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COVID-19: the window for infection

Have you ever wondered how long people are contagious with COVID-19 and are still able to spread the virus to others? Scientists at the Public Health Agency of Canada’s (PHAC) National Microbiology Laboratory (NML) tried to find the answer to this sticky question.

Dr. Jim Strong is a Senior Research Scientist at NML, working to understand the mechanisms that make COVID-19 so infectious. His team of researchers recently collaborated with Manitoba’s Cadham Provincial Laboratory to publish a paper on how long people infected with COVID-19 can spread the virus to others.

How viruses attack

"Viruses can not survive on their own, so they look to us for help" says Dr. Jim Strong, Senior Research Scientist, NML. "However, once viruses invade a person’s living cells, they can multiply exponentially and cause severe disease. If your immune system is not robust enough to fight off the virus when it first enters your body, then the bug is able to set up camp and begin wreaking havoc."

On their own, viruses are not necessarily harmful, but they can bring deadly disease with them. The virus that causes COVID-19 brings with it symptoms like coughing and sneezing that can spread the virus and infect other people. It seems, according to new research from NML, that the virus that causes COVID-19 only has a limited window of time to infect others. However, these are initial findings and more research is required to confirm results.

The deadline for infection

NML Scientists led by Dr. Jim Strong conducted the largest and most diverse COVID-19 infectivity study to date in the world, looking at samples from over 90 patients. In the study, researchers compared nasal and throat samples of patients at various times from symptom onset, and measured the ability of the samples to infect lab-grown cells. The researchers used the COVID-19-positive samples to grow the virus in a controlled environment.

After growing the virus, the researchers monitored how infectious the samples were. The researchers found that patient samples did not contain infectious material after eight days following the onset of symptoms. This means, that up until eight days, the patients can still spread the disease. After that point, patients are unlikely to spread the virus. However, these findings do not mean the virus that causes COVID-19 disappears after 8 days. The researchers found that the genetic material of the virus remained in the samples, but it was unable to grow and therefore should not infect others.

Before this research study, there was a lack of laboratory data to understand this very important factor in the spread of COVID-19.

"This study continues previous research conducted by other experts from around the world, but with much larger sample sizes," says Dr. Strong. "The next step is to replicate this study on an even larger scale. The reason we do this is to ensure the findings are consistent between different and larger groups, so that we can confidently adjust public health guidelines to best protect the health of Canadians."

Until more studies are done, the 14-day isolation period will remain in Canada and likely many other countries. While more research will need to be conducted, this study is a major stepping-stone on the road back to normalcy.

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