Australia has had 100 times more flu cases in January and February than the first two months of last year.
Unlike COVID-19, influenza hits younger demographics hardest, and pediatrician Daryl Cheng says a mix of vaccine fatigue and hesitancy among parents has put young children at greater high risk of ending up in hospital.
"Flu is a very different beast," he told AAP.
"It affects kids under five, lands them in hospital and has significantly disproportionate impacts on younger children."
A Royal Children's Hospital Melbourne survey found half of Australian children were likely vaccinated against the flu last year.
It revealed one in two parents didn't know it was safe to have the flu and COVID vaccines together, while 43 per cent believed the flu shot wasn't as important and a third didn't know the flu could seriously affect children.
"There were many parents who didn't get their infants vaccinated in the first year of life because of restrictions or other priorities, so that population immunity has not built up," Dr Cheng said.
Over the past three years, the country has recorded fewer flu cases because measures taken to prevent the spread of COVID also protected Australians from influenza.
But as the world emerged from the pandemic, there was a resurgence of flu.
In the first two months of 2022, the federal Department of Health and Aged Care recorded 79 cases. In the same period this year, it counted 8453.
The compounding impact of more human interaction, international travel and reduced vaccine uptake fuelled the surge.
Flu traditionally moves from hemisphere to hemisphere with the winter seasons and if that timing is interrupted, it impacts the number of cases and level of flu community immunity.
During the pandemic, travel restrictions stopped the disease entering Australia and caused the 2022 flu season to start in late March, months before it would have pre-COVID.
"In terms of duration, peak and timing ... everything has changed," Dr Cheng said.
"There is a time window, where it is optimal to get this vaccination, it is not something that should be postponed indefinitely.
"If the vaccine isn't ready early enough and the flu season starts earlier, immunity can be affected."
Last year 56 per cent of hospital patients with flu were children younger than 16.
Those aged five to nine were most likely to be hospitalised, making up 2154 admissions per 100,000 members of the population, followed by children under five who made up 1859.
Many younger patients ended up on ventilators or feeding tubes as the flu affected their immune system, exposing them to secondary infections like pneumonia.
"In severe cases, both heart and lung machine support may be required to allow kids' bodies to recover. Unfortunately, some don't recover and a very small proportion die," Dr Cheng said.
"Vaccination is important. It prevents severe disease and some transmission and it's something parents need to be aware of."
