Australia is headed for a severe flu season after a big increase in the number of confirmed cases of influenza in summer and autumn.

There have already been 40,000 laboratory-confirmed cases of influenza in 2019 - about three times the usual number recorded at this time of year.

In an average year, 3,000-4,000 people will die from flu-related complications.

Those most at risk from flu are the very young and the very old - but Australian society suffers greatly as a whole.

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Influenza is a major cause of both absenteeism and presenteeism during the autumn and winter months, with more than 48,000 cases of influenza being reported across the country in 2018 - and that was a quiet year.

Absenteeism - when a worker is absent from work due to illness - costs $7 billion dollars each year in lost wages, while presenteeism - that's when a worker turns up while unwell - costs $34 billion dollars in lost productivity.

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Professor Robert Booy from the Sydney Medical School said this year's flu season may be quite different to 2018.

"2019 has been really strange. There has been a sustained and rising autumn surge that began at the end of last year and is continuing to increase. The best explanation - and it's only an immunological guess - is that 2018 was so quiet that we have reduced community immunity."

Professor Booy said that those most at risk from flu are the very young and, particularly, the elderly.

"More than 90 per cent of the deaths were in people over 65, especially over 80," Professor Booy said.

"A quarter of the flu notifications occurred in children and one-third of hospitalisations were in children under two years of age; so it's the very old and the very young. The very young get hospitalisation but rarely die, the very old - hospitalisation and death."

Professor Bill Rawlinson is a senior medical virologist at the University of New South Wales. He said flu should not be underestimated and vaccination can make a significant difference to outcomes.

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"The at-risk groups - people with diabetes, people with asthma that's severe, people with heart disease, Aboriginal and Torres Strait Islanders - those risk groups are those who get more severe disease and die from flu so it's particularly important that they get vaccinated every year," he said.

"And of course it's important in the elderly, particularly those over 65 years of age, that they get vaccinated and it's important to think about the risk to them of acquiring it from their family."

Professor Rawlinson said simple measures such as washing your hands, coughing into your arms and using tissues can also help prevent the spread of the disease.

Meanwhile new research is showing that there are other groups who are at risk - those who are overweight or obese, and those who are suffering from diabetes.

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Dr Kirsty Short is an Influenza Virologist at the University of Queensland.

She said 75 to 80 per cent of middle aged men are overweight - and diabetes rates in Australia, both type one and type two, are increasing - particularly type two. Diabetes affects one in ten Australian adults.

She said those metabolic conditions have emerged as a significant risk factor.

"In the 2009 so-called 'swine flu' epidemic, what we saw was that both obesity and diabetes for the first time emerged as susceptibility factors for severe influenza."

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"What we saw was that in this pandemic people with either obesity and diabetes were significantly more likely to be hospitalised with the flu, they were significantly more likely to be admitted to the ICU and were significantly more likely to die from the virus."

Dr Short said that obesity can also impair how effective the flu vaccination can be.

Professor Booy said key to protecting at risk groups is herd immunity - in other words, getting as many people effectively vaccinated against the disease as possible.

"You don't need 95 per cent like with measles - if you have 50 per cent of the population vaccinated there will be at least some degree of herd immunity and that's something to aspire to"

America's Centres for Disease Control and Prevention said flu vaccines have a good safety record.

Hundreds of millions of people have safely received flu vaccines over the past 50 years, and there has been extensive research supporting the safety of flu vaccines.

The organisation said a flu vaccine is the first and best way to reduce your chances of getting the flu and spreading it to others who may be more severely affected than you are.

Additional reporting: AAP