Australia may have recorded first case of Covid-19 reinfection

A case of Covid-19 in Victoria is being treated as a rare case of reinfection, the first case classified as such in Australia.

Reinfection with Covid-19 is rare, with only a six cases reported among the 40m cases worldwide to date, including in the US and Hong Kong. It seems that in most cases of Covid-19, people develop immunity to the virus after being infected, though it is still unclear how strong this immunity is or for how long it remains.

In the Hong Kong case, a 33-year-old man was diagnosed with the virus more than four months after he recovered from his first infection of the disease, with genomic sequencing finding the man had been infected with two different strains. However, his reinfection was asymptomatic, indicating he may have had some clinical immunity.

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The Victorian case is less clear. Genomic sequencing has not yet been completed, but the premier Daniel Andrews said the man first tested positive to the virus in July. He tested positive again in October. In many cases it is unclear whether a second positive test is truly a reinfection or merely dead virus being shed.

Andrews announced the possible reinfection on Wednesday saying the man was being treated as a reinfected case “out of an abundance of caution”.

An expert panel had “reviewed this particular case and concluded there wasn’t enough evidence to say that the positive test presented viral shedding, so the case is being monitored closely, and it is through an abundance of caution that we are assuming that it is a positive case, rather than the person shedding after the original infection,” he said.

“There have been very few reported cases of reinfection around the world. It is also the case that persistent shedding over a long period of time can be a feature of this virus. This is understandably frustrating for everyone involved, whether this is in fact a positive case or not, but we do take a very cautious approach, and I think that is the best way to go.”

Dr Larisa Labzin, from the University of Queensland’s Institute for Molecular Bioscience, said for the sake of Victoria’s accurate tracking of Covid-19 transmission, it was important for the department of health to determine if this is indeed reinfection or a persistent original infection.

“It is important to note that after we clear an infection we are not necessarily protected from reinfection if we are re-exposed to the virus, but we anticipate that we are protected from developing severe disease,” Labzin said.

“This can be determined by genetically sequencing the virus during the first and the second test and comparing the viral sequences. The virus accumulates enough small changes with time that we can distinguish a virus that was caught in July from a virus that was caught now.

“The alternative is that the person who caught Covid in July never truly cleared that original infection, in which case this wouldn’t be a ‘new’ case of Covid-19.”

Guardian Australia has contacted the department of health to ask whether genomic sequencing is currently under way, and what methods were used to determine the case as one of reinfection, but is yet to receive as reply.

Although the Australian case is “interesting,” Associate Professor in epidemiology at La Trobe University, Hassan Vally, warned there was so far little detail to go on.

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“It’s really important not to attach too much weight to this finding,” he said. “We do know that whilst infection by some viruses leads to lifelong immunity, there are others for which infection only leads to relatively short-term immunity.

“Since we have only coexisted with SARS-CoV-2 [Covid-19] for less than a year, it is not certain what the immunological characteristics of infection are yet. So far, however, there have been very few cases of reinfection reported. We also know about the law of truly large numbers, which says that with a large enough number of samples improbable events do occur. We have obviously seen a very large number of infections now around the world and so we will start to see unlikely events occur.

“No doubt there will be more done to try and understand more completely what has happened in this case, with it being recognised that if we start to see more reinfections as time goes on — which points to immunity being short-term in some individuals — this has obvious implications for how effective a vaccine or vaccines will be.”

Associate Professor Sanjaya Senanayake, a specialist in infectious diseases, said some people can shed the virus for a prolonged period after an infection, sometimes for more than 100 days.

“This may yet be the case here,” he said. “If, however, this turns out to be true reinfection with Covid-19, this will be an important case globally, especially to see if the second illness is milder than the first. Hopefully, they will get better soon.”