Norovirus — Outbreaks and Surveillance

What we know about norovirus tracking, and why wastewater surveillance is coming

What Is Norovirus?

Norovirus is the most common cause of acute gastroenteritis (inflammation of the stomach and intestines) in the United States. Often called the "stomach flu" — though it is not related to influenza — norovirus causes an estimated 19–21 million illnesses each year in the US, along with 109,000 hospitalizations and 900 deaths, primarily among young children and older adults.

Norovirus is remarkable for its extreme contagiousness. As few as 18 viral particles are enough to cause infection. It spreads through contaminated food and water, contact with infected surfaces, and close contact with infected people. It survives on surfaces for days and is resistant to many common disinfectants.

Why CDC Doesn't Track Norovirus in Wastewater Yet

Wastewater surveillance for norovirus is scientifically feasible — the virus is shed at high levels in feces of infected individuals — but several methodological challenges have slowed its integration into the CDC's National Wastewater Surveillance System:

• Strain diversity: Norovirus has numerous genotypes (GI, GII, and others), and different strains require different PCR primers. A single assay doesn't capture all circulating strains equally.
• Normalization complexity: The PMMoV normalization method used for respiratory viruses works less cleanly for norovirus, whose shedding patterns differ significantly from respiratory pathogens.
• Baseline establishment: Norovirus circulates year-round at low levels with sporadic outbreaks, making it harder to define "normal" vs. "outbreak" levels without years of standardized data.

CDC-affiliated researchers are actively publishing on norovirus wastewater methodology, and several pilot programs are underway. We expect norovirus to be added to the NWSS within the next 1–2 years. When it is, Outbreak Radar will incorporate it.

Current Norovirus Outbreak Patterns in the US

Norovirus outbreaks peak in the winter months (November through April) in the US, which is why it's sometimes called the "winter vomiting bug" in other countries. However, unlike flu or RSV, norovirus never fully disappears — outbreaks occur year-round in closed settings like cruise ships, schools, nursing homes, hospitals, and restaurants.

The CDC's NoroSTAT surveillance program tracks laboratory-confirmed norovirus outbreaks reported by participating states. Unlike wastewater data, NoroSTAT represents only reported outbreaks — a significant undercount given that most norovirus illness never reaches a diagnostic lab. You can view current NoroSTAT data at the CDC norovirus surveillance page.

Symptoms and Duration

Norovirus illness typically begins 12–48 hours after exposure and lasts 1–3 days. Symptoms include:

• Sudden onset nausea
• Vomiting (often forceful and projectile)
• Watery diarrhea
• Stomach cramps
• Low-grade fever, headache, and body aches in some cases

Most healthy adults recover fully without medical treatment. The primary risk is dehydration, especially in young children, older adults, and those with underlying conditions.

When to Seek Medical Care

Seek care if you or someone you are caring for shows signs of dehydration: dry mouth, decreased urination, dizziness, or in infants, fewer wet diapers and no tears when crying. Severe dehydration may require IV fluids. Older adults and young children dehydrate faster and should be monitored carefully.

Prevention

There is no vaccine for norovirus. Prevention relies on:

• Handwashing: Wash hands thoroughly with soap and water for at least 20 seconds — hand sanitizer is less effective against norovirus
• Food safety: Wash fruits and vegetables, cook shellfish thoroughly (norovirus is common in raw oysters), and avoid preparing food when sick
• Surface disinfection: Use bleach-based products (at least 1,000 ppm chlorine) — many common disinfectants do not kill norovirus
• Isolation: Stay home for at least 48 hours after symptoms resolve — you remain contagious during this period
• Laundry: Wash contaminated clothing and linens immediately using the hottest available cycle

What Outbreak Radar Tracks Now

While we await norovirus wastewater data, Outbreak Radar provides real-time surveillance for the four pathogens currently in the CDC NWSS: COVID-19, Influenza A, RSV, and Measles. These cover the major respiratory illnesses affecting US communities today.