Seasonal influenza: What the US Data Actually Shows

The published figures

Every number below is reproduced from a primary source and carries that source beside it. Where a figure is absent it is shown as unavailable rather than estimated — the reasoning for each gap is set out further down this page.

2024-25 was classified 'high severity' — the most severe season since 2017-18. CDC's own long-run range (2010-2025) is 9.4M-51M illnesses and 6,300-52,000 deaths, so this single season sits at the top of that range. case_fatality_rate_pct is a same-record derivation (deaths ÷ illnesses, both CDC 2024-25-season modeled estimates) per SITE-PLAYBOOK.md §12a; both inputs are model-based CDC burden estimates, not raw notified counts, since seasonal influenza isn't individually reportable. The illnesses and deaths trends use CDC's modeled disease-burden estimates (same method each season), one point per season from 2017-18 through 2024-25 — deaths: 52,000 (2017-18), 28,000 (2018-19), 22,000 (2019-20), 6,300 (2021-22), 21,000 (2022-23), 28,000 (2023-24), 45,000 (2024-25). The 2020-21 season is intentionally absent: CDC published no burden estimate for it because influenza circulation was minimal under COVID-19 mitigation. These modeled burden figures are NOT the smaller coded death-certificate counts, which are a different series and are not mixed in.

How these figures were calculated

One figure above is arithmetic rather than a number lifted straight from an agency page. Both inputs are given below with their own source, geography, and reporting period, so the calculation can be checked rather than taken on trust. A derived figure is never rated more reliable than its weakest input.

What this data cannot tell you

2 of the five funnel metrics have no publishable US figure for Seasonal influenza. That is a deliberate result, not an oversight. Where no primary source publishes a comparable figure, this site leaves the cell empty rather than filling it with an estimate, a number borrowed from a different population, or a single-study result presented as a general rate.

• Severe Outcome Rate — no comparable figure published by the record's primary source as of 2026-03-12.
• Hospitalization Rate — no comparable figure published by the record's primary source as of 2026-03-12.

This matters when comparing viruses. A blank cell is not a low number: it means the measurement does not exist, and treating it as zero would make Seasonal influenza look safer than the evidence supports. The comparison table on the Virus Risk Perspective page applies the same rule — unavailable figures are excluded from ranking rather than sorted to the bottom.

How Seasonal influenza compares

Ranked only against viruses in this collection that publish the same metric. Viruses without a published figure are excluded from each comparison rather than counted as zero, so the denominator differs from metric to metric.

Read the case comparison carefully. Records in this collection do not all measure exposure the same way. Some report confirmed cases counted by a surveillance system (measles, West Nile), others a modelled estimate of total annual illness including cases that are never reported (norovirus, seasonal influenza). A modelled burden figure will always dwarf a confirmed-case count, so a gap on this metric can reflect how a disease is counted as much as how often it occurs. The per-figure sources above state which basis each number uses.

How the figures have moved

Each observation below is a figure as it stood on a given date, with the source that published it. Points are not interpolated: where an agency published a figure once, one point is shown rather than a line drawn through invented values.

Global picture

WHO estimates ~1 billion seasonal influenza cases worldwide each year, including 3–5 million cases of severe illness and 290,000–650,000 respiratory deaths. Fact sheet re-read 2026-09-15: dated 28 February 2025 (the date now carried here) and unchanged — the ~1 billion cases and 290,000–650,000 deaths are WHO's standing estimates, restated rather than re-derived.

Source: WHO — Influenza (seasonal) fact sheet · Data as of 2025-02-28 · Surveillance confidence: moderate

Sources

• CDC — 2024-2025 Influenza Season Summary: Severity, Disease Burden, and Burden Prevented primary source · retrieved 2026-03-12
• WHO — Influenza (seasonal) fact sheet primary source · retrieved 2025-02-28
• CDC — Estimated Flu Disease Burden, 2017–2018 Season primary source · retrieved 2018-05-31
• CDC — Estimated Flu Disease Burden, 2018–2019 Season primary source · retrieved 2019-05-31
• CDC — Estimated Flu Disease Burden, 2019–2020 Season primary source · retrieved 2020-05-31
• CDC — Estimated Flu Disease Burden, 2021–2022 Season primary source · retrieved 2022-05-31
• CDC — Estimated Flu Disease Burden, 2022–2023 Season primary source · retrieved 2023-05-31
• CDC — Estimated Flu Disease Burden, 2023–2024 Season primary source · retrieved 2024-05-31
• CDC — 2023-2024 Influenza Season Summary: Disease Burden primary source · retrieved 2024-05-31

Sourcing tiers, and the rule that a derived figure inherits its weakest input's tier, are set out on the Our Sources page. The process behind every figure is described in How We Answer Questions.