Post COVID-19 condition - better known as "long COVID" - has been described since the earliest stages of the pandemic, but even two years later the reasons why some people struggle to fully recover are not known.
University of NSW's Kirby Institute is among several Australian institutions investigating the profile and prevalence of long COVID. Professor Gail Matthews who heads up the institute's therapeutic vaccine and research program, says there are a substantial number of symptoms that could be experienced, which makes it hard to settle a definition for what some people are experiencing.
"In fact, up to 100 different symptoms have been described as part of the long COVID spectrum," Professor Matthews said recently.
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One study - dubbed "ADAPT" - is investigating the persistence of COVID-19 symptoms well beyond the anticipated four-week recovery timeframe among patients at NSW's St Vincent's Hospital.
"Some of our patients who've been very unwell when they've been in hospital with COVID-19 have certainly taken a long time to recover. It could be because they've got scarring in their lungs, or just because they've been very sick in hospital. And that's not too surprising.
"But we also see many people with long COVID, who, in fact, were never hospitalised. They may have had some symptoms at home, but they were managed in the community. It was not severe enough to go to hospital, but they still have symptoms some months afterwards."
Post COVID-19 condition refers to a set of symptoms that remain with someone for a period after recovering from COVID-19.
As well as being described by people still experiencing COVID-19 symptoms, studies have shown some people continue to experience the effects of the disease three months later.
A large 2021 survey by the Office of National Statistics (UK) found more than one in five Britons reported symptoms five weeks after infection, dropping to just under 10 per cent after 12 weeks. A more recent study published in Nature Communications found symptoms extending beyond 12 weeks were reported in 7.8-17 per cent of cases.
Research published in The Lancet Regional Health - Western Pacific in 2021 found five percent of those with COVID-19 in New South Wales continued to experience symptoms after three months.
The Centres for Disease Control and Prevention (US) says post-COVID health problems could last weeks, months or even years.
It's hard to land on an exact proportion of cases which experience long COVID because there's no universally accepted definition. The WHO lists fatigue, shortness of breath and cognitive dysfunctions among others in its long COVID definition. The Australian Department of Health definition adds heart palpitations, chest pain or tightness, changes to taste and smell and joint and muscle pain.
The British National Health Service adds insomnia, pins and needles, depression and anxiety, tinnitus and earaches, gastrointestinal issues and other symptoms on its guidance. The persistence of any one of these symptoms after recovery from COVID-19 could indicate a person has long COVID. Authorities recommend re-testing to rule out a new COVID infection as a repeat infection can occur within 28 days.
There are no specific treatments for long COVID, and because medical understanding of post-COVID symptoms is still developing, health authorities recommend seeing their healthcare provider or general practitioner.