Norovirus Outbreaks: Surveillance, History, and High-Risk Settings
Norovirus is the world's most common cause of viral gastroenteritis outbreaks. The CDC counts about 2,500 reported US outbreaks a year through its National Outbreak Reporting System (NORS) — and its genotype and outbreak-tracking systems, updated monthly, show that the 2024–2025 season was unlike any since 2012.
U.S. Outbreak Burden
According to the CDC's Norovirus Facts and Stats page (updated May 8, 2024), norovirus causes each year in the United States, on average:
- 19 to 21 million illnesses
- 2,270,000 outpatient clinic visits, mostly in young children
- 465,000 emergency department visits, mostly in young children
- 109,000 hospitalizations
- 900 deaths, mostly among adults aged 65 and older
- ~2,500 reported outbreaks, most common from November through April
- $2 billion a year in foodborne norovirus costs (lost productivity and healthcare expenses); total US societal cost across all transmission routes is higher — see the Economic Impact page
Where those numbers come from matters, because none of them is a count. Norovirus is not individually reportable in the United States — the CDC's reporting page states that health departments "are not required to report individual cases of norovirus illness to CDC," and most clinics do not test for it. The medical-visit, hospitalization, and death figures are modeled estimates from Burke et al., 2021 (Clinical Infectious Diseases, DOI 10.1093/cid/ciaa438), who analyzed 2001–2015 administrative data and reported the confidence intervals the CDC page omits: 900 deaths (95% CI 650–1,100), 109,000 hospitalizations (80,000–145,000), and 465,000 ED visits (348,000–610,000). The 19–21 million illness estimate traces to Hall et al., 2013 (Emerging Infectious Diseases, PubMed 23876403). The Norovirus Risk Perspective page shows these figures with their provenance and explains why a case-fatality rate is deliberately not computed from them.
The CDC also publishes the burden by the time a child turns five: 1 in 7 will have visited an outpatient clinic for norovirus, 1 in 40 an emergency department, 1 in 160 will have been hospitalized, and 1 in 110,000 will have died from it.
The Current Season: What CDC's Live Surveillance Shows
Two CDC systems publish norovirus outbreak data monthly, and together they tell a story the static burden figures above cannot.
NoroSTAT: outbreak counts
NoroSTAT collects suspected and confirmed outbreaks from 14 participating state health departments within seven business days. Its August 26, 2026 update reports:
- 1,392 outbreaks in the 2025–2026 seasonal year (August 1, 2025 – July 31, 2026) — "within the middle 50%" of the 2012–2025 seasonal years, i.e. a normal season.
- 2,650 outbreaks in the same period a year earlier (2024–2025) — nearly double, and well above the historical interquartile range.
CaliciNet: which strains caused them
CaliciNet genotypes outbreak specimens submitted by public health laboratories. Its September 3, 2026 data table explains the outbreak spike:
| Season (Sep 1 – Aug 31) | Typed outbreaks | Leading genotype | Second | Third |
|---|---|---|---|---|
| 2024–2025 | 411 | GII.17[P17] — 306 (74%) | GII.4 Sydney[P16] — 43 (10%) | GII.6[P7] — 16 |
| 2025–2026 | 153 | GII.4 Sydney[P16] — 73 (48%) | GII.17[P17] — 27 (18%) | GII.7[P7] — 14 |
Percentages are our arithmetic from CDC's counts (e.g. 306 ÷ 411 = 74%).
Read together: in 2024–2025 a genotype that had never dominated the United States, GII.17, caused three-quarters of typed outbreaks and the NoroSTAT states recorded nearly twice their usual outbreak count. In 2025–2026, GII.4 Sydney — the strain that had led every season since 2012 — returned to the top and outbreak numbers fell back inside the normal range. The CDC's Facts and Stats page (last updated May 2024) still reads "since 2012, the same virus (GII.4 Sydney) has been the dominant strain worldwide" and notes only that GII.17 had "temporarily replaced GII.4 viruses in several Asian countries"; its own CaliciNet data now shows the same displacement happened in the US for one season. The CDC's general observation that "in years when there is a new strain of the virus, there can be 50% more norovirus illness" is consistent with what NoroSTAT recorded.
Outbreak Seasonality
Norovirus circulates year-round but, per CDC, US outbreaks are most common from November through April. This seasonal pattern is consistent enough that norovirus has earned the colloquial name "winter vomiting bug" in the United Kingdom. Globally, the CDC notes outbreaks run November–April north of the equator and May–September south of it, with less seasonality near the equator.
The reasons for seasonal peaks are not fully understood but may relate to closer indoor contact during colder months, changes in host immune function, and the stability of the virus in cooler temperatures.
High-Risk Settings and Outbreak Dynamics
The CDC's Norovirus Outbreaks page (April 22, 2026) describes where reported US outbreaks concentrate. This table records only what the CDC publishes.
| Setting | What CDC Publishes | Key Risk Factors |
|---|---|---|
| Healthcare facilities (long-term care and hospitals) | "The most commonly reported setting"; over half of all reported US norovirus outbreaks occur in long-term care facilities | Vulnerable residents; communal dining; shared bathrooms; outbreaks "can sometimes last months"; illness "can be more severe — and occasionally even deadly" |
| Restaurants & catered events | Norovirus causes about 50% of all foodborne-illness outbreaks; most occur in food-service settings | Infected food workers touching ready-to-eat food bare-handed; food contaminated at the source (oysters, field-sprayed produce) |
| Schools, childcare, colleges | Outbreaks "frequently occur"; some "have even led to campus closures"; no percentage published | Close quarters, shared spaces, high-touch surfaces |
| Cruise ships | Over 90% of cruise diarrheal-disease outbreaks are norovirus, but cruise ships are only about 1% of all reported norovirus outbreaks | Close living quarters; shared dining; rapid passenger turnover; repeated outbreaks on consecutive cruises from infected crew or persistent contamination |
The routes behind these settings are set out on How Norovirus Spreads; the handwashing, bleach and isolation measures that interrupt them on Prevention; and caring for someone who is ill on Treatment.
Corrections on this page
September 2026: earlier versions attributed "~65%" (long-term care), "~15–20%" (restaurants), and "~10%" (schools) to CDC. The CDC's published wording is "over half" for long-term care facilities and it publishes no share for the other settings on its outbreak pages. The figures were replaced with the CDC's actual language. August 2026: an earlier version used an incorrect, capitalized name for the CDC's cruise-ship inspection programme; it is the Vessel Sanitation Program (VSP), and the seasonality window was corrected to November–April at the same time. Both entries are in the corrections log.
Norovirus Cruise Ship Outbreaks: CDC Vessel Sanitation Program
The CDC's Vessel Sanitation Program (VSP) inspects cruise ships and investigates onboard gastroenteritis outbreaks. It posts an outbreak when a ship is under VSP jurisdiction (a voyage touching both US and foreign ports) and 3% or more of passengers or crew report gastrointestinal symptoms to the ship's medical staff. Counting the outbreaks on CDC's published lists (as of July 23, 2026):
| Year | Outbreaks posted | Norovirus-confirmed | Other / unknown agent |
|---|---|---|---|
| 2023 | 14 | 13 | 1 (Salmonella and E. coli) |
| 2024 | 18 | 15 | 3 (Salmonella; E. coli; unknown) |
| 2025 | 23 | 18 | 5 (E. coli; presumed ciguatera; 3 unknown) |
| 2026 (through July 23) | 9 | 5 | 4 (2 E. coli; 2 unknown) |
Our tally of CDC's per-voyage listings; CDC does not publish annual totals on the page. Attack rates in the 2024 listings ranged from about 3% to 13.5% of passengers (Queen Mary 2, December 21, 2024 – January 3, 2025: 346 of 2,565).
These counts are why the CDC's "cruise-ship virus" nickname is misleading: 15–18 confirmed norovirus outbreaks a year at sea, against roughly 2,500 reported outbreaks a year on land. See the dedicated Norovirus on Cruise Ships page for cruise-specific risks and precautions.
Norovirus Strains and Variant Emergence
Norovirus belongs to the genus Norovirus within the family Caliciviridae. It is classified into multiple genogroups, with GI and GII causing most human illness. CaliciNet now reports strains by "dual typing" — the capsid genotype and the polymerase type together, written as, for example, GII.4 Sydney[P16] — because the two genes can reassort independently.
Per the CDC, GII.4 viruses have caused the majority of outbreaks worldwide since 2002. Between 2002 and 2012 new GII.4 variants emerged about every 2 to 4 years (Sydney 2012, New Orleans 2009, Minerva 2006 among them), each typically driving a global rise in outbreaks until population immunity accumulated. Since 2012 GII.4 Sydney has been the dominant strain worldwide — with the one-season US exception, GII.17 in 2024–2025, documented in the CaliciNet table above. GII.17 had earlier displaced GII.4 temporarily in several Asian countries (CDC), so its 2024–2025 US season followed a known pattern rather than an unprecedented one.
Notable U.S. and Global Outbreak Events
1968
Norwalk, Ohio — The Original Outbreak
An acute gastroenteritis outbreak at an elementary school in Norwalk, Ohio led to the collection of stool samples that, in 1972, allowed Albert Kapikian and colleagues at the NIH to identify the "Norwalk agent" — now known as norovirus — using immune electron microscopy. This was the first viral gastroenteritis pathogen identified.
2002
Global Pandemic Wave — GII.4 Emergence
A new GII.4 variant caused a global wave of outbreaks, including widespread cruise ship outbreaks that brought intense public attention to norovirus for the first time. Per CDC, GII.4 viruses have caused the majority of outbreaks worldwide since this year.
2012
GII.4 Sydney Variant Pandemic
The GII.4 Sydney 2012 variant caused a major global outbreak wave and, per CDC, remained the dominant strain worldwide for more than a decade afterward.
2024–2025
The GII.17 Season in the United States
GII.17[P17] caused 306 of 411 outbreaks typed by CaliciNet (74%), and NoroSTAT-participating states reported 2,650 outbreaks — nearly double the 1,392 of the following season and above the 2012–2025 interquartile range. GII.4 Sydney returned to the top in 2025–2026.
Global Burden
Per the CDC's Facts and Stats page, norovirus causes worldwide, on average, 685 million cases of acute gastroenteritis a year, including 200 million in children under five, and about 50,000 child deaths, "mostly in developing countries." The 685 million is a modeled estimate, not a count; the same figure is used by Bartsch et al., 2016 (PLOS ONE, DOI 10.1371/journal.pone.0151219), the source this network's risk data records for it, with a 95% uncertainty interval of 491 million to 1.1 billion cases — a reminder that the global number is far less certain than the US one. Worldwide, norovirus accounts for about one in five cases of acute gastroenteritis and, per CDC, an estimated $60 billion a year in healthcare costs and lost productivity.
Compared to What?
About 2,500 reported outbreaks and 19–21 million illnesses a year make norovirus the highest-volume virus in the network's dataset — and roughly 900 deaths make it among the least lethal per case. The hub's Virus Risk Perspective tool sets these figures beside 14 other viruses on the same five measures.
This content is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider for medical decisions.
Sources & References
- Centers for Disease Control and Prevention. Norovirus — Outbreaks. cdc.gov/norovirus/outbreak-basics (page dated April 22, 2026).
- Centers for Disease Control and Prevention. Norovirus Facts and Stats. cdc.gov/norovirus/data-research (updated May 8, 2024).
- Centers for Disease Control and Prevention. NoroSTAT Data. cdc.gov/norovirus/php/reporting/norostat-data (updated August 26, 2026).
- Centers for Disease Control and Prevention. CaliciNet Data. cdc.gov/norovirus/php/reporting/calicinet-data (updated September 3, 2026).
- Centers for Disease Control and Prevention. Norovirus Reporting and Surveillance. cdc.gov/norovirus/php/reporting (January 15, 2026).
- Centers for Disease Control and Prevention, Vessel Sanitation Program. Outbreaks on Cruise Ships in VSP's Jurisdiction and Earlier Outbreaks. cdc.gov/vessel-sanitation/cruise-ship-outbreaks (July 23, 2026) and earlier-outbreaks (February 23, 2026).
- Burke RM, Mattison CP, Pindyck T, et al. Burden of norovirus in the United States, as estimated based on administrative data: updates for medically attended illness and mortality, 2001–2015. Clin Infect Dis. 2021;73(1):e1–e8. DOI: 10.1093/cid/ciaa438. PubMed 32291450.
- Hall AJ, Lopman BA, Payne DC, et al. Norovirus disease in the United States. Emerg Infect Dis. 2013;19(8):1198–1205. PubMed 23876403.
- Bartsch SM, Lopman BA, Ozawa S, Hall AJ, Lee BY. Global economic burden of norovirus gastroenteritis. PLOS ONE. 2016;11(4):e0151219. DOI: 10.1371/journal.pone.0151219. PubMed 27115736.
