Why Different Health Data Sources Sometimes Disagree
If you've ever checked flu data from two different sources and gotten two different answers, you're not doing anything wrong. Different surveillance systems measure different things, and they often tell different parts of the same story.
The Main Surveillance Systems
ILINet — Outpatient Doctor Visits
ILINet tracks the percentage of visits to outpatient healthcare providers (doctors' offices, clinics) where patients have influenza-like illness (ILI): fever plus cough or sore throat. This tells you how many people are going to the doctor with flu symptoms. It doesn't confirm that patients actually have flu — some may have COVID, RSV, or other respiratory viruses.
NSSP — Emergency Department Visits
The National Syndromic Surveillance Program tracks emergency department visits with respiratory symptoms. ED visits tend to capture more severe illness than outpatient visits. If NSSP shows high activity but ILINet doesn't, it may mean people are skipping the doctor and going straight to the ER.
NREVSS — Lab Results
The National Respiratory and Enteric Virus Surveillance System tracks actual lab test results — what percentage of respiratory samples tested positive for specific viruses. This is the most specific measure (it confirms the virus) but only covers samples that were actually sent for testing.
Wastewater Surveillance
Wastewater (sewage) testing detects viral genetic material in sewage. It captures everyone — including people who never go to the doctor. Wastewater signals often appear before clinical data because people shed virus before they feel sick enough to seek care.
Common Reasons They Disagree
| Scenario | What It Might Mean |
|---|---|
| Wastewater rising, ILINet flat | Virus is spreading but people haven't started going to the doctor yet. Wastewater often leads clinical data by 1-2 weeks. |
| ILINet high, NREVSS low for flu | People have flu-like symptoms but testing shows it's not flu — it might be COVID, RSV, or another virus causing similar symptoms. |
| NSSP high, ILINet moderate | More people are going to the ER rather than their regular doctor. This can happen during holidays or when doctor offices are closed. |
| Lab positivity rising, visits flat | A higher proportion of sick people test positive, but the overall number of sick people hasn't changed much. Could indicate a new variant spreading among those who do get sick. |
What This Means for You
No single data source gives the complete picture. When sources agree, you can be more confident in the signal. When they disagree, it usually means the situation is more nuanced — not that one source is "wrong." Local Health Signal primarily uses ILINet data for flu activity levels because it's the longest-running, most geographically complete surveillance system. We note when other sources tell a different story.