WHEN Sienna Rose Pereira caught a deadly strain of the common influenza virus, it was just too overwhelming for her little body.

Her heartbroken parents noticed the tiny two-year-old had become visibly unwell earlier this month and her mum rushed her to see a doctor.

Deeply concerned about the symptoms Sienna was showing, the GP immediately sent the her to Sydney Children’s Hospital at Westmead.

But her condition deteriorated and she died just 24 hours later last Sunday when her life support was withdrawn.

According to a statement on a fundraising website established to support her grieving family, Sienna had developed a high fever and swelling developed around her brain while she was being treated in hospital.

She was not vaccinated and she was among 19 other kids admitted to Sydney Children’s Hospital at Westmead between April 3 and July 8 with the flu.

Worryingly, out of all these children, only two were fully vaccinated.

The state’s authorities say Sienna’s death should fire a warning shot for parents who haven’t had their kids vaccinated — because the worst of this year’s flu season is yet to come.

“We know that 15 of the children [admitted to Westmead] were eligible for the free flu vaccine but only two of them had been fully vaccinated against flu,” NSW chief health officer Dr Kerry Chant said in a statement.

“Flu case numbers across the state are starting to rise and influenza A (H1N1), which mostly strikes children and young adults, is the key strain circulating in the community.”

Sienna’s tragic death is the first of the 2018 flu season affecting a child and it was caused by the deadly influenza A strain, which was at the centre of the 2009 pandemic.

It’s been a slow start to the flu season in NSW, with 256 cases confirmed in the week ending July 8, compared with 6449 cases in the equivalent week in 2017.

Almost all of the 256 cases had contracted influenza A, the key strain circulating in NSW and mostly striking children and young adults.

In May, doctors across the country had to ration shots for the most vulnerable because of unprecedented demand for the jab.

Health authorities said they had become a victim of their own success, with doctors, the Australian Medical Association and governments’ push to encourage people to get vaccinated working so well they now can’t keep up.

However, after the tragic death last Sunday, Dr Chant was keen to stress there were “plentiful” supplies available of the free flu jab for under-five-year-olds.

“This is an important reminder to parents who have not yet vaccinated their children that influenza can be life-threatening and it’s not too late to vaccinate,” she said.

HOW DO I KNOW IF IT’S A COLD OR FLU?

Flu symptoms can hit children from nowhere, but there are often hints that can let you know if it’s a common cold or worse, according to Australian Medical Association vice-president Dr Tony Bartone.

“The virus can progress quite quickly in people of all ages, usually within in a few hours or a day,” Dr Bartone told news.com.au in response to last year’s killer flu season. “It usually starts with feeling sniffily and there will be minor aches and pains; they are also the symptoms of a common cold, so it’s often hard to tell the difference at first.

“However, if you notice that your child starts to have a high fever, muscle spasms at the back of the head or legs and they are struggling to get out of bed, then it is likely to be the flu and you should probably make a call and get some medical advice.

“If they are not responding to Panadol or other medication for treating high fever, they are not responding to fluids or keeping them down, they become dizzy and find it difficult to catch their breath, then the symptoms of flu have become severe and you should contact your doctor.”

Although the flu subtype influenza A (H3N2) — which was named as the deadliest strain of the virus in last year’s record-breaking flu season — is harder to protect against via a vaccine, it’s still our best barrier against the virus.

“It’s the only thing we have to protect ourselves and our children,” Dr Bartone said.

“We know that children, compared to the rest of the population, don’t tend to be immunised — with take-up of free vaccinations as low as 10 per cent in some areas of WA. And, we know that children between six months and two years are more susceptible to the complications of flu.

“Children also tend to make a lot of physical contact with one another and their parents, so we’ve got to make sure we’re being hygienic as possible. This means, coughing into tissues and disposing of them, washing our hands and keeping our immune systems in check by not running ourselves down and eating healthily.”