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What we know about COVID-19 reinfection

It's been six months since COVID-19 was declared a pandemic by the World Health Organization. Since then, millions of people across the world have caught SARS-CoV-2, the virus causing the disease.
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It's been six months since COVID-19 was declared a pandemic by the World Health Organization. Since then, millions of people across the world have caught SARS-CoV-2, the virus causing the disease.

While researchers continue to work on understanding the virus, there's still a lot they don't know. One of the many questions revolves around reinfection.

“With other viruses, like the flu or the common cold, it's not infrequent that you get another infection every year... because your immune response is not perfect,” said Dr. Lisa Barrett, a physician and clinician scientist with expertise in infectious diseases.

In late August, a study confirmed the first COVID-19 reinfection in Hong Kong. Two reinfection cases followed in Belgium and the Netherlands. This week, Dr. Robert Strang, Nova Scotia's chief medical officer of health, announced a suspected reinfection in the province.

“The case that was reported yesterday is definitely not a confirmed reinfection, yet,” Barrett said. “There's still a lot of work being done at public health to sort out exactly what's going on.”

Barrett added that COVID-19 reinfections have not been common.

“This is not a panic point,” she said. “Out of all the infections worldwide, we're only documenting a handful, if any, of real reinfections.”


Proving it's a reinfection isn't easy

Determining whether a case is positive due to a reinfection can be challenging. People who are considered recovered from COVID-19 can still have non-infectious virus fragments in their bodies.

“Those bits and pieces can linger and not necessarily signal that this is a new infection.”

So, when a PCR test shows as positive, it could be detecting virus that's leftover from a previous infection.

One way to confirm that a positive case is due to reinfection and not leftover virus is by identifying every piece of the viral genetic material in a process called genetic sequencing. The process enables researchers to find subtle differences in the genetic material corresponding to different strains of SARS-CoV-2.

“Unless that's the case, it's often very difficult to prove it's not leftover virus.”

When investigating whether the reinfected case in Hong Kong was confirmed, researchers at the University of Hong Kong used genetic sequencing on the virus from the patient's two infections and found that they did not match, meaning the new infection was not tied to the old one.

“That capacity is available for us as Canadians through a resource nationally and so we can do that,” said Barrett.

But this method may not be helpful in the suspected Nova Scotian case.

The difference between the confirmed Hong Kong reinfection and the potential case under investigation in Nova Scotia is their travel history. The Hong Kong patient tested positive after travelling to Spain through the United Kingdom. He had recovered from COVID-19 in April, more than four months ago.

“Between Mexico and here, for example, the virus would probably look different enough that we could see the difference in this molecular analysis,” said Barrett.

However, the case in Nova Scotia didn't travel to a distant place.


Questions around severity, immunity

Having very few cases of COVID-19 reinfections worldwide means that researchers don't know a lot about the severity of the disease and how contagious it is the second time. For now, Barrett said we get tentative clues from other viruses.

“It's likely that partial immunity within three months of a past infection may be somewhat protective and mean that you get less severe infection the second time, but we're not sure yet.”

It's also not known whether a reinfection indicates that immunity from the initial exposure to the virus is not long lasting.

When the immune system fights COVID-19, it produces antibodies, which are proteins that attach to foreign invaders like a lock and key. Antibodies that attach to the spike protein on SARS-CoV-2 prevent the virus from entering the cells and are called neutralizing antibodies. They're the ones that could protect people from getting sick.

Barrett said researchers expect there is a relationship between high levels of these antibodies and protection from reinfection, but nothing is certain.


What reinfection means for potential vaccines

The uncertainty around the longevity of the immune response against COVID-19 also raises questions about how effective a vaccine could be. But Barrett said it's still important to continue  working on vaccines.

“The goal of most vaccines under development is to produce neutralizing antibodies,” said Barrett.

Barrett said our bodies may develop more neutralizing antibodies from the vaccine, resulting in a more protective immune response compared to getting infected with the virus.

“Even if it's not perfectly protective... it might not protect them from getting infected all together, but it might make them far less sick.”

Potential reinfection can tell us one thing for sure. It's that people who previously got COVID-19 should continue to practise physical distancing, handwashing, and wearing masks.