Flu season 2026–27: when activity rises, and how to read it here

When flu season starts — and when it peaks

The pattern the Centers for Disease Control and Prevention describe each year is consistent: influenza activity in the United States is low through late summer, most often begins to increase in October and November, and peaks most often between December and February, with activity sometimes continuing into May. The CDC's respiratory reporting season is built around that shape — weekly surveillance runs from early October (roughly MMWR week 40) through late May (week 20).

No two seasons are identical. Some rise early and peak over the holidays; others stay quiet until January and peak in February, which is the single most common peak month. The year's timing is only knowable in retrospect — which is why this site shows what the current data says, not what a season is "supposed" to do.

What FluTrack shows during the season

Every week, FluTrack reads the CDC's public respiratory surveillance — emergency-department visit percentages, laboratory test positivity, and wastewater viral activity — and combines them into a single 0–4 threat level with a rising-or-falling trend, for every state. During the season those numbers move weekly; every figure on every page is labeled with the week it represents, and figures inside the usual one-to-two-week settling window are marked as still firming up.

The clearest October signal for most states is usually wastewater viral activity: sewershed measurements are published for many parts of the country and often move before clinical reporting does, which makes them an early hint that a local rise may be starting — even while the combined index is still rated low.

Where to watch it

• Your state's page — the weekly threat level, trend, and per-pathogen read for flu, RSV, and COVID-19.
• Atlanta, Boston, and SF Bay Area — metro pages that pair the state-level read with the official metro-scale sources that exist, including wastewater programs.
• Season alerts — sign up to hear when your state's level moves.
• Methodology and data sources — exactly which CDC systems feed the index and how they are combined.

Holiday weeks read lower than reality

The season's steepest stretch overlaps its least reliable reporting weeks. Around Thanksgiving, Christmas, and New Year, testing volume, care-seeking, and public-health publishing all drop — so surveillance for those weeks can read artificially low and then jump once reporting catches up. FluTrack labels those gaps rather than smoothing over them; a "falling" trend across a holiday week deserves skepticism in both directions.