Norovirus is climbing to high levels across much of the United States, with recent wastewater surveillance showing an upward trend and hikers in Southern California among the people newly sickened. The signal is strongest in the Northeast, where levels are still rising.
The timing matters because the virus is already moving widely enough to show up in the sewage even as many cases never make it into clinic records. Amanda Bidwell said national wastewater levels are still in the HIGH category because of high concentrations over the last 21 days, a reading that has drawn attention to how much illness can be missed when people stay home and recover.
That broad rise comes with a sharp regional mix. Wastewater numbers also highlight an outbreak in the San Francisco Bay Area, while CDC tracking from Aug. 1 to May 7 recorded 1,194 outbreaks, far below the 2,534 logged over the same stretch the previous year. The gap does not mean the virus has disappeared; it means the current season looks different from last year’s surge, even as activity remains elevated in enough places to matter.
The CDC says national norovirus levels are not unusually high compared with prior seasons, a reminder that wastewater and confirmed outbreak counts do not always tell the same story. But the sewage data is now catching a pattern that clinical reporting alone can miss, and Bidwell said monitoring it is especially useful because this highly contagious virus often sends people home to ride it out without seeing a healthcare professional.
Strain mix helps explain why the virus keeps finding new pockets of spread. Several norovirus strains are circulating, including GII.4 and GII.17, and during the 2024-25 season GII.17 overtook GII.4 as the dominant strain in the U.S., causing about 75% of outbreaks. Dr. Linda Yancey said there is nothing unusual about the California cluster and that the hikers simply got unlucky, but she also noted that the newer variant can move more easily because fewer people have partial immunity to it.
Norovirus is often called the winter vomiting disease, yet it can spike in late spring, which leaves room for more cases even as the weather warms. The question now is not whether the virus will keep circulating — it will — but how much of that spread is still being missed by the systems that count only people who seek care.
