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Norovirus: How contagious is it?


There is a good chance that anyone reading this article has likely been previously infected with the virus because it is known to be awfully contagious. A tiny amount, the size of a pinhead (18 to 2 800 viral particles) can induce symptoms. This incapacitating gastrointestinal virus which causes sudden onset of vomiting and diarrhea, is infamous for running rampant in closed and semi-closed environments, including cruise ships, long term care facilities, acute care facilities and schools, accounting for 60-95% of outbreaks of nonbacterial acute infectious diarrhea (PHAC). In fact, the first outbreak of norovirus, previously known as Norwalk virus, occurred in 1968 in a small town in Ohio called Norwalk where students and teachers attending the Bronson Norwalk Elementary School were affected by this virus. During a two-day period, 50% of the students and teachers developed a gastrointestinal illness. There from, the origin of the name, Norwalk virus.

It can cause very unpleasant symptoms

Although its course is brief, lasting 24 to 48 hours after the start of the first symptom, and is usually not deadly in the general healthy public, the unpleasant symptoms it causes can keep you intimate with a toilet for a couple days. In the very young, the elderly or the immunocompromised however, it can cause severe disease with associated dehydration and electrolyte imbalance that may require hospitalization.


Noroviruses are a genetically diverse group of single-stranded non-enveloped RNA viruses belonging to the Caliciviridae family. Although there are six recognized norovirus genogroups, three are known to affect humans (GI, GII, and GIV). Among them, genogroup GII is the most prevalent human genotype, responsible for 95% of all infections. More than 25 different genotypes have been identified within these three human genogroups and new genotypes replace circulating dominant type every 2-3 years due to recombination and antigenic drift (PHAC).

It's important to prevent and control its spread

These pathogens generally present wintertime seasonality and every year healthcare institutions (across Canada) are faced with the risk of fighting outbreaks of viral gastroenteritis caused by a norovirus. These outbreaks can be challenging for healthcare institutions like many institutions around the country have been experiencing for the past couple of weeks. Prolonged asymptomatic shedding up to two weeks after recovery and a short incubation period of 12 to 48 hours both play a part in the rapid spread of the virus.

In addition, norovirus is persistent in the environment (non-enveloped, heat stable and can resist disinfectants). The virus has multiple routes of transmission including person-to-person transmission, airborne transmission of aerosolized infectious vomit, contact with contaminated surfaces, and sometimes ingestion of food contaminated by an infected food handler. These characteristics all contribute to the highly contagious nature of the norovirus.

Because it is easily transmissible and hard to kill it’s important to prevent and control its spread. For acute and long term care facilities the focus is on surveillance especially during the winter months, education, rigorous attention to hand hygiene with soap and water or alcohol-based hand rub containing a minimum of 70% ethanol, contact precautions, accommodations and transfers, increased environmental cleaning focusing on high touch areas and bathrooms or commodes, as well as guidelines for visitors and staff. Since the virus is spread mainly by the oral-fecal route, measures to contain vomitus and stool are crucial. Dedicated bathroom or commode is essential. Handling and disposal of stool should employ methods that minimize environmental contamination such as single-use bedpans and kidney basins, liners or macerators.