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COVID Flow

STATE SURVEILLANCE

Illinois

Statewide community wastewater signal from CDC NWSS sewersheds across Illinois, aggregated weekly after cleaning and quality review.

Activity index

-2.31

Week over week

-27.0%

FallingMay 25, 2026

County map

Activity index by county for sewersheds reporting this week. Hover or focus a county for detail. Neutral fill means no NWSS coverage in that county. Data for the week of May 25, 2026.

Week of May 25, 2026Latest
31 of 102 counties reporting
Nov 22, 2021May 25, 2026
Illinois counties, wastewater activity index, week of May 25, 2026
Activity index
-1.750+3
No NWSS coverage

Current metrics

Activity index

-2.31

Week-over-week -27.0%

Compared to each sewershed's own history (0 = typical week)

Sites reporting

58

58 of 101 registered sewersheds · 9.0M population represented

Data quality

73%

Latest week composite score. See quality panel for flags.

How to read the activity index

01

Near zero

Typical shedding for that sewershed compared with its own history.

02

Below zero

Less viral RNA than the historical baseline for that location.

03

Above zero

More viral RNA than the historical baseline for that location.

Week-over-week change shows direction only. Trend labels (rising, falling, stable) apply only when enough samples meet our quality thresholds.

Historical trend

Weekly wastewater signal for Illinois from CDC NWSS samples after cleaning and quality review.

Wastewater activity over time
Weekly weighted activity index for Illinois. Higher values indicate elevated viral RNA relative to the regional baseline.

Source: CDC NWSS via COVID Flow weekly pipeline. Index is relative to historical sewershed baselines, not a case count.

Quality and provenance

Data quality
How complete and reliable the underlying wastewater samples are for the latest reporting week. Lower scores mean more caution when interpreting trends.
73%quality score
  • Some samples below limit of detection
  • Grab sample rather than composite collection
  • Sample location is not a wastewater treatment plant
  • Missing flow or population normalization
  • Fewer than three samples in the last 28 days

Wastewater monitoring reflects community shedding, not individual diagnoses. Gaps in sampling, lab methods, or sewershed coverage can shift the index without a true epidemic change.

Data provenance
Metrics are rebuilt each week from CDC NWSS open data. We harmonize sewershed identifiers, apply Illinois cleaning rules, and roll samples up to site and region level.

Source: CDC NWSS program

Projected trend (next few weeks)

Our best estimate of where the wastewater activity index for Illinois is heading over the next few weeks, shown with an uncertainty band. This tracks wastewater activity, not confirmed case counts.

Short-horizon forecast
Ensemble baseline for Illinois from the week of May 25. Shaded band ≈ 80% interval from backtest errors.

Viewing: Ensemble baseline

Rising
ObservedEnsemble~80% interval

Activity fell sharply this week (-27.0% week-over-week). The ensemble forecast shows a rebound, and simple baselines often mean-revert after big moves. Treat the next few weeks as uncertain.

1-week outlook (ensemble_v1): Rising · confidence high. Surveillance context only, not for clinical decisions.