With the rise of the highly transmissible Omicron variant, getting reinfected with COVID-19 appears to be increasingly common.
According to Dr. Saqib Shahab, Saskatchewan’s chief medical officer of health, public data shows that as many as 10 per cent of Canadians who were recently infected with the Omicron BA.2 sub-variant were previously infected with COVID-19.
"That shows just because you got Omicron once doesn't mean you're bulletproof now," Shahab said earlier this month.
As a result, it’s important to keep in mind what constitutes a reinfection, said Dr. Dale Kalina, an infectious disease doctor at Joseph Brant Hospital in Burlington, Ont. According to Kalina, it’s possible for someone who has been infected with COVID-19 to continue to shed the virus for months after their initial infection.
“You can certainly continue to shed the virus and that's not necessarily a marker for a reinfection,” Kalina told CTVNews.ca on Tuesday in a phone interview. “That's one of the difficulties, or one of the pitfalls, of PCR testing.”
This means that someone could continue to test positive using a polymerase chain reaction (PCR) test, for example, up to about 90 days after their infection due to existing residual virus, even if the viral material isn’t active and the person is not necessarily contagious, Kalina said. This makes it difficult to tell whether the virus being detected is from the primary infection, or a subsequent one.
What is increasingly being used as a marker of reinfection instead is the progression of COVID-19 symptoms, said Dr. Lisa Barrett, an infectious disease specialist and professor at Dalhousie University in Halifax. If someone who was infected with COVID-19 has noticed a clear improvement in their symptoms to the point where they have been resolved, followed by the worsening of symptoms yet again, this is likely a COVID-19 reinfection, she said.
“The definition that most folks are using is resolution of symptoms,” Barrett told CTVNews.ca on Tuesday in a phone interview. “If available, test negative either by PCR or a [rapid] antigen test, with a subsequent clear link to exposure and new infection with symptoms.
“Usually, we say that it has to be greater than 60 days before it's considered a reinfection but honestly, that's a work in progress.”
According to Public Health Ontario, a reinfection is considered a subsequent infection of SARS-CoV-2 that has taken place at least 90 days after the initial infection. The National Institute of Public Health of Quebec has a similar definition, with SARS-CoV-2 reinfection expected to occur at least 90 days following the onset of symptoms from the primary infection.
According to the Public Health Agency of Canada (PHAC), because it’s possible for PCR test results to remain positive or fluctuate for weeks or even months after they were first conducted, positive results obtained within three months of a previous infection might not actually represent a true reinfection. But it’s not yet clear how soon after a COVID-19 diagnosis that reinfection can take place, PHAC says.
Results displayed using rapid antigen tests (RATs), on the other hand, are not expected to remain positive for as long as PCR tests, Barrett said. This is because they target specific proteins on the surface of the virus as opposed to viral genetic material. According to PHAC, the ideal frequency of repeat testing using an RAT has not yet been determined.
Despite some of the challenges that may come with detecting COVID-19 reinfection through PCR testing, what’s useful is its ability to detect the positivity level of a viral sample, Kalina said. When using PCR tests, a positive reading is generated through fluorescent signals, which are measured to determine a cycle threshold value. This corresponds to the number of cycles necessary for the signal to exceed the amount that would normally be found in a test sample.
As the cycle threshold value increases, this translates to a lower amount of target nucleic acid detected in the sample, and vice versa. Therefore, if the cycle threshold value is higher, chances are the viral genetic material is from an older infection; if it’s lower, this signals that the material is linked to a more recent infection, Kalina said.
Looking at this data, combined with symptoms, helps doctors to provide a diagnosis and guide care. If someone suspects they are reinfected with COVID-19 and have symptoms consistent with the virus, Kalina recommends speaking with a doctor.
“[Part] of the investigating that one does as a health-care provider is to look at the cycle threshold, mixed with the symptoms to put the whole story together,” Kalina said. “If you can, ask your doctor because … it’s our job to help you navigate that.”
With files from The Canadian Press.
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