According to Dr.  Saqib Shahab, Saskatchewan’s chief medical officer, public figures show that up to 10 percent of Canadians recently infected with the Omicron BA.2 subtype had previously been infected with COVID-19.
“This shows that just because you took Omicron once does not mean you are bulletproof now,” Shahab said earlier this month.
As a result, it is important to keep in mind what constitutes re-infection, said Dr. Dale Kalina, an infectious disease physician at Joseph Bradt Hospital in Burlington, OD.  According to Kalina, it is possible for someone infected with COVID-19 to continue to shed the virus for months after their initial infection.
“You can definitely continue to eliminate the virus and this is not necessarily an indicator of re-infection,” Kalina told CTVNews.ca in a telephone interview on Tuesday.  “This is one of the difficulties or one of the pitfalls of PCR testing.”
This means that one could continue to be positive using a polymerase chain reaction (PCR) test, for example, up to about 90 days after infection due to an existing residual virus, even if the viral material is not active and the individual is not it is necessarily contagious, Kalina said.  This makes it difficult to distinguish whether the virus being detected is from the primary infection or from a later one.
What is increasingly being used as an indicator of re-infection is the development of COVID-19 symptoms, said Dr. Lisa Barrett, an infection specialist and professor at Dalhousie University in Halifax.  “If someone who has been infected with COVID-19 has noticed a clear improvement in their symptoms to a point where they have resolved, followed by worsening of the symptoms again, this is likely a COVID-19 re-infection,” he said.
“The definition most people use is symptom resolution,” Barrett told CTVNews.ca in a telephone interview on Tuesday.  “If there is, the test is negative either by PCR or by a [rapid] antigen test, with clear association with exposure and re-infection with symptoms.
“Usually, we say it has to be longer than 60 days before it can be considered a re-infection, but honestly, this is an ongoing project.”
According to Ontario Public Health, a re-infection is considered a subsequent infection of SARS-CoV-2 that has occurred at least 90 days after the initial infection.  The Quebec National Institute of Public Health has a similar definition, with SARS-CoV-2 re-infection expected to occur at least 90 days after the onset of symptoms from the primary infection.
According to the Public Health Service of Canada (PHAC), because PCR test results are likely to remain positive or fluctuate for weeks or even months after their first test, positive results obtained within three months of a previous infection may in fact do not represent true re-infection.  But it is not yet clear how soon after COVID-19 diagnosis this re-infection can occur, says PHAC.
Results shown using rapid antigen (RAT) tests, 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 repeated tests using RAT has not yet been determined.
Despite some of the challenges that can arise in detecting COVID-19 transfection through PCR, what is useful is its ability to detect the level of positivity of a virus sample, Kalina said.  When PCR tests are used, a positive indicator is generated by fluorescence signals, which are measured to determine a cycle threshold value.  This corresponds to the number of cycles required for the signal to exceed the quantity normally found in a test sample.
As the threshold value of the cycle increases, this translates into a lower amount of target nucleic acid detected in the sample and vice versa.  Therefore, if the threshold value of the cycle is higher, the chances are that the viral genetic material is from an older infection.  if it is lower, it indicates that the material is associated with a more recent infection, Kalina said.
Examining this data, combined with the symptoms, helps doctors provide a diagnosis and guide care.  If anyone suspects that they have been re-infected with COVID-19 and have symptoms that match the virus, Kalina recommends talking to a doctor.
“[Part] “One of the researches that one does as a healthcare provider is to look at the threshold of the cycle, in combination with the symptoms, in order to combine the whole story”, said Kalina.  “If you can, ask your doctor why… it’s our job to help you navigate it.”
With files from the Canadian Press.
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