What constitutes the outbreak of COVID-19?

Some social media insulted seemingly low numbers when the public health sector in southwestern Ontario declared the outbreak of the COVID-19 community after five students in the area tested positive over the weekend. Did.

Did they wonder if the five cases could really make up an outbreak? If not, how many cases do you need for that classification?

According to infectious disease experts, the classification of outbreaks is unclear, including the one declared by the London Middlesex Health Unit on Sunday after a positive test from a student at Western University.

Also, the magnitude of the outbreak is not always the most important factor in considering whether to call it it.

“(Outbreak) means that there is some uncontrolled, undesired transmission of an infection,” said Dr. Andrew Morris, a professor of medicine at the University of Toronto. “And what we want people to understand is that if they fail to act, whether it’s a community or a concentrated facility like a school or hospital, the consequences are huge. There is a concern about the outbreak because it is possible.

“5 cases can move very quickly to 25 cases, and (of which) 625 cases very quickly.”

Declaring the outbreak, Morris said, indicated that something needed to be done to stop the infection quickly.

Most outbreaks consist of two or more positive cases, but the spread of viruses that already exist in the community affects how cases are classified.

Therefore, even one positive test can be categorized as an outbreak in situations where COVID cases are not expected, such as in a small isolated community.

That’s also why an outbreak occurred following a positive case at a hospital in Langley, British Columbia last month. Morris says it’s actually unexpected for patients to be hospitalized without a COVID and get infected with the virus inside the hospital’s walls because hospitals are strict about infection prevention protocols.

“When public health officials make that call (to classify outbreaks), they see what they are seeing, what they expect to see, and ongoing. See what the potential for exposure is: “So a very small town … it would be very different from Toronto.

“Not every new outbreak in Toronto occurs, but (it) when we find epidemiologically concentrated infections in institutions and communities,” he said.

Dr. Saverio Stranges, an infectious disease expert in Western, agrees that a relatively small number of simultaneous occurrences at a location can be classified as an outbreak as long as those cases are related to each other.

If the infection is likely to come from one source (person or event), it is an outbreak.

“If there is an epidemiologically unrelated case in the (community), we usually define it as an outbreak, not an outbreak,” Stranges said.

Although the terms are sometimes used interchangeably, Morris states that it is important to distinguish between clusters and outbreaks.

But it’s not always easy to tell which is which, he added.

Phylogenetics, which is widely used in New Zealand but rarely used in Canada at present, can help distinguish between viral gene strands by examining clusters of positive tests, he says. ..

Morris explained that similar SARS-CoV-2 strands can be considered to be from the same source. However, if different mutations appear, it probably means that the case is irrelevant.

“If there is a good reason to believe that the case is related and there is potential for growth, it is called an outbreak,” Morris said.

Outbreaks at Western University can be more troublesome than the five cases already reported. The health unit said the currently isolated students live off campus and have never attended classes or events on campus. But they went to restaurants and bars in the heart of downtown London before they got a positive reaction.

Curiously, he said it was difficult to predict how many more cases would occur.

He added that this is where contact tracking becomes increasingly important when testing people who may have been exposed to five positive tests.

“We know that SARS-CoV-2 is highly contagious, but it also depends on the pattern of movement and the type of precautions these students may have taken,” he said. .. “We don’t know the strength of their social interaction. We don’t know if they were masked or if they were at a distance.

“Therefore, it is very difficult to predict how many other people may be infected.”

This report by the Canadian Press was first published on September 15, 2020.