When Nicola Straiton and Sarah Hall went to a dinner party on Christmas Eve, everything was in place to minimise the risk of a COVID outbreak.
The group of eight dined in the backyard, as health experts had urged, for just under four hours and all attendees were regularly doing rapid tests.
So it was a "bit of a shock" three days later when the guest sitting next to Nicola tested positive.
The Sydney couple immediately launched into close contact mode: The new national definition, which would have excused them from isolation, hadn't yet been announced.
They tested negative on rapid tests the following day but continued isolating together in their Newtown home.
Three days later, Nicola tested positive. But, for Sarah, who had spent the past week by Nicola's side, the positive result would never come.
"We were completely together," Nicola says, of the days before the positive result.
Once that happened, they immediately isolated on different floors and kept to separate bathrooms.
"I was really scared of passing on the virus," Nicola says.
As Nicola's symptoms worsened, their resolve to prevent Sarah from falling ill hardened.
"There was a point where I was like, 'Why are we even bothering with this, because surely I've got it'," Sarah says.
"But then, as I watched [Nicola] get more ill, I stopped feeling that … then it was just a case of, over seven days, trying to pre-empt everything."
An effective pandemic response requires clear rules that tell people how to act in every possible circumstance with as little ambiguity as possible: A close contact is someone who has spent more than four hours in a household-like setting with a case; COVID-positive people need to isolate for seven days; your antibodies will start to wane three months after infection.
But these time frames, while a useful guide in the majority of cases, disguise the fact the virus doesn't always follow the same script.
While we know the new Omicron variant, which has taken over as the dominant strain in Australia, is the most transmissible yet, not all infections are created equal.
It's this variability that means Nicola could, as she assumes, catch the virus at an outdoor gathering that only lasted a couple of hours but not pass it on to her partner who she shared a home with her for the week before she tested positive.
It's also why some people become "superspreaders", while others don't pass it on to anyone.
And while scientists understand some of the ingredients that go into creating a household outbreak — viral loads, individual immunity and environmental factors — there are still many things they don't yet know about how the virus operates.
House 1: The partner who didn't get sick
By most measures, Sarah was high-risk for getting the virus; the couple shared a bed every night, including during the days before Nicola developed symptoms, a period when epidemiologists say infectiousness is high.
We also know the home is the most high-risk environment for transmission, which is exactly why the close contact rule focuses on "household-like settings". So, what happened here?
While age and vaccination status can play a role in how likely someone is to get sick, on the surface they shouldn't apply here. Both Sarah and Nicola are in their 40s, and fully vaccinated — they even got their booster shot on the same day, five hours apart.
"While we talk about antibodies declining after five months or so, that's an average … for some people, it's going to be two months, for others it's going to be 12 months," says Stuart Tangye, an immunologist with the Garvin Institute who is working to understand why some people get severely ill with COVID-19, while others don't get it at all.
At a population level, he says, health policy relies on "the rule of averages". It's like saying, on average, women live longer than men, he explains, "but we all probably know women who have died younger than men".
To figure out why some people get sick while others don't, we need to first understand how viruses make their way into the body. We now know that COVID-19 is an airborne disease, so most people encounter it in the air they breathe — but what happens next?
Firstly a person has to breathe in enough of the virus to reach what Stuart Turville, an associate professor at the Kirby Institute, calls a "threshold of viable virus".
"The thing about SARS-CoV-2 is that threshold is very low," he says. "The more transmissible it becomes, the lower number of particles you need to inhale to get an infection."
Once this happens, the virus is met with "quite a hostile environment": layers of naturally occurring defences, evolved over time to fight back against viral intruders.
By now we're familiar with COVID-19's effective reproduction rate — what scientists call the "Reff" or "R number" — which measures how many people a positive person will typically pass the virus on to. For an outbreak to be controlled, this number needs to be below one.
Inside the body, at a cellular level, there's another "reproductive rate" going on.
When talking about the threshold mentioned above, Dr Turville is referring to a "decent amount of infected cells" that kick start the process of infecting others.
"The first few cells infect three or four, the next few cells infect another three or four of the cells, to get a real roll on momentum," he says.
"And, with SARS-CoV-2, it only takes a few to get in to start that."
Once enough cells have been infected, the acute infection phase begins.
So what factors influence whether a COVID-19 exposure meets the threshold needed to kick off this process?
The length and location of exposure are one element. For example, sitting outside means airborne particles are more likely to become diluted, reducing the likelihood of breathing in enough virus to become infected.
But it also depends on the positive person's viral load.
"Individuals with a really high level of SARS-CoV-2, they've been called 'superspreaders' because their ability to transmit it is phenomenal compared to somebody with a really low viral load that may have a sniffle," Dr Turville says.
This is in line with what Professor Michael Good, one of Australia's top infectious disease experts, calls the 20-80 rule: "Eighty per cent of transmission is in general caused by 20 per cent of those who are infected," he told ABC Everyday.
Research is currently underway to understand why this is.
Dr Turville says there are a number of possibilities at play: It might relate to how big a dose of the virus someone is exposed to initially, or to deficiencies in people's immune response that give the virus an early head start.
"We don't really know," Dr Turville says.
"At the end of the day, everybody is a bit different in their genetic make-up."
House 2: The family isolating together but didn't all catch it
On the same day Nicola tested positive, another family in Armidale, a city in regional New South Wales, was figuring out the logistics of isolating within the home.
Kate Simpson's 21-year-old daughter, Madeline, returned a positive rapid test on New Year's Eve, after a few days of mild symptoms she had mistaken for hayfever.
The familiar routine began: Madeline went straight into bedroom isolation, while Kate and her 18-year-old son, Andrew, prepared to hunker down at home for a week.
Four days later, they both took a rapid test despite not having symptoms. Kate's came back negative, but Andrew was positive.
"He'd probably been positive all along," Kate says. "And he'd obviously been with me in the house, while Madeline was in her bedroom."
PCR tests confirmed the rapid results: Madeline and Andrew were positive, but Kate hadn't contracted the virus.
But after days of binge-watching TV together on the couch, Kate, who is 54, assumed she too would soon test positive.
They made the decision to bring Madeline out of her room, so the family could isolate together.
"I had to isolate for 10 or 11 days, and I didn't bloody have it," Kate says.
And she's pretty sure of this because, over the isolation period, she returned four negative rapid tests and a negative PCR.
"I'm pretty healthy. I don't have any underlying conditions, but I was surprised."
Similarly to Nicola and Sarah, all members of the Simpson family are fully vaccinated and Kate had recently had her booster shot.
Following the family's isolation, Kate has returned to her role as a caregiver for her 92-year-old mother and has been left wondering whether this mean she'll never get the virus.
Around the world, scientists are racing to establish whether there is a clear genetic reason some people may be naturally resistant to COVID-19.
Among them is the COVID Human Genetic Effort, an international consortium of experts seeking to understand the genetic and immunological factors that influence SARS-CoV-2 infections.
As part of the study, the team of co-authors is recruiting hundreds of people who have been exposed to a COVID-positive person for an extended period of time without testing positive — people such as Kate and Sarah.
If they are able to understand why some people are naturally resistant to the virus, the hope is the knowledge could be used to develop virus-blocking treatments.
So far, no gene has been formally identified that makes someone resistant to COVID, but Professor Tangye, who is a member of the research group, predicts it's only a matter of time.
"It's not completely unexpected or without precedent," he says.
As an example, he references a small cohort of people who are naturally resistant to HIV infection because of a genetic variant.
"There are resistant genes in the population which do protect you against some pathogens, so there's probably going to be something there," he says.
One theory, he says, revolves around a hormone called Type-1 Interferon, a key player in our innate immune response that can be "very effective at holding viral infection at bay".
"When it comes to resistance, one prediction would be that people who have overactive Part-1 Interferon production could be resistant, or less vulnerable, to SARS-CoV-2 infection," Professor Tangye says.
But this is only speculative, he stresses, and studying COVID-19 resistance is challenging.
You need large numbers of participants from high-risk groups, who can prove they were exposed to the live virus while also undergoing regular testing.
"When [the results] land, they'll land big time," Dr Turville says.
House 3: The sharehouse who all caught it
When the virus came knocking at Cat Wratten's four-person Redfern sharehouse, there were a few days of cautious attempts at infection control before a second positive test led them to surrender.
The older, two-storey terrace has two bathrooms on separate floors, which initially allowed the first positive case to isolate in his bedroom, away from the other two 20-something housemates. A fourth housemate was, luckily, away with family at the time.
But three days later, when Cat also tested positive, the call was made — "majority rules" — to let him out. They would now isolate together, accepting this meant they would all likely test positive.
On the sixth day of isolation, as the first housemate was coming to the end of their illness, the final housemate returned a positive test.
The recent wave has created an ethical and practical minefield for sharehouses.
Current health rules centre around the household as a unit but, for those who live with non-family members, it's a framework that doesn't always fit neatly onto their lifestyles.
"Living with so many people, we all have different jobs and have to be in contact with people," Cat says.
While she is able to work from home, her two housemates work in hospitality and retail and need to get shifts covered when they can't come in.
"At first, every two days, someone would message in the group chat and say, 'This person has it, I'm a close contact," the 25-year-old says.
"It's kind of good to be in a house where everyone's on the other side now."
Many houses are also not set up to manage multiple people isolating at once, with older homes often less ventilated.
"The way the house is laid out, I would have had to walk through the entire house to go to the bathroom," Cat says.
Architecture professor Geoff Hanmer has spent the past two years studying the role buildings play in COVID-19 outbreaks.
His research began during Victoria's second wave by looking at aged care architecture and whether environmental factors meant they were uniquely predisposed to outbreaks. It has continued ever since.
"What led to the problem in aged care in Victoria, and is still causing problems, is there is insufficient ventilation to dilute the virus in the air," he says.
"If you've got good ventilation, the chance of you breathing in enough virus particles to infect you is low. If you have poor ventilation, there's a much greater probability that you're breathing in virus particles."
When it comes to homes, similar issues exist — in both the winter and summer months.
Most homes are designed for windows to provide ventilation. If they are closed because of extreme temperatures outside, virus particles can build up.
"We've got a pandemic of buildings," he says.
Ideally, to limit the chance of a household outbreak, he says, you want cross-ventilation, created by opening windows on both sides of a building, even if an air conditioner is on.
"In summer, it's tricky, because you open a window and the air conditioner loses its effectiveness," he says.
"But, if you have COVID in a household, you just have to put up with that, because if you close a window you'll massively increase the probability that infections will spread."
With so much unknown about why household outbreaks can play out so differently, Professor Hanmer has one final message for people going through it right now.
"If you have someone who is COVID positive in the house, don't give up," he says.
"If you are triple-vaxxed, and you do your very best to ventilate a space and be careful about interactions, there are examples of people to have managed to confine an infection to one person. So, don't give up."
