In late February, a woman in Japan tested positive for COVID-19 after being declared virus-free before. In China, similar reports have come out, where people have recovered from the virus once and then been tested positive for it again. This has raised some questions in the scientific community about just how this virus works and whether humans can catch it twice.

Following these reports, a small-scale study was done in China following four patients with a mild or moderate COVID-19 infection. They were provided with the antiviral drug Tamiflu, to help treat it. After they tested negative for the illness on two consecutive days they were put in isolation. Five days later, they all tested positive again. The study writes that this “implies a proportion of recovered patients are still virus carriers”. So, what exactly is going on? The short answer is we don’t know for sure.

Normally, when your body is infected with a virus, it mounts an immune response to fight off the disease. After you recover, your immune system can remember the virus,  providing some level of protection against it if you are re-infected. Therefore, it is unlikely that any of the reported cases in China and Japan are due to catching the disease twice. Mark Harris, a professor of virology at the University of Leeds writes: “It is unlikely that they would have been re-infected having cleared the virus, as they would most likely have mounted an immune response to the virus that would prevent such reinfection.”

One explanation is that the virus persisted at low levels in the body, even after recovering from the illness. This is observed in both the Zika virus and the Ebola virus. Ebenezer Tumban, a virologist at Michigan Tech University, says the Tamiflu they were taking as a treatment could have pushed the number of viruses in the body down to just a few. After this, the test might not have been sensitive enough to detect the virus. The virus may then have begun replicating again to the point where the test could detect them.

This has implications regarding how contagious people are. If you still have the virus in your body, you could potentially still pass it on. The patients in the study showed no coughing or sneezing symptoms after testing positive for the second time, so they were less contagious. However, activities such as sharing drinks or prolonged intimate contact could potentially pass the virus on.

Professor Paul Hunter, a Professor of Medicine at the University of East Anglia, discussed the case in Japan. “There is so much we do not know about this case to give a properly informed opinion. I would caution against reading too much into this report given the lack of information.”

We are still in the early days of research into this virus. There is lots we don’t know about it and jumping to conclusions about the behaviour of the virus, based on a single case, is both irresponsible and dangerous. We must keep our speculation grounded in science.

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