Norovirus FAQ: Your Questions Answered

Norovirus is most contagious when you are actively sick and during the first few days after you recover. Per CDC guidance, the virus can be found in your stool for 2 weeks or more after you feel better, meaning you can still spread the virus to others even when you feel completely well.

The CDC recommends not preparing food for others for at least 48 hours after symptoms resolve. Immunocompromised individuals may shed the virus for weeks to months.

Yes. Unlike many viral infections, norovirus does not confer long-lasting immunity. After infection, most people develop some protection against the specific strain that infected them — typically lasting 6 months to 2 years. However, you can be reinfected by a different norovirus strain or by the same strain once your immunity wanes.

The evidence behind these figures — the challenge studies, the model that says immunity may last years, and the FUT2 gene — is set out on our immunity and reinfection page.

This is why some people seem to get norovirus "every year" — they are likely being infected by different strains or reinfected after immunity fades. There is also some evidence that certain people with specific blood group antigens (particularly those who are blood type O) are more susceptible to GII.4 norovirus strains.

Alcohol-based hand sanitizers are less effective against norovirus than soap and water. The CDC specifically recommends washing hands with soap and water for at least 20 seconds as the preferred method of hand hygiene when norovirus exposure is possible.

This is because norovirus is a non-enveloped virus — it lacks the lipid membrane that makes many other viruses (like influenza and coronaviruses) susceptible to alcohol disinfection. The physical scrubbing action of handwashing with soap removes viral particles from the skin more effectively.

Hand sanitizers may be used when soap and water are genuinely unavailable, but should not be relied upon as primary protection during norovirus outbreaks.

Norovirus is the most common cause of what people colloquially call "stomach flu," "stomach bug," or "a 24-hour bug." However, the term "flu" technically refers to influenza, a respiratory illness caused by influenza A or B viruses. The two conditions are not related.

Norovirus causes gastrointestinal symptoms (nausea, vomiting, diarrhea). Influenza primarily causes respiratory symptoms (cough, congestion, sore throat) along with fever and body aches. You can have both infections simultaneously, but they are distinct diseases caused by different viruses. For the comparison people usually mean — norovirus against rotavirus and against bacterial food poisoning, by onset, duration, who, season and what changes management — see our three-way table.

Norovirus contaminates food through two main pathways. The most common is an infected food handler who touches ready-to-eat food without properly washing hands. Because the infectious dose is so small (as few as 18 viral particles), even a tiny amount of contamination can cause illness in many people simultaneously.

The second major foodborne route is shellfish, particularly raw oysters, clams, and mussels. These filter-feeding animals concentrate norovirus from contaminated water. Shellfish from sewage-contaminated growing areas carry significant norovirus risk, and even cooking does not guarantee complete inactivation — thorough cooking to 145°F (63°C) internal temperature is required, and some risk remains with raw shellfish.

Norovirus is remarkably stable in the environment. The CDC's statement is that it "can persist on surfaces for days or weeks and is resistant to many common disinfectants." CDC does not publish separate survival times for hard versus soft surfaces, so neither do we — treat any surface touched by vomit or stool as contaminated until it has been disinfected.

The virus also survives freezing temperatures, meaning frozen food contaminated with norovirus remains infectious after thawing. Standard household disinfectants containing chlorine bleach are required to inactivate norovirus on surfaces — many common disinfectants (including those based on quaternary ammonium compounds alone) are not reliably effective.

For most otherwise healthy pregnant women, norovirus illness follows the same course as in non-pregnant adults and resolves within 1–3 days. The primary concern is dehydration, which can affect both the mother and fetus.

Pregnant women who cannot keep any fluids down, experience reduced fetal movement, or have symptoms lasting more than 2–3 days should contact their healthcare provider. Severe dehydration may require IV rehydration. There is no evidence that norovirus itself crosses the placenta or directly harms the developing fetus.

Most otherwise healthy adults and older children with norovirus illness do not need emergency care. Home management with oral rehydration is appropriate for mild-to-moderate illness.

Seek emergency care if you or someone you are caring for experiences: inability to keep any fluids down for more than 24 hours, signs of severe dehydration (no urination for 8+ hours, extreme dry mouth, confusion, rapid heart rate), blood in stool or vomit, fever above 102°F (38.9°C) persisting more than 24 hours, or symptoms lasting more than 3 days without improvement. Infants, elderly adults, and immunocompromised individuals have a lower threshold for seeking care.

No. As of September 2026 there is no approved norovirus vaccine. The most advanced candidate on the public record is Moderna's mRNA-1403, an mRNA vaccine in a Phase 3 efficacy trial of 37,864 adults aged 18 and over (ClinicalTrials.gov NCT06592794; status "active, not recruiting," primary completion estimated February 2027, record last updated May 2026). Its Phase 1/2 dose-ranging study enrolled 1,407 adults (NCT05992935). No results from the Phase 3 have been published as of this review, and this site will not characterize its efficacy until they are.

Correction (September 2026): an earlier version of this answer named a different candidate as the most advanced; it was out of date and has been replaced — see the corrections log.

Development is complicated by the number of circulating genotypes — the CDC notes that "infection with one type of norovirus may not protect you against other types" and that "it is not known exactly how long protection lasts" — and by genotype turnover such as the 2024–2025 GII.17 season described on the Outbreaks page. A vaccine would likely be most valuable for older adults in long-term care, where over half of US outbreaks occur (CDC), and for young children, who account for most emergency-department visits.

Human noroviruses (GI and GII) do not infect dogs, cats, or other common household pets. Animals have their own calicivirus strains that can cause illness in them, but these do not cause norovirus illness in humans.

However, pets can theoretically act as mechanical vectors — meaning a pet could carry norovirus on its fur or paws after contact with contaminated surfaces, and then transfer it to human hands. This is considered a low-probability transmission route, but it's a reason to maintain good handwashing habits around animals as well.

The CDC's advice for everyone: "wait at least 2 days (48 hours) after symptoms stop" before preparing food or caring for others — "this is important if you work in a restaurant, school, daycare, long-term care facility, or other places where you may expose people to norovirus."

For food workers the answer has legal weight. Under the FDA Food Code 2022 (§§ 2-201.12–2-201.13), an employee with vomiting or diarrhea must be excluded from the establishment. If they were never tested, they may be reinstated after 24 symptom-free hours. If they were diagnosed with norovirus, reinstatement requires approval from the local regulatory authority and either more than 48 symptom-free hours or a health practitioner's written statement that they are free of infection; and in facilities serving highly susceptible populations (nursing homes, hospitals, daycares) the exclusion is retained until then. Full detail on the Food Safety page. Remember that shedding continues for weeks after recovery, so handwashing discipline matters most in the first days back.

Not from ordinary breathing the way you catch a cold — but yes, from vomiting. The CDC's transmission page lists "tiny drops of vomit from a person with norovirus spray through the air, landing on surfaces or entering another person's mouth" as a route. The classic evidence is a hotel-restaurant outbreak (Marks et al., 2000, DOI 10.1017/S0950268899003805) in which one diner vomited and the share of people who fell ill at each table declined with distance from that person; no food could be blamed. The virus is still ultimately swallowed — the authors' phrase is "inhalation with subsequent ingestion" — which is why it is called fecal–oral rather than airborne. Vomit is potent: a 2014 challenge study measured a median of about 41,000 genome copies per millilitre in vomit against a 50% infectious dose of about 1,300. See Transmission.

The CDC publishes this monthly through CaliciNet. For the 2025–2026 season (September 2025 – August 2026, 153 typed outbreaks as of September 3, 2026) the leading strain was GII.4 Sydney[P16] at 48%, followed by GII.17[P17] at 18%. That is a return to the long-running pattern: in 2024–2025, GII.17 caused 74% of typed outbreaks — the first season since 2012 in which GII.4 Sydney was not on top — and the NoroSTAT states reported nearly twice as many outbreaks as the following year (2,650 vs 1,392). Details and the full table are on the Outbreaks page.

Partly luck, partly prior immunity — and partly genetics. The CDC notes that "your likelihood of getting a norovirus infection is also determined in part by your genes." In a 2003 human-challenge study (Lindesmith et al., Nature Medicine, DOI 10.1038/nm860), about 29% of volunteers were "non-secretors" — people with two inactive copies of the FUT2 gene who do not make the gut-surface sugar that Norwalk virus attaches to — and none of them became infected at any dose. That protection was shown for one genotype (GI.1), and the CDC is clear that "infection with one type of norovirus may not protect you against other types," so non-secretors should not assume they are immune to every strain. Separately, about a third of infected adults in challenge studies shed virus without ever developing symptoms, so a household member who "never got sick" may nonetheless have been infected and contagious.