Background: Global comprehensive assessments of hospital admission and mortality among patients with influenza across diverse populations and geographical settings remain limited. To support an update of WHO influenza clinical guidelines, we aimed to assess the baseline risks of hospital admission and case fatality among patients with seasonal, pandemic, or zoonotic influenza.

Methods: In this systematic review and meta-analysis, we systematically searched MEDLINE, Embase, Cochrane Central Register of Controlled Trials, Cumulative Index to Nursing and Allied Health Literature, and Global Health for observational studies published from Jan 1, 2000 to Oct 30, 2025, that reported hospital admission or mortality among patients with laboratory-confirmed influenza. Paired reviewers independently identified studies, extracted data, and assessed the risk of bias. We calculated hospital admission risk and case fatality risk among patients with laboratory-confirmed influenza. We did inverse-variance, fixed-effects model, meta-analyses to summarise the evidence and assessed the certainty of evidence using the GRADE approach. We registered the protocol with PROSPERO, CRD42023456645.

Findings: Of 35 808 records, 468 studies with 19 139 404 patients (mean age 0·1-87·9 years) were eligible. The hospital admission risk was 1·08% (95% CI 1·07-1·08; moderate certainty) for patients with seasonal or pandemic influenza and 93·72% (90·12-96·58; low certainty; based on a single study of 223 patients) for patients with zoonotic influenza. The case fatality risk was 0·19% (0·18-0·20; high certainty) for patients with non-severe seasonal or pandemic influenza, 4·59% (4·55-4·64; moderate certainty) for patients with severe seasonal or pandemic influenza, and 38·72% (36·79-40·66; moderate certainty) for patients with zoonotic influenza.

Interpretation: Our findings provide estimates of hospital admission and case fatality risks among laboratory-confirmed influenza cases identified in health-care or surveillance settings relevant for clinical and public health decision-making. Even when these risks are modest for seasonal or pandemic influenza, widespread transmission can still result in substantial absolute numbers of hospital admissions and deaths.