Post-influenza syndromes: a hidden burden in clinical practice

INTRODUCTION: Influenza is still taught and managed as a self-limited acute respiratory illness. The COVID-19 pandemic forced a reexamination of every respiratory virus and made a longer time horizon impossible to ignore. Comparator-controlled cohort data now indicate that influenza, like SARS-CoV-2, is followed by persistent, new-onset multi-system disease that is largely invisible in practice. AREAS COVERED: This perspective reframes influenza as a systemic infection with a distinct post-acute footprint. Drawing on autopsy, experimental and comparative-cohort evidence, a working construct of post-acute influenza syndrome (PAIS) is outlined across its cardiovascular, pulmonary, infectious, neurological, metabolic, renal and immunological components. Literature was identified through PubMed searches combining post-acute, post-influenza and post-viral terms with influenza (run to June 2026), with citation tracking, prioritizing studies with a non-influenza comparator. Influenza is framed as a cardiovascular stress test and a trigger of long-term multimorbidity in the already vulnerable, alongside the unresolved conflict over whether its cardiac burden exceeds that of COVID-19. EXPERT OPINION: Post-influenza sequelae are real, clinically consequential, and systematically missed because of delayed onset, attribution bias and fragmented care. Vaccination, longitudinal follow-up of high-risk survivors and integrated care pathways should become standard practice within five years.