Coronavirus: Can you catch it twice?

If you're unlucky enough to catch COVID-19, but lucky enough to recover, can you catch it again? 

There have been reports of people recovering from the disease, which has killed more than 27,000 people to date, then testing positive again

While most people are able to develop an immunity to the majority of viruses after having them once, it's still not clear if that's the case with SARS-CoV-2, the virus which causes COVID-19.

It's early days yet, with the disease's existence only reported to the World Health Organization less than three months ago, but the signs are good.

"A recent study conducted in China reports that monkeys infected with SARS-CoV-2, the virus that causes COVID-19, are protected from symptoms when infected a second time a month later," explains John Taylor, senior lecturer in virology at the University of Auckland.

"Not only were there no symptoms following the second exposure, researchers couldn't detect any virus in the re-infected animals, suggesting they were unable to spread infection."

Like humans, monkeys develop defence proteins called antibodies when a pathogen is introduced. 

"Viruses present hundreds of different antigens to the immune system so that when we become infected with a virus like SARS-CoV-2, hundreds of different antibodies build up in the blood of an infected person, usually peaking two-three weeks after infection," says Dr Taylor.

"Some of these antibodies are more important than others in ending the infection. Neutralising antibodies bind to the virus and block its attachment to the cells in which it needs to replicate, putting the brakes on further spread of the virus. We can think of antibodies as the body's own self-made antiviral drug."

Some researchers are now looking into whether the blood of previously infected patients could help in developing effective treatments, like they have for Ebola and SARS (which is closely related to the virus causing the current outbreak, as its name suggests). 

"To know for sure that having COVID-19 makes people immune to getting reinfected we need to see if those who have recovered get it again. So far there is no good data that this happens, although there can also be exceptions," said Helen Petousis-Harris, a vaccinologist also at the University of Auckland.

"The general thinking is that protection for at least a few years is likely, based on what we know about other coronaviruses. With the common human coronaviruses that cause colds we know that people make immune responses and have antibodies, but after a few years they can get reinfected. We do not know if this is the case with SARS and MERS.

"Assuming people become immune for a while, there is no way to know how long the protection will last, but I imagine it will be at the very least a few years."

That's because viruses can mutate. Some mutate quickly and new vaccines are needed every year - like the flu. Others don't and can be effectively wiped out, like SARS. 

"A lot of people are getting the virus and recovering, and when scientists have looked in their blood, they've seen traditional signs of a mature immune response - good levels of IgG antibodies, and evidence that immune cells have activated and responded to the virus," said Nikki Moreland, senior immunology lecturer at the University of Auckland.

"But there is still not enough research published for us to have a clear understanding idea of the immune response to COVID-19."

While there remain so many unknowns, if you do manage to recover from COVID-19 the keys to avoiding reinfection are somewhat obvious - eat well, sleep well and try to avoid stress. 

"If someone gets infected the best defence they have is their immune system," said Prof Petousis-Harris.

"There are some scientifically proven ways to optimise the function of the immune system. Those backed by really good research are: Eating as well as possible; getting a bit of exercise (not ultra-marathons); and the value of sleep on the performance of the immune system cannot be understated.

"Also, as we know, over time stress can have a negative impact on immunity. During this very stressful time we need to value the importance of looking after our physical and mental health. Spending a fortune on vitamins and potions is unlikely to be helpful."