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Norovirus – a highly contagious virus known for causing gastrointestinal distress – is making headlines this winter due to a sharp rise in cases and widespread outbreaks , the , and .

Nelson Lee is an infectious disease physician who is director of the ���߲ݴ�ý’s , a U of T , and a professor in the epidemiology division at the Dalla Lana School of Public Health.

He says the re-emergence of a particularly contagious strain combined with low population-level immunity are driving the current spike in cases.

U of T News spoke with Lee about how norovirus spreads, how to protect yourself and your loved ones and why alcohol-based sanitizers are ineffective.  

What is norovirus?

Norovirus is a type of virus that typically infects the gastrointestinal tract and can infect individuals of all ages.

It's very contagious and it has a “shell” (capsid) surrounding the virus structure. It is the most common cause of gastrointestinal infections in adults worldwide – including U.S. and Canada.

Most infected individuals will have diarrhea, which typically lasts one to three days. In some high-risk individuals, the symptoms can be prolonged for five days or even a week. The most vulnerable populations include children under five years old, the elderly and those with compromised immune systems. Vomiting is a prominent symptom experienced by at least two-thirds of infected individuals and about one-third may develop low-grade fever.

One characteristic that I need to point out is that this virus is very contagious and tends to cause outbreaks, especially in congregate or confined settings. For example, schools, nursing homes, long-term care facilities and even on cruise ships.

Why is this norovirus strain so contagious? Why are there more cases than usual this year?

I’ll start with the microbiology side. This predominant strain – we call it GII.17 – is re-emerging after it caused a worldwide outbreak about 10 years ago. After disappearing for some time, the population-level immunity to it has substantially decreased – thus, we are becoming more susceptible to the infection.

Another reason for its high transmissibility is the extremely low infective dose, meaning only a small amount of the virus is needed to infect an individual.

The other thing is that the characteristic symptom of vomiting can help transmit the disease because a patient’s vomit can more easily spread in an open environment than diarrhea. Vomiting can contaminate a lot of surfaces and increases the risk of infection.

What can you do to protect yourself and others? 

Start with proper food handling and consumption – wash your vegetables and fruits. There are some high-risk items such as . Make sure food is cooked properly.

Proper hand washing with soap and water is important too. Alcohol sanitizers are not effective. If there’s a contaminated environment such as the bathroom, clean and disinfect the area with bleach.

Why don’t sanitizers work against the norovirus?

The structure that encloses the virus – the capsid – protects the virus and makes it more resistant to alcohol-based sanitizers. With other viruses like the flu, there’s an envelope around the virus that can be dissolved with sanitizers that contain alcohol, but that’s not the case with the norovirus.

Who is most at risk?

Some individuals – especially young children, the elderly and those with underlying conditions – are at a higher risk of severe illness. In some cases, they may require hospitalization because of profuse diarrhea, dehydration and electrolyte imbalance.

There are no specific anti-viral treatments at this point, so if an individual catches it, make sure to keep hydrated and replace electrolytes.