Cecilia* feels like she's been unwell most of the year.
After recovering from COVID in April, she suffered through a handful of colds over winter.
"I felt like I'd just get over one before the next one hit," the triple-vaccinated Melbourne mum of two said.
"It was really rough for a while."
At the end of October, her eldest child tested positive for COVID. Shortly after, Cecilia did too.
"I couldn't believe it. It feels never-ending."
Infections soon after COVID-19 — whether it be another stint with the SARS-CoV-2 virus or a different pathogen altogether — are commonplace.
And it's not entirely unexpected, according to Paul Griffin, an infectious disease physician and microbiologist at the University of Queensland.
"With other viral infections, sometimes [a person] can be at slightly greater risk of getting an additional infection or even reactivating latent infections.
"For example, we see some people that, when they have a nasty viral infection, their cold sores might come back."
Plus the Omicron subvariant "soup" currently doing the rounds means the antibodies you generate in the face of one infection don't necessarily protect you against other subvariants for long.
But the reasons some people seem to be more susceptible to reinfection or other diseases after COVID are complex, Dr Griffin said.
"There are a lot of factors at play," he said.
"Our immune systems are different at baseline, and those differences increase as we get exposed to different things.
"We know past exposure to other viruses might be relevant, for example.
"So some immune systems are overactive, while others are less equipped to protect against certain types of infections."
What does COVID-19 do to your immune system?
The SARS-CoV-2 virus tends to infect us via our lungs, but can wreak havoc in all corners of our body, says Simon Barry, a molecular immunologist at the University of Adelaide.
That's because the ACE2 receptors the virus hooks onto and uses to infect us aren't just found in lungs: they're everywhere from our gut to our blood vessels.
To fight these infections, our immune system springs into gear. It has different cells that work in different parts of our body to try to expel the virus.
And it appears that even when we might feel recovered after a bout of COVID-19, our immune system is still on the mend.
Professor Barry was part of a study published in January that, over 6 months, measured the different types of immune cells in people who were hospitalised with the Delta variant.
"In general, in most people, there's an awful lot of change immediately after getting COVID, and then it sort of slowly normalises over three to six months," he said.
"But there are some subtle immune changes which seem to last a long time."
They found one type of immune cells, called T cells, were among the slowest to return to normal.
"T cells are where you get an immune memory response, and that's critical for the next time you see the virus — you want to knock it off before you get any symptoms, or you knock it off quickly so you don't get many symptoms."
Similar studies have found other immune cell populations don't bounce back quickly either.
These changes aren't necessarily a drop in number, but more a shift in how immune cells act, Professor Barry said.
"The classic thing in immunology is that the number of the cell type you're interested in has not changed … but then you look at what they do, and you find out actually they've reprogrammed."
So immunologists like Professor Barry are teasing out how COVID-19 changes the function of our immune cells and any knock-on effects in the body.
"We're trying to follow up and ask, what are the actual functional impacts of having had COVID inside individual immune cell types? How do they talk to each other, and is that driving the pathology? Is that what we need to repair?"
Why are some people more likely to get COVID more than once?
Once considered a relative rarity, COVID-19 reinfections are now a dime a dozen, with some people racking up four infections or more.
In July, the Australian Health Protection Principal Committee, which provides advice during health emergencies, revised COVID-19's "reinfection period" from 12 weeks to 28 days, meaning a positive test result more than a month after a previous infection should be counted as a new one.
So who is more likely to get COVID-19 more than once?
People who come in contact with the virus such as healthcare workers are, obviously, more likely to be reinfected, as are people with HIV, diabetes and obesity, according to a Saudi Arabian study that examined Delta reinfection.
Older or immunocompromised people, whose immune system may not be as proficient at fighting off subsequent infections, are at higher risk too.
But regardless of your job or physiology, you can decrease reinfection risk by doing all the things that have been drummed into us since the pandemic started, Dr Griffin said.
"We know people can get reinfected with COVID, and we can't assume reinfections don't cause problems.
"So it's still worthwhile trying to do what you can to prevent each and every infection, and the best way of doing that is some simple strategies about reducing your risk, like social distancing and mask wearing, as well as being up to date with your vaccines."
Having "hybrid immunity" — prior infection plus vaccination — does seem to shore up our defence against reinfection from Omicron subvariants, at least for a little while.
For instance, a Danish study found a combination of previous Omicron infection plus three doses of mRNA COVID vaccine gave people high levels of protection against subsequent BA.5 and BA.2 infections.
A Canadian study during the country's BA.2 wave found healthcare workers who had two doses of mRNA vaccine plus previous BA.1 infection were protected against BA.2 reinfection "for a prolonged period".
Cecilia is keen to get her fourth dose in the New Year, in line with recommendations from the Australian Technical Advisory Group on Immunisation.
"I'll do whatever I can to lower my odds of getting COVID, or even getting sick, as long as possible."
*Name changed for privacy
