It has emerged, for example, that the symptoms of COVID-19 can last for months. Doctors and patients are now dealing with "long Covid".

Research suggests that one in five people who catch COVID-19 have symptoms for five weeks or longer. For around one in ten people, the symptoms last for 12 weeks or longer.

Catching COVID-19 may damage the body in ways which linger. It's not always just a case of catching it and getting over it quickly.

The question scientists are trying to answer is: who is likely to get "long Covid" and why?

"We do know that 80 per cent of people with COVID-19 are likely to have at least one problem still remaining two weeks after the acute effects have stopped," Professor Adrian Esterman, an epidemiologist and biostatistician at the University of South Australia, told this paper.

"And the most common longer term problems are fatigue, headache, difficulty concentrating, hair loss and shortness of breath."

Professor Esterman says that statistically "those with five or more symptoms in the first couple of weeks are more likely to get 'long covid', as are older people and females".

Researchers from St Vincent's Hospital in Sydney followed 78 of the first COVID-19 patients and found that two months after getting sick, 31 had chest difficulties, from fatigue to shortness of breath.

The Australian Heart Research Institute says there is evidence of an increased risk of strokes and heart attacks. "As well as causing severe respiratory problems, there is mounting evidence COVID-19 causes abnormalities in blood clotting," it says.

As the British Heart Foundation explains, "In the initial (acute) phase of the illness, severe COVID-19 can cause pneumonia and respiratory failure, which can result in permanent damage and scarring to the lungs."

Severe flu can also cause complications which linger long after the initial illness.

The sufferers of the most serious attack of COVID-19 are those most likely to have lingering ill-effects.

Italian researchers followed 147 patients who had been hospitalised and found that 87 per cent had symptoms 60 days after they were sent home from hospital.

Another study of patients in Wuhan found that 76 per cent of those taken to hospital had symptoms including breathing problems six months later.

In this Wuhan study, doctors examined patients' lungs with CT scans. "Many of the scans showed splotches called 'ground glass opacities'," according to Stephanie LaVergne, an infectious diseases scientist at Colorado State University.

She thought these splotches on the lungs were inflammations after the COVID-19 had caused pneumonia.

"The people in this study who had severe COVID-19 could not walk as fast as those whose illnesses were less severe - these lung problems reduced how much oxygen was moving from their lungs into their bloodstream. And remember, this was all measured six months after infection."

There is a sizable minority of long Covid sufferers who didn't suffer a bad initial bout, but nobody quite knows why symptoms persist in some patients who have only had a mild case.

One puzzle is why some sufferers from long Covid have a worsened sense of smell.

Researchers wonder if a mild case still manages to alter the immune system so the patient's body then fails to deal with other ailments.

Professor Gail Matthews in Sydney is leading research to "evaluate the effects of COVID-19 on the immune system, heart, lung, brain function and mental health of patients of St Vincent's Hospital, Sydney. Patients will attend the hospital six times over the 12 months following their COVID-19 diagnosis to be monitored by their doctor and to provide blood samples, collect swabs and complete questionnaires."

Vaccination may help. A British study of 44 long Covid sufferers found improvements among those who were vaccinated (which vaccine they had didn't matter).

And even if there were no improvement, there was no evidence of vaccination actually worsening anyone's illness.

As the researchers put it: "Receipt of vaccination ... was not associated with a worsening of long Covid symptoms, quality of life, or mental wellbeing. Individuals with prolonged COVID-19 symptoms should receive vaccinations as suggested by national guidance."

On the latest figures, there have been 29,220 cases of COVID-19 in Australia, with 909 deaths, adding up to 36 deaths per million people.

In the UK, it's 1885 per million; in Germany, it's 905 deaths per million; in the US, it's 1650.

Germany has extended its lockdown for three weeks, imposing an almost complete halt over Easter in response to a "third wave" of coronavirus infections.

France, Italy and Spain have introduced a new clampdown.

The European Union and Britain are in an increasingly acrimonious dispute about access to vaccines.

At least there's some light at the end of the Australian tunnel.

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