A doctor explains why you haven’t caught Covid yet

If you’ve been lucky enough to get through the pandemic without a dreaded double red line, this is probably why.

By now, it’s likely you’ve either had Covid or you’ve been a close contact of somebody who did. If you’re in the latter group and you haven’t caught it yourself, chances are you’re feeling very smug right now — but scientists are trying to work out why.

That was the case for Sydney man Craig Jeffs who has yet to catch the dreaded virus, despite dodging it three times. The first time took place at the start of the pandemic when he shared a bit of steak while at dinner with a work colleague.

Despite the dubious interaction, he didn’t contract the virus.

“We shared a steak and a glass of wine and (I) actually had some steak off his plate. He was positive at the time,” Mr Jeffs told A Current Affair on Monday.

He also didn’t appear to contract the virus after sitting next to an infected friend during a Waratahs game in March, and more recently completely his quarantine period with his wife, who was hit with Covid-19.

However, Australian National University lecturer and epidemiologist Dr Katrina Roper suggests there are three main factors that could be helping you avoid Covid.

Being exposed to the common cold

As it turns out, previous infections of any kind may have helped you escape Covid, according to research.

“Having a prior infection to another cold virus can confer some protection to Covid, or other respiratory viruses,” said Dr Roper.

”Exposure to other respiratory viruses can prime parts of the immune system, leading to better defence against infection by the SARS CoV-2 infection”.

This theory was confirmed in a recent study from the Imperial College of London. Published in Nature Communications journal in January 2022, they found that T cells gained from other coronaviruses (like the common cold) can protect people from Covid-19 infection.

“Being exposed to the SARS-CoV-2 virus doesn’t always result in infection, and we’ve been keen to understand why,” said Dr Rhia Kundu who authored the study.

“We found that high levels of pre-existing T cells, created by the body when infected with other human coronaviruses like the common cold, can protect against COVID-19 infection.”

It’s not a foolproof system, however. Roper notes that some virus can live in your body together — like influenza and SARS CoV-2.

PCR and Rapid antigen tests likely missed a lot of infected people, so you may have had Covid without knowing. Picture: iStock

Of course, there’s also a good chance you did have Covid and just didn’t realise it, according to Immunologist, Professor Stuart Tangye.

“When we first started doing PCR testing, it was really done on symptomatic people … so we were obviously missing a lot of those asymptomatic people,” Tangye told the ABC.

“I’m sure we missed a lot of positive cases over December and January too, where there was a supply and demand problem in terms of getting tests”.

Your immunity

Another, of course, is the simple fact your immunity may just be better suited to avoiding Covid, or if you’re vaccinated and have been exposed, your vaccine may have been working more strongly at the time.

“Immunity to infection varies between people,” Dr Roper explained to news.com.au. “For example, compare the immune level of a younger person versus an elderly person.

“Immunity can also vary according to a person’s health status at any point in time. If a person is stressed — be that emotionally or physically — this can lead to reduced immunity and increased susceptibility to illness”.

Scientists are trying to work out why some people haven't caught Covid – and it might have more to do with luck. Picture: iStock

While many scientists are talking about underlying health conditions, obesity and old age being the main factors that make a person more susceptible, Roper says that even the healthiest among us could be suffering from a weakened immune system.

“Even elite athletes, if they start over training, can stress their body and result in reduced immunity — despite being very fit for their sport”.

The circumstances of your exposure

I caught Covid at the start of this year from my boyfriend, when we were isolating in a studio apartment, but I know lots of people who avoided the virus despite being in close proximity with infected people.

We’ve all heard of boyfriends who never caught it from their girlfriend even though they live together. Or six-year-old boys who never passed it on to their sister or parents, despite them catching nearly every other cold under the sun from him.

So why are some people so lucky? Roper suggests it can have a lot to do with factors other than immunity.

“In households, there would also be other factors,” she explained, “such as how much time one person spends in close proximity to an infected person in comparison with another.

Another illness, or high levels of antibodies from vaccines may have help. Picture: iStock

“The size of the household would also have an impact on why there is more transmission in some homes compared with another. Two people living in a one-bedroom apartment is not the same as two people living in a three-bedroom house, and their opportunities for exposure will be different”.

Or, it could just be your genetics

Earlier in 2022, UK researchers performed the first human-challenge trial of its kind for Covid.

The study found 36 young and healthy people who had no evidence of previous Covid infection or vaccination. It exposed all 36 of these people to the virus, and only about half of them actually caught it — which was defined by two positive PCR tests in a row.

Of those who didn’t catch Covid, about half of them did briefly show low levels of the virus. This suggests their immune systems shut it down pretty quickly.

“There’s probably a few people … who would have a really strong innate immune response [that] just quells the infection, without enabling the virus to get too far ahead,” said Tangye.

“There are going to be people who are less susceptible to viral infection because they have differences in their genes, such as genes that are important for viral entry into your cells”.

Although that group would be very small, Tangye also suggested this wasn’t the first time some people have been found to have a genetic resistance to diseases.

“With HIV, for example, there is a very, very small number of people who are genetically resistant to infection,” he said.

“That’s because they have naturally occurring genetic mutations in a certain gene so the virus can’t physically infect their T cells.”

So maybe you’re genetically lucky, but more likely you were just lucky at the time of your exposure to Covid — or never realised when you had it.

Originally published as A doctor explains why you haven’t caught Covid yet