COVID-19: When can we talk about an outbreak?

When a public health unit in southwestern Ontario declared a community outbreak of COVID-19, last weekend, after five students in the area tested positive, some people laughed at the number, considered small, on social media.

Can five cases really constitute an outbreak, they asked? And if not, how many cases are needed to be able to use the term?

Infectious disease experts say the number of cases isn’t always the most important factor in deciding whether to talk about an outbreak.

“What (an outbreak) means is that there is some kind of uncontrolled and unwanted transmission of the infection,” said Dr. Andrew Morris, professor of medicine at the University of Toronto. “And what we want people to understand is that whether it’s in a community or in an institution like a school or a hospital where there is a high concentration of people, we are concerned about outbreaks because the consequences, if we do not act, can be very significant.

“Five cases can very quickly increase to 25 cases, (which) can very quickly increase to 625 cases.”

Declaring an outbreak, notes Dr Morris, means something needs to be done to stop transmission quickly.

Although most outbreaks consist of two or more positive cases, the prevalence of the virus in a community has an impact on how cases are classified.

So even a single positive test can qualify as an outbreak in an environment where you wouldn’t expect a case of COVID-19, such as in a small island community.

This is also what happened last month at a hospital in Langley, British Columbia, where an outbreak was declared following a positive case. Since hospitals are strict with their transmission prevention protocols, Dr Morris says it’s actually unexpected that a patient will arrive COVID-19 free and contract the virus within hospital walls.


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