Methodology

How COVID Flow works

COVID Flow ingests CDC NWSS data each week, cleans Illinois sewershed records, and publishes indices you can read on the regional dashboards. This page documents what we compute and how to interpret it.

Primary data source

All metrics derive from CDC NWSS SARS-CoV-2 wastewater sample records. Our pipeline downloads open releases on a weekly schedule, harmonizes sewershed identifiers, and stores cleaned observations in a versioned database before computing indices.

Source: CDC NWSS program

The activity index compares this week to each sewershed's own history. It describes relative change in viral RNA, not how many people are infected.

What wastewater measures

Each NWSS sample reflects SARS-CoV-2 RNA in wastewater from a sewershed: the area served by a wastewater treatment plant or sewer network. The signal integrates shedding from the connected population. It is not a case count, hospitalization estimate, or diagnostic test result.

Weekly activity index

For each sewershed and region we compare the current week to that location's own historical baseline. The index is centered near zero (roughly −1 below typical to +1 above). Values are aggregated across sites using population-informed weighting where available. The index describes relative viral load, not absolute prevalence.

Trend labels

Rising, falling, and stable labels apply week-over-week changes only after minimum sample and coverage thresholds are met. When data are sparse or quality is low, we label trends as insufficient data rather than forcing a direction.

Quality review

Quality scores incorporate detection limits, sample type, missing flow or population normalization, and reporting gaps. A large index change paired with a low quality score should be interpreted cautiously. Hydrology, industrial inflow, and sewershed boundary changes can move readings without a true epidemic shift.

In plain terms

Wastewater measures the whole community, not individuals. Levels can rise before clinics see more cases in some outbreaks. They can fall as immunity or behavior changes. The data cannot tell you your personal risk.

Use these dashboards for community context alongside official public health guidance and clinical reporting. They are not a substitute for medical advice.

How to cite

COVID Flow (2026). Community COVID wastewater dynamics dashboard. Data: U.S. CDC National Wastewater Surveillance System (NWSS). Accessed via public NWSS releases.