If you’ve ever been hit with a bout of vomiting and diarrhea that resulted in you hugging the porcelain throne for a few days, you might’ve referred to it as “stomach flu” or “food poisoning”. But in all likelihood, it was norovirus, the leading cause of vomiting and diarrhea from acute gastroenteritis, which is inflammation of the stomach and intestines. The CDC (Centers for Disease Control and Prevention) shares that in the U.S., norovirus causes 19–21 million cases of vomiting and diarrhea illnesses annually.
Norovirus is a very contagious virus that causes vomiting and diarrhea. Anyone can get infected and sick with norovirus. The American Medical Association shares that norovirus kills about 900 people—mostly adults 65 or older—and causes 109,000 hospitalizations each year. Norovirus is also responsible for 465,000 emergency department visits and nearly 2.3 million outpatient clinic visits annually, mostly in young children, says the CDC.
How infectious is norovirus?
You’ve likely become familiar with the R0 value of viruses over the past few years due to the discussion of Covid-19 and its transmissibility. The R0 value (R-naught value, meaning how many people could be infected from one other individual, on average) for norovirus is estimated to be between 1.1–7.2, but is often pinpointed to be closest to 2.75.
Why the wide range? It largely depends on the time of year- studies have uncovered that outbreaks are most likely to occur during the wintertime, with those cases having higher transmissibility. People being in close quarters during the rainy, cold, snowy months contributes to the spread of illness. But despite the higher likelihood of infection during the winter months, people can become infected at any time of the year.
Another factor that impacts transmissibility is the setting. While studies conflict on how impactful the setting is to the spread of norovirus, some settings by nature of their operation are more likely to experience an outbreak. In one study conducted by the CDC, 75% of cases that were followed occurred in long-term care and assisted living facilities. Surprisingly, that same study found that outbreaks in schools, colleges, and universities had lower estimated transmission, but transmission varied little across all other settings (including private homes). Researchers noted that factors like the mixing patterns of people within settings, as well as control methods once infections were detected were not accounted for in determining the r0 value, ultimately providing less clarity to the true r0 of the virus. But it is clear that it has the potential to spread quickly, and to impact many people.
Norovirus is often thought to be the “cruise ship virus,” and is often pinpointed as the culprit (responsible for 90% of diarrheal cases) when large amounts of people aboard a ship experience illness. Despite its infamy, norovirus outbreaks on cruise ships account for only a small percentage (1%) of all reported norovirus outbreaks.
There are limitations to studies that have evaluated the R0 value of norovirus, with many not accounting for different genotypes (variants) or tracking of asymptomatic people and their impact to the spread of the virus. To give some perspective, here are some commonly known viruses and their R0 values:
| Virus | R0 value |
| Common Cold | <0 |
| Influenza | 1.3-1.5 |
| Ebola | 2 |
| “Original” Wuhan Covid-19 strain | 3.3 |
| SARS | 4 |
| Covid-19 Delta strain | 5.1 |
| Smallpox | 7 |
| Covid-19 Omicron BA1 strain | 9.5 |
| Mumps | 12 |
| Measles | 18 |
| Covid-19 Omicron BA4 and BA5 strains | 18.6 |
Infection
Some ways you can get norovirus are:
- Having direct contact with an infected person
- Touching contaminated surfaces and then putting your unwashed hands in your mouth
- Consuming contaminated food or water
Joanna Turner Bisgrove, MD, a family physician and assistant professor of family medicine at Rush University Medical Center, and member of the AMA Council on Science and Public Health shares that “Twenty million people a year get norovirus in the food industry—it’s the most common cause of foodborne illness,”. And “the biggest reason is that food workers come to work sick or they get sick at work.
“There’s a vomiting episode or a diarrhea episode and it’s very hard to clean up,” she added, noting that “70% of norovirus outbreaks for food is caused by workers, and the CDC talks about how observation of food-service workers has shown that they only practice proper hand washing about one in four times.”
Diagnosis
The most common symptoms are:
- Diarrhea
- Vomiting
- Nausea
- Stomach pain
Noroviruses are not related to the influenza virus, despite it commonly being referred to as the “stomach flu”. “The most common method of diagnosis is based on symptoms,” says Dr. Robyn F. Chatman, MD, MPH, a family physician in Cincinnati, and delegate in the AMA (American Medical Association) House of Delegates for the Ohio State Medical Association. “It’s the physician evaluating the patient and listening to their symptoms and making a clinical diagnosis based on their symptoms, ” but “there is a [also] a viral stool culture that can be ordered so that you know exactly what you’re treating.”
Prevention
As with many other illnesses, prevention is key. Hand hygiene remains one of the best methods of reducing the chances of contracting norovirus. Other ways to help prevent norovirus spread are:
- Rinse fruits and vegetables
- Cook shellfish thoroughly
- Clean high-traffic surfaces often (countertops, remote controls, door knobs, etc).
- Stay home when sick and for two days after symptoms stop
- Avoid preparing food for others when sick and for two days after symptoms stop
The CDC shares the video below, which highlights best practices when cleaning up surfaces that may have been contaminated by norovirus (or how to clean to prevent contamination).
We can all likely remember a time (or two), where we experienced the unpleasant symptoms of norovirus and would prefer to not go through that again. The best bet to stay well? Maintain the same basics that help keep us healthy against other illnesses- get adequate sleep and eat well to keep your body strong, wash your hands regularly, avoid being around sick people, and stay home if you are sick to prevent the spread.
-https://www.ama-assn.org/delivering-care/public-health/what-doctors-wish-patients-knew-about-contagious-norovirus
-https://www.cdc.gov/norovirus/index.html
-https://pubmed.ncbi.nlm.nih.gov/22317811/
-https://pubmed.ncbi.nlm.nih.gov/24098406/
-https://ajph.aphapublications.org/doi/abs/10.2105/AJPH.2013.301704
-https://wwwnc.cdc.gov/eid/article/26/8/19-1537_article#:~:text=Several%20transmission%20modeling%20studies%20in%20different%20settings%20have,occurs%20and%20R%200%20ranges%20from%201.1%E2%80%937.2%20%2810%29.
-https://pubmed.ncbi.nlm.nih.gov/24098406/
-https://www.cdc.gov/norovirus/lab/index.html
-https://pubmed.ncbi.nlm.nih.gov/24898166/
-https://pubmed.ncbi.nlm.nih.gov/16642006/