The unnamed man, reported in a Chinese study that has not yet been rigorously peer-reviewed on Wednesday, is among the first to be diagnosed with COVID-19 three separate times. But many other patients have cleared the virus, only to later test positive again. This has raised concerns among many about the risk of getting reinfected with the dangerous virus – and bigger questions about what it would mean for the development of a vaccine if we did not get a strong and lasting immunity to the virus. Becoming immune Generally, when our immune system catches and kills a virus, it keeps a record – a bit like a fingerprint on a police database. As soon as that virus comes around again, the immune system recognises it and quickly kills it. This painting depicts a coronavirus just entering the lungs, surrounded by mucus secreted by respiratory cells, secreted antibodies, and several small immune systems proteins. Credit:David Goodsell, Protein Data Bank

At least four coronaviruses circulate among humans, where they can cause symptoms associated with a common cold. When we encounter one, our immune system catches and kills it, and we develop immunity. But for reasons scientists have not yet unpicked, coronavirus immunity fades over time. After a couple of years it is gone. SARS and MERS are caused by superpowered coronaviruses gone savage. They can kill us. Luckily, they “do induce robust immunity”, said Professor Stephen Turner, head of microbiology at Monash University.

Studies suggest that immunity lasts several years, longer than for a cold-causing coronavirus, but again does appear to fade over time. What about SARS-CoV-2, the virus that causes COVID-19? Loading “We know it’s inducing an immune response,” said Dr Claire Gordon, an infectious diseases physician at Austin Health. “Scientists have been able to detect antibodies.” Scientists told The Age that probably meant people who had fallen sick enjoyed a period of immunity, just as with other similar viruses.

The cases of reinfection – like the 68-year-old – were probably false positives, they said. “Reinfection can’t be ruled out, but it’s possible the test is picking up lingering viral material in the cells, even after the virus has been killed,” said Professor Jamie Triccas, head of infectious diseases and immunology at the University of Sydney. Loading Replay Replay video Play video Play video The big question then remains how long that immunity lasts for. In SARS and MERS, it tends to wane slowly over several years. Professor Triccas suspects – although we have no evidence of this yet – that immunity to SARS-CoV-2 behaves in a similar way.

“It could only be a matter of years. It’s probably not life-long,” Professor Triccas said. Fighting mutants There is another virus we don’t develop lasting immunity to: the flu. Unlike SARS or MERS, where immunity fades over time, the flu evades our immune system by constantly mutating.

When it enters a human cell, it harnesses the cell’s machinery to make new copies of itself, each a little different. In our fight against COVID-19, scientists have discovered one big piece of good news: SARS-CoV-2 does not appear to mutate as quickly. The virus has inbuilt proofreading machinery, ensuring that each new copy of the virus is almost identical. That’s good news. It’s easier to make a vaccine for a virus that does not change, and it should mean the vaccine will provide lasting protection. “It is pretty stable,” says Professor Turner. “But it’s such early days. We don’t know. The challenge with developing a vaccine is it is just so new.”