When Lyndall Heather caught COVID-19 for a third time this year, she initially didn't believe she had it.
Just six weeks on from her second COVID infection, the Darwin nurse was well within the immune period that precluded her from testing.
"I just thought it's pretty unlikely, it's probably just the cold," she said.
But after becoming seriously unwell, she went for a PCR test and found out she had contracted a separate COVID-19 infection.
"Even my manager found it confusing. I guess things are constantly changing," Ms Heather said.
Hampered by long COVID symptoms like fatigue and brain fog, Ms Heather said her latest reinfection has left her anxious for the future.
"I feel like I can't possibly get it a fourth time but unfortunately now that I've had it a third time, it's a very real chance that I could get it again," she said.
How common is COVID reinfection?
Ms Heather is one of thousands of Australians who have now battled COVID-19 multiple times, but reliable figures on reinfections are hard to find.
Counting the reinfections relies heavily on self-reported data submitted to health departments in each state when someone tests positive on a RAT, figures which have been historically under-reported.
Ms Heather's second COVID-19 infection was asymptomatic and was only picked up by routine workplace testing.
Health authorities believe the true number of Australians who have been infected is much higher than official tallies.
States hardest hit by COVID-19 such as New South Wales and Victoria have rough figures detailing reinfections in the tens of thousands.
Other states, such as Tasmania, have only recently begun tracking reinfection or, in the case of Queensland, are not tracking it at all.
Despite a lack of quantitative data, audience submissions to the ABC show a large swathe of Australians are battling their second and even third COVID infections.
Looking internationally may provide insight into how reinfections have surged in 2022.
What do we know about reinfections globally?
A two-year study in the Serbian province of Vojvodina found a sharp increase in COVID-19 reinfections following the rise of the Omicron variant at the start of 2022.
Stanford University epidemiologist John Ioannidis co-authored the study, published in The Lancet Regional Health - Europe, and said the research was an effort to combat the lack of documentation about reinfection.
"It's clear that the number of reinfections, much like the number of infections, is underestimated," Professor Ioannidis said.
The results from the study were staggering.
Of the 13,792 COVID reinfections recorded in the province between March 2020 and January 2022, almost 87 per cent occurred in January 2022 alone.
Almost all third infections recorded in the study also occurred after October 2021, during the period of Omicron circulation.
It's illuminating data that points towards the explosive transmissibility of Omicron when it enters a community.
Data suggests the initial BA.1 Omicron strain is three times more transmissible than the original Wuhan strain of COVID.
Professor Ioannidis said that while increased cases could be an artefact of increased testing, the impact of the Omicron strain was evident.
"It’s real, with Omicron we have far more infections," he said.
"I think this probably continues downstream with the BA.4 and BA.5 strains that are now dominant in many countries including Australia."
Early data from South Africa suggests the BA.4 and BA.5 strains could be almost six times as transmissible as the Wuhan strain.
Chief medical officer of the government-funded healthdirect service Nirvana Luckraj said COVID infections could now occur more rapidly in succession than before.
"The COVID sub-variants are more likely to evade the immunity gained from previous infection, and reinfection is possible just weeks following a past infection," Dr Luckraj said.
As a result, Australia has now dramatically shortened its reinfection period for testing from 12 to four weeks.
Who is getting reinfected?
The Serbian study found reinfected patients were significantly younger, more commonly female and more frequently employed as healthcare workers.
Australia's health workforce is predominantly female and the average worker is aged between 20 and 34.
It means people like 28-year-old nurse Ms Heather are some of the most likely to be reinfected.
Professor Ioannidis said more regular testing schedules for working young adults and higher interactions with others was reflected in the numbers.
"I think it's probably a reflection of the fact that younger people, particularly younger adults, had much higher levels of exposure," Professor Ioannidis said.
"The good news is that for that population, the risk of serious disease is very low."
What are the health impacts of reinfection?
There is conflicting data on just how much more dangerous second and third COVID infections are.
A United States study of almost 39,000 reinfections from the Department of Veteran Affairs found "reinfection adds risk of all-cause mortality and adverse health outcomes".
"Studies are showing that reinfection has higher health risks, particularly for those with underlying health conditions, and is linked with a higher risk of long COVID," Dr Luckraj said.
However, Professor Ioannidis said data from the Serbian study appeared to show a less severe health impact from reinfections.
"What we have seen, at least in our analysis, is that with reinfection the risk of hospitalisation is four times lower compared to the original infection and the risk of death is 10 times lower," he said.
"So far it seems like [reinfection] is very frequent, but it's not severe."
Will the reinfections ever end?
Given the spike in Omicron reinfections almost two years after the pandemic, concerns have shifted to COVID's next evolution.
Dr Luckraj said the BA.4 and BA.5 sub-variants would likely not mark the end of the virus.
"The natural history of the virus is that it constantly evolves to survive, so I think we can expect to see more sub-variants emerging," Dr Luckraj said.
Professor Ioannidis said while the virus was here to stay, Australians should not panic.
"There's no guarantee of what the next variant will look like but what we have seen so far is commensurate with an evolution to an endemic phase," he said.
"We can live with that. It would be wonderful if we could get rid of this coronavirus completely, but it’s very unlikely."
