Can you catch Covid-19 twice or are ‘reinfections’ just remnants of the virus showing up again?

Were 163 people in South Korea so unlucky they caught Covid-19 twice, or is there another explanation?

This question is being asked after around 2 percent of those who had already recovered from the virus in that country tested positive again.

It’s not the first time this has happened. It has been reported in China, although details of how many people were affected aren’t entirely clear. Early on in the outbreak, the same thing happened to a woman in Japan.

For Sweden, which opted for a herd immunity approach, this could mean it’s time for an urgent strategy change.

If reinfection is possible, this also has potential to derail plans for an ‘immunity passport’ approach discussed by some countries, where those who have recovered from Covid-19 are allowed to return to work because they’re no longer vulnerable to the virus.

But are people really catching it twice? 

Faulty testing, different virus strains or the virus reactivating are all theories for the illness double-whammy. Catching the virus twice falls onto the less likely side of the equation.

How can you be negative and then positive?

What’s considered to be the most likely explanation to a positive test after recovery is that, for a time, the virus in a person’s body dipped below levels which could be detected in a test.

University of Auckland research fellow Dr Janine Paynter said one paper has shown what could be the answer. 

Using the term ‘reinfection’ to describe what has occurred could be incorrect, she says. Rather than catching the virus twice, a person may not have entirely rid themselves of their original infection.

“There’s a point at which the virus has fallen so low that the test doesn’t work but it doesn’t mean the virus is not there.”

The paper, which has been peer-reviewed, looked at samples from nine patients with mild cases of Covid-19 over a number of days. 

“What was happening was the virus numbers were varying, particularly towards the end of the infection, so it dropped below the sensitivity of the test.”

This could lead to somebody testing negative one day and then positive another. 

“It seemed like people were getting reinfected but they weren’t. It’s a virus that hangs around.”

Are these ‘reinfected’ people infectious?

One thing Paynter found interesting about the paper was when the researchers tested the swabs taken from participants over the course of their illness to see whether the viral load was likely to be able to cause an infection in another person.

“Even though the virus came out for about 20 days, they could only get reinfection until about the seventh day.”

Could it be the tests, not the people?

University of Otago Professor David Murdoch sees a number of likely reasons for people who recovered to test positive again, with reinfection not being at the top of the list.

“There’s a quote among labs which says ‘ordering a diagnostic test is like picking your nose in public. You need to know what to do if you find something’. That’s probably what they’ve done, they’ve gone, ‘we’ve got to keep testing’ when they don’t really need to.”

His take is it’s highly unlikely people would get a second infection in such a short timeframe. 

“I would be surprised if you tested the number they have in Korea and not got a proportion with that result if they’re going to do that sort of testing.”

He feels the positive tests are more likely to be explained by the infection not being over, or issues with the test itself.

“If someone’s not taking a good sample, you could miss a few times … sometimes if the swab is not quite in the right place it might not get the right sample. Especially later on in the infection.”

If it’s not likely to be reinfections are ‘immunity passports’ a good idea?

Immunity passports based on antibody or serological tests have been touted as one way communities could get people back into the workforce safely. Only those who have been exposed to the virus and have passed an antibody test would get the green light to give up physical distancing.

British officials ordered four million antibody tests from China, however, they all turned out to be faulty. In New Zealand, officials are not planning to use them.

Paynter said she would “unambiguously advise” against the idea of immunity passports because so little is known about immunity yet.

Antibody tests can show who has been exposed, but they don’t measure the level of antibodies, these might be too low to offer immunity, or could wane over time. 

“How long immunity lasts is what we don’t know. What we’re using at the moment to answer that question is how long the antibodies last for in people’s blood.”

With this virus being new, the length of time a person will be immune to the virus isn’t known. For other Coronaviruses, immunity generally lasts up to a year. Tests in four rhesus monkeys showed immunity occurs, but it is unknown yet how long it lasts.

How does immunity work with different strains of the virus responsible for Covid-19?

Not a huge amount of work has been done on this subject. 

For viruses like a rhinovirus – the main cause of the common cold –  there are around 100 different strains. This is why it’s so hard to create a cold vaccine which will be effective against all of them and you can catch several colds during one winter.

“This [the virus responsible for Covid-19] hasn’t reached that point because it’s still evolving and evolution takes a long time.”

Paynter had read a suggestion on social media the virus might have evolved to target people with diabetes. Her response was: ”Not on a three-month time scale. That’s just silly.”

Murdoch said while small changes to the virus had resulted in different strains of the virus these didn’t appear to be behaving differently at present. 

“Seasonal influenza changes more than most viruses. That’s why we always play catch-up with the vaccine, trying to keep ahead of it. But we don’t see major changes through the season.”

If the pandemic stretches on for an extended time, though, bigger changes could occur. This could affect both immunity gained from catching a different strain and the race to develop a vaccine.

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