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.
“When public health officials make that call (to declare an outbreak), they’ll consider: what they should expect to see; what they plan to see; and what is the potential for continuous exposure, ”explained Dr Morris. “So in a very small town, (…) it will be very different from Toronto.”
“Outbreaks are not declared every time a person contracts a new infection in Toronto, but (it does) when we see them epidemiologically targeted in a facility or community.
Dr Saverio Stranges, infectious disease expert at Western University, clarifies that relatively small numbers of cases occurring at the same time in one place can be classified as outbreaks, provided those cases can be linked to each other. .
If the infections are likely to come from only one source – a person or an event – then it is an outbreak.
“If you have cases within a (community) that are not epidemiologically linked, we usually define this as a cluster, not an outbreak,” explained Dr Stranges.
While the terms are sometimes used interchangeably, Dr. Morris says it’s important to distinguish between a cluster and an outbreak.
But it’s not always easy to determine which is which, he added.
“When we have strong reasons to believe that the cases are related and that there is potential for growth, then it is called an outbreak.”
– Dr. Andrew Morris, Professor of Medicine at the University of Toronto
He says phylogenetics, a practice widely used in New Zealand but very little so far in Canada, can help make this distinction by examining the genetic strands of the virus among groups of positive tests.
One might think that strands of SARS-CoV-2 that are similar come from the same source, Dr Morris explained. However, if different mutations appear, it probably means that the cases are not related.
“When we have strong reasons to believe that the cases are related and that there is potential for growth, then it is called an outbreak,” noted Dr. Morris.
The Western University outbreak could grow larger than the five previously reported cases. The health unit said the students, who are now in isolation, live off campus and have not attended classes or events on the university grounds. But they had been to restaurants and bars in downtown London before they tested positive.
Dr Stranges said it was difficult to predict how many more cases could be detected.
This is where contact tracing becomes increasingly important, he added, to test people who may have been exposed to the five positive people.
“We know SARS-CoV-2 is very contagious. (…) But it also depends on the mobility models and the type of precautions that these students were able to take, ”he declared. “We don’t know the intensity of their social interactions. We don’t know if they wore masks, if they kept a distance, etc. ”
“So it’s very difficult to predict how many other people might be infected.”
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