Two out of three Australian patients who become seriously ill with Covid-19 are left with ongoing health problems six months later, say experts tracking patients' recoveries.
The researchers shared early results from a study that covered 30 hospitals in Australia, with collaboration from researchers, physiotherapists, doctors, and nursing staff who work in intensive care units.
Dubbed the Covid Recovery Study, experts have been interviewing about 200 patients to learn more about their health outcomes six to 12 months after infection.
Monash University professor Carol Hodgson said the team had yet to analyse the final dataset, but the early results showed about 70 percent of patients report ongoing problems six months after contracting the virus, though most were mild.
These symptoms included mild shortness of breath and weakness, and a small number have a persistent cough, headache, or loss of taste and smell.
About 30 percent of people were "disability free" six months after contracting the virus.
Patients who had already been critically ill with a wide range of infections were more likely to report lingering issues, Hodgson said.
The study was aiming at figuring out whether there were any "unique symptoms and long-term outcomes" of Covid-19 disease that differ markedly from patients recovering from other illnesses.
"The question isn't whether Covid can have an impact, the question really is whether Covid is any different from what we usually see with critical illness," Hodgson said.
'Very specific symptoms' of Covid-19
Various international researching teams are also watching the long-term impacts of Covid-19, and published studies are beginning to emerge.
One Swedish study found 15 percent of people experienced mild Covid-19 suffered symptoms for at least eight months after infection.
And a separate US study found people who tested positive to Covid-19 were more likely to need future medical help for a range of issues such as respiratory conditions, nervous system disorders, mental health problems and fatigue.
Burnet Institute medical research fellow Joseph Doyle said there were "definitely some very specific symptoms", such as weakness and tiredness, which lingered following Covid-19 infection in some patients.
But Dr Doyle said other serious viral infections could also result in "chronic fatigue-type symptoms".
Patients who had been critically ill with influenza, for example, could have lingering issues such as tiredness and reduced physical function for up to five years post-recovery.
Dr Doyle stressed Covid-19 was "far more serious" than influenza as both an initial infection and its potential longer-term impacts.
"[Covid-19] clearly has a much higher chance of killing you when you get it than the flu does, and a much higher chance of getting you into hospital and intensive care," he said.
But not everyone will experience these ongoing issues.
"Most people do recover, and most people do recover fully," Dr Doyle said.
"If you were fortunate to have a very mild Covid infection you're probably not going to get any of these problems at all, you're probably going to make a complete recovery."
How Covid-19 causes ongoing symptoms
Dr Doyle explained that because Covid-19 disease was predominantly a lung infection, there was a risk of lung damage.
"For people who have had more severe [Covid-19] disease sometimes there's this problem [post-infection] where they lack the exercise capacity, the fitness, the lung reserve, and that can take weeks and months to recover," he said.
"Some people clearly get these things worse than others, and we don't understand why."
And we don't yet know when, or if, they will recover.
For some very sick patients their immune system becomes overactive, which means their body can't tell the difference between normal cells and infection.
"Covid doesn't just affect the lungs and make you sick like some other viruses do."
"[You] end up getting blood clots, you can end up getting heart attacks, you can end up getting strokes, and those diseases themselves can have long-term consequences as well," Dr Doyle said.
And hospitalisation itself causes independent issues.
"If you spend a long time in the ICU, you always have those problems, no matter what condition puts you in the ICU," Dr Doyle said.
Professor Hodgson agreed and said half of the critically ill Covid-19 patients in her study needed ventilators while in the ICU, which meant they were hospitalised for long periods.
"The length of care for patients who were ventilated is about two weeks," she explained.
"Four weeks [total] in hospital, that does result in significant ICU-acquired weaknesses."
Professor Hodgson said this weakness was "more than just the result of bed rest."
It's partly due to a "cytokine storm", where the body's cells release a type of proteins called cytokines, which cause inflammation and a breakdown of proteins in the muscles.
"We know that this [muscle breakdown] can be quite prolonged and can last up to five years in other types of critically ill patients," Hodgson said.
What about the mental impact?
Professor Hodgson said so far the symptoms being reported in their Australian study were more likely to be physical problems, like tiredness or weakness, rather than psychological issues such as anxiety.
But that's not to say that those recovering from Covid-19 disease aren't struggling with mental health challenges.
One UK study found that in the six months after a Covid-19 infection, one in three people had experienced a psychiatric or neurological diagnosis.
Professor Hodgson said based on preliminary data, the rates of anxiety and depression in Covid-19 patients were less than 20 percent - on par with other patients recovering from critical illness.
"One thing we are seeing is ... an increased number of patients who test positive to being at-risk of post-traumatic stress," she said.
Dr Doyle said a lot of people who'd been critically sick for long periods of time come out of hospital with psychological sequelae - mental health conditions that can result from disease or trauma, such as acute stress disorder. And these could emerge days, months, or years afterward.
He said the infection itself had a high risk of stroke, which led to a lack of oxygen and could impact a patient's mental health and brain function.
When will we know more about the long-term impact?
Time will tell.
To understand the impact Covid-19 disease could have in five years, experts need to literally wait five years to look at the patient data.
"The research is starting to paint a picture, but it's early days," Dr Doyle said.
"It will take a number of years to really know whether people who have been sick in the last year end up with lingering problems, or whether it's just a few months of symptoms."
Professor Hodgson said for now, the best thing people could do was get vaccinated.
"The one thing that we can do to protect Australians at this point in time is to have as many people vaccinated as possible for the coming winter," she said.
Dr Doyle agreed.
"I think that anyone who is offered a vaccine should be seriously thinking about taking it up," he said.
"If you can avoid getting infected and avoid getting a more serious type of the illness, you're more likely to avoid getting more of these long term consequences of the virus.
"Vaccines are going to be our best protection in the long run."