As COVID-19 cases continue across the globe, an intriguing phenomenon has emerged, including in Australia.
Some people have been able to dodge the disease despite close exposure to infected cases, including isolating at home with family members and friends who are unwell.
Scientists have been researching cases like these to form a more detailed understanding of how COVID-19 spreads and to devise better strategies to combat viral outbreaks in the future.
James Cruickshank, 30, shares an apartment in Sydney with his partner Steph Grant, 28.
When Mr Cruickshank fell ill with COVID-19 earlier this year, they figured it was only a matter of time before Ms Grant got sick as well due to them living so closely together.
Sydney couple James Cruickshank and Steph Grant. Source: Supplied
"And then you get that second line and go 'argh.'"
But to their surprise, Ms Grant never got COVID-19 herself.
"I get colds more than anyone I know," she said. "And I thought, if anyone is going to get it, it's for sure going to be me. But I didn't. It was super surprising."
Dr Vanessa Bryant is the head of the immunogenetics laboratory at Melbourne's Walter and Eliza Hall Institute of Medical Research.
She too avoided infection when one of her children contracted COVID-19 earlier this year.
Dr Vanessa Bryant (right) is involved in a project that is looking to unravel how COVID-19 spreads within Australian households. Credit: Walter and Eliza Hall Institute
"As it ended up, everyone in the house fell sick with Covid, except for me."
Dr Bryant is involved in the 'First Few X' project, which is studying the spread of COVID-19 within Australian households.
"It's a question that's got a lot of people scratching their heads; why was I the last one standing? What's special about me?"
Research indicated a number of factors were at play, Dr Bryant said, including how recently a person was vaccinated and how well they were able to isolate themselves from others.
"There's so much individual variability when it comes from person to person. And we know most of that really comes down to their genetics."
It is well known that factors like age, gender, and the presence of other medical complications can influence whether a person is likely to contract an infectious disease.
But these factors don't explain scenarios such as an older person who recovers from COVID-19 with ease, or an otherwise healthy, younger person who gets surprisingly sick.
Studies have suggested a small number of people have genetic mutations that make them more resistant to the COVID-19 virus.
Others have genetic mutations that appear to make them more susceptible to severe infection.
Professor Stuart Tangye leads research into inflammatory diseases at the Garvan Institute of Medical Research.
He said the presence of genetic defects helps to explain cases where otherwise healthy people became surprisingly ill after getting infected.
Stuart Tangye has been researching genetic factors that can make a person more susceptible to Covid infection. Credit: PETER SECHENY PHOTO
"That pointed to a genetic possibility. It was unlikely to be a co-existing disease or an environmental factor."
In a recent review, a team including Professor Tangye estimated as many as five per cent of severe COVID-19 cases in people under the age of 70 could be caused by previously undiagnosed genetic defects.
"There's over 400 genes which, if they don't work properly, can result in what we call an inborn error of immunity - or a disease which presents clinically as a recurrent, or severe, or adverse reaction to viral infections or some vaccines," he said.
"That's a real indicator that there is something in your genes that is not quite right."
Associate Professor Sanjaya Senanayake from the ANU Medical School in Canberra, said many people probably wouldn't know they were genetically susceptible to COVID-19 until they were infected.
"That's why I always encourage people to not go and deliberately get Covid – even if they’re young and healthy – because they might have some unrecognised risk factor that could give them severe disease."
It's difficult to get a firm number on how many people might be genetically protected from COVID-19, or how many people might be genetically susceptible.
In both cases, it's likely to be only a small fraction of the general population.
Scientists hope that by better understanding these mechanisms they can design better treatments for COVID-19 and similar viral outbreaks in the future.
People who are unwell are advised to contact the Healthdirect 24-hour health advice helpline on 1800 022 222 or their doctor. In an emergency call 000.
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