The face of Sydney's deadly flu season

Her father Neeraj and mother Anjali had little warning. Their vivacious daughter was in high spirits two days earlier when her father surprised her at pick-up outside her daycare in Ryde.

"She saw me and came running to me in excitement," Mr Idnani said. "She was so happy and alert and full of life. I remember she reminded me, 'Papa we forgot my water bottle.' "

"Her not being here three days later ??? it's something we could never have imagined," he said.

Vanika died with influenza on Saturday, July 29, just as this year's flu season was building to a crescendo.

Three children have died of influenza-related illnesses in NSW in 2017 so far. NSW Health recorded 288 flu deaths overall from January 1 to September 15.

An eight-year-old Victorian girl died of flu on September 15 while a Victorian mother remains in an induced coma six weeks after she gave birth to a boy on August 28. A young Victorian father also died from flu on his first Father's Day on September 3. In Victoria, 121 influenza-related deaths have been recorded.

The infectious disease is the most common cause of hospitalisation among vaccine-preventable diseases, and killed more children in Australia than any other such disease between 2005-2014, followed by meningococcal, a NSW Ombudsman report found. Children up to four years old are most at risk.

Seven children under five died during the 2007 flu season. Eleven died during the H1N1 pandemic, according to the report.

On Friday, July 28, Vanika had a fever, was lethargic and complained of joint pain and a tummy ache, her father said. She vomited twice during the day, but rallied briefly.

"She was singing in the shopping centre when we were buying gifts and decorations for her birthday," Mr Idnani said.

Her mother took her to their GP, who took a throat swab. Mrs Idnani said the GP also checked her breathing and other vital signs, and mentioned she had the symptoms of an ear infection.

"We thought she would be fine," Mr Idnani said. "We thought 'it's just a normal virus.' "

On the Saturday night, Vanika fell asleep with her father next to her.

"I woke up in the night and I suddenly realised she was not moving or breathing ??? I tried to move her but her hands were like stone," Mr Idnani said.

Vanika was rushed by ambulance to Westmead Children's Hospital, but she did not survive.

It took another day before her parents discovered she had influenza when they received the pathology results from her throat swab.

"We have so many questions ... we don't know the complete picture," Mr Idnani.

Her case has been referred to the coroner.

Vanika's parents hope her case will serve as a warning to other families that even healthy children can be severely affected by the flu.

"We want there to be some awareness that these things could happen to anyone," he said.

Infectious diseases expert at the University of Sydney Robert Booy said about half of the children who died from influenza had no known underlying condition.

"The majority of flu deaths in children are before they get to school age," said Professor Booy, Head of Clinical Research at the National Centre for Immunisation Research and Surveillance in Australia.

Often these children died of a secondary infection, including meningococcal disease. Flu and other viral respiratory infections are known risk factors for meningococcal disease, Professor Booy said.

Almost everyone will carry harmless meningococcus bacteria in their nose and throat at some stage in their lives. The flu can damage the lining of their respiratory tract, allowing the bacteria to "invade" their bloodstream, becoming a harmful infection.

Professor Booy said antibiotics may mute the presence of meningococcal and skew the test results so it didn't get picked up.

"This year we have had an increased rate of meningococcal disease," Professor Booy said. "We should be asking, could this be because we have had so much flu?"

But flu and meningococcal deaths among children were still rare, Professor Booy said.

NSW Health director of communicable disease Vicky Sheppeard said if parents suspected their child might have the flu they should give them plenty of fluids, rest, pain relief for aches and pain, and take them to a GP if they were worried.

Parents should bring their child to hospital if they had severe headaches or neck stiffness, difficulty breathing, if they were lethargic and not drinking enough fluids, Dr Sheppeard said.

Professor Booy said rapid breathing and becoming pale or blue should also prompt families to seek medical attention.

Professor Kanta Subbarao, director of the WHO Collaborating Centre on influenza at the Doherty Institute, said the influenza A strain H2N3 - responsible for 60 per cent to 65 per cent of cases this year - could severely affect young children, as could influenza B strains.

WHO has recommended modifying the H2N3 component of the influenza vaccine next year, after finding this year's vaccination provided "suboptimal" protection as low as 40 per cent against the strain.

"We all hope we won't see anything like this [year's flu season]," Professor Subbarao said, adding the strain could mutate, significantly affecting the effectiveness of the new vaccine.

Like most children, Vanika was not immunised against influenza. The annual flu vaccine is recommended for children aged six months and over to reduce the likelihood of becoming ill with influenza, but it is not part of the national immunisation program.

West Australia has offered a free annual flu vaccine for all children under five years old since 2008 but fewer than 10 per cent were immunised last year.

Professor Booy said the program should be rolled out nationally.

"It is certainly time to look at the double benefit of vaccinating young children to directly protect them and also prevent transmission to adults, including the elderly," he said.

Mr and Mrs Idnani laid their daughter to rest in her birthplace of Bangalore before returning to Sydney.

"It's very hard ... to be surrounded by so many memories of her," Mr Idnani said.

Flu symptoms in children include cough, fatigue, muscle aches and high fever that can last two to three weeks. Exhaustion is an early prominent feature of flu and children may also have nausea and diarrhoea.