"We slept in the same bed." Why some household contacts aren't testing positive to COVID-19.

When my boyfriend tested positive to COVID-19 four days into the new year, I had no doubt in my mind I was going to test positive as well. 

We had just spent a week bouncing between each other's houses, sleeping in the same bed, going to the same shops, and sharing meals. 

"At least when I get COVID we can look after each other in isolation," I said over the phone, expecting a positive RAT result that day. 

But after two more RAT tests, the positive line never came. 

As baffled as we both were, this experience isn't exactly rare. 

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"My boyfriend and I shared a bed and worked from home together for a full 12 hours. He tested positive the next day while I remained negative," Lucy told Mamamia.

"My husband got COVID but my two daughters and I didn’t. We were in the same house and weren't isolating separately," Hannah shared. "We assumed we’d all just get it but we never did."

It's a familiar story.

Lily also couldn't believe that her partner managed to avoid COVID-19 after she tested positive three days before Christmas. 

"We'd been sleeping in the same bed, kissing, hugging, and sharing a water bottle just the day before. He either had a super immune system, or the vaccine was really working," she wrote in a piece for Mamamia. 

Gemma, who recently recovered from COVID-19, was in the same boat. 

"I slept in the same bed, used the same towel, laid on the same couch, drove in the same car and kissed my fiancé while COVID-positive," she shared. 

"When I found out, I immediately isolated in our bedroom - which has its own ensuite - but we expected him to get it given how close we were. I also had a friend sleepover at my house while I was infectious and she is negative as well."

Meanwhile, Emma has dodged COVID-19 after being a close contact not once, but three times. 

"At Christmas, I was seeing someone, and he tested positive after we had spent the whole week together (in the same bed etc). In hindsight, it was not the right move (obviously) but to avoid my housemates being exposed to me, I temporarily moved in with him," she shared. 

"For the next two weeks, all of my PCR and RAT results were negative despite staying in the same room. Then one of my housemates got back from her holiday and tested positive so I had to isolate for another week once I was back at home. However, I was still negative. Now, I have been seeing a different person, and he just tested positive today. I am still negative... I've only had two doses of AstraZeneca four weeks apart. I'm very confused."

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Others have also isolated with positive cases and still remained negative. 

"My bestie was living with me to avoid getting COVID off her roommate. She ended up contracting it at work and we had been sharing a bed for over a week, including while she was symptomatic. There was no available room for her to isolate in so we gave in to the fact that she would spread it in our house. She didn’t, and I remained negative the whole time as well as my parents," Susannah said. 

So why are some people who live with or spend a lot of time with a COVID positive cases still not contracting it themselves? 

Well, according to Professor Michael Good, one of Australia's top infectious disease experts, there's been a "very uneven transmission" of the virus.  

"It's not like flu, where the transmission is more uniform from person to person," he told the ABC.

While research is still underway to understand more about the transmission of the virus, Professor Good says COVID-19 appears to follow a 20-80 rule. 

"Eighty per cent of transmission is in general caused by 20 per cent of those who are infected," he told the publication. 

"The reasons for that aren't, to my knowledge at least, well understood." 

Professor Good said possible biological differences, mask wearing, or people who expel more air in the household are factors that could explain why some people contract COVID-19 while others don't.

“For example, we know some people speak more loudly... some people are larger than others and larger people expel more air," he told Melbourne radio station 3AW.

"Some people wear masks, others do not, even in households. Some people keep further away from other people when they talk."

Professor Tony Cunningham, an infectious diseases physician, clinical virologist and scientist from The University of Sydney, has also offered an explanation. 

"If there isn’t spread in a family that is 100 per cent it may be because of the levels of immunity of the person who’s got it, so the virus has fallen off quickly, it may be the level of immunity in the person receiving it and that could be 'specific' because they’re immunised or 'not specific' because they’re just lucky enough to have the right genes," he told news.com.au.

Meanwhile, experts around the world are currently trying to understand if there is a genetic reason why some people aren't contracting COVID-19. 

The COVID Human Genetic Effort, an international consortium of experts, have been recruiting hundreds of people who have been exposed to a COVID-positive person but haven't tested positive to the virus. They're particularly interested in people who shared a home or bed with an infected partner. 

The team of scientists, who span from 10 research centres across the world, hope that identifying the genes protecting people from contracting COVID-19 could help develop virus-blocking drugs. 

In the meantime, Professor Good told the ABC, "vaccination is by far the best way to prevent the disease" in combination with social distancing measures. 

Feature Image: Getty/Mamamia.