/medical-practice/three-quarters-of-people-with-flu-have-no-symptoms/