The Centers for Disease Control and Prevention raised the benchmarks it uses to sort COVID-19 wastewater levels into categories in August, so a reading that once counted as high transmission may now appear as moderate, NBC News reported. The change affects the color-coded maps many families check before visiting older relatives or deciding whether to test.
The underlying virus levels did not change. What changed is where the CDC draws the lines between very low, low, moderate, high, and very high. The categories are labels on a relative scale, not counts of cases.
For caregivers and high-risk households, that matters. Someone who learned last year that "high" meant it was time to take extra precautions may now see a calmer label for a similar amount of virus and could ease up sooner than intended.
The August Recalibration, Explained
The CDC's explainer on the wastewater viral activity level says the agency re-evaluated its thresholds using the most recent 24 months of data. COVID-19 previously used thresholds based on percentiles, while flu and RSV used a method that looks for natural breaks in the data. As of August, all three use the natural-breaks method, and the CDC checked the new cutoffs against emergency department data from its syndromic surveillance program.
"Minor updates were made to WVAL thresholds to maintain continued alignment with recent wastewater activity," the page states. Under the current scale, a COVID wastewater viral activity level up to 2.6 is very low, above 2.6 to 4.9 is low, above 4.9 to 7.9 is moderate, above 7.9 to 11.6 is high, and anything above 11.6 is very high.
The agency made other changes at the same time. Extreme readings are now capped at the 99th percentile instead of being removed. New COVID sampling sites use a baseline drawn from their testing laboratory's data for their first 182 days to avoid wild swings. The national level is now the median of state and territory levels rather than regional levels. The CDC applied all of these updates to past data too, so its own charts stay internally consistent.
The scale is relative. For each site, the CDC sets a typical low level from the past two years, then measures how far the current reading sits above it. Each virus has its own cutoffs, so the same value can mean different things: A reading of 10.0 counts as moderate for flu, high for COVID, and very high for RSV. State levels are medians of local sites, which means a hot spot in one city can be hidden by quieter readings elsewhere.
The CDC says it reviews the metric every summer and describes the process as routine recalibration, similar to updates in other respiratory tracking systems. Because baselines use the most recent two years of data, the range of values can shift as new data replace older data.
Since MedicalDaily's Earlier Report
MedicalDaily previously reported that COVID was growing in 27 states, down from 35, with hospitalizations low and Texas showing the highest wastewater levels. That story tracked the direction of infections. This one addresses how to read the wastewater labels themselves.
The recalibration can also help explain why readers see different pictures from different trackers. Other programs, such as WastewaterSCAN, use their own methods and categories, so their labels may not match the CDC's. The CDC itself advises pairing wastewater readings with clinical data, such as those on its respiratory illnesses data channel, before drawing conclusions.
Reading Your Local Wastewater Map This Fall
The most useful number is the trend, not the color. A site moving from low to moderate over two weeks tells you more than a single label. The CDC updates its state-level respiratory virus wastewater data weekly, and recent weeks can be revised as more sites report. The agency also posts current and historical values in a public dataset on data.cdc.gov.
State figures may not reflect every community. Residents of large metro areas can often find local sewershed data through city or county health departments, which may be more relevant than a statewide label.
The people who should pay closest attention are adults 65 and older, people with weakened immune systems or chronic heart, lung, or kidney disease, pregnant people, and families with young infants. For most healthy adults, a one-step change in category is not a reason to cancel plans.
Practical steps do not depend on the label. Testing before visiting a vulnerable relative, staying home when sick, and keeping up with recommended vaccines all work regardless of how the map is colored. High-risk people who test positive should contact a clinician quickly, because antiviral treatment works best when started early. Trouble breathing, chest pain, new confusion, bluish lips, or difficulty staying awake require emergency care.
The next scheduled change is coming soon. The CDC recalculates COVID baselines each April 1 and Oct. 1, which means site comparisons will shift again in early October. Families who watch the trend at their local site, rather than the color alone, will have the clearest picture heading into fall and winter.
Key Questions Answered
What did the CDC change? In August, it recalculated the thresholds that sort COVID wastewater levels into categories, using the last 24 months of data and a new statistical method.
Does this mean there is less COVID? No. The change affects the category cutoffs, not the virus levels measured in wastewater, so a similar reading can now carry a lower label.
What are the current COVID thresholds? Very low is up to 2.6, low is above 2.6 to 4.9, moderate is above 4.9 to 7.9, high is above 7.9 to 11.6, and very high is above 11.6.
Why do other trackers show different levels? Programs such as WastewaterSCAN use different methods and categories, so their labels can differ from the CDC's.
How should I use wastewater data? Watch the trend at your local site over several weeks, and use it alongside testing, vaccination, and local health department guidance.
When will the scale change again? The CDC recalculates COVID baselines on April 1 and Oct. 1 each year, so the next update is due in early October.
Published by Medicaldaily.com
