Fewer than half of young children in WA are vaccinated against influenza this year, prompting health experts to urge parents to give their kids the jab.
WA's flu season officially started when health authorities recorded a spike in influenza cases in the past week indicating it is spreading.
Uptake of the flu jab has increased this year and, though still much lower than targets, the number of young children immunised also rose significantly.
Flu vaccination rates for children plunged in the State from 2010 when WA child Saba Button was permanently brain damaged and 250 children had serious reactions.
Associate Professor of Paediatrics at the Telethon Institute for Child Health Research Chris Blyth said more than 13,000 children under five have had flu jabs so far this year compared with 8000 last year.
"The uptake is still lower than we need to protect all children," he said. "The rate is way below 50 per cent but encouragingly I think people are being reassured it is a safe vaccine and children can benefit from it."
About 440,000 doses of the flu vaccine were distributed across WA this year, compared with 360,000 last year.
Dr Blyth, a paediatric doctor at Princess Margaret Hospital, said the only way to protect against influenza was immunisation, which had to be given two weeks before infection to be effective.
WA was in the last few weeks for effective jabs, with the influenza season just starting so a rapid increase in affected patients was expected.
Dr Blyth said the winter virus season, when colds and respiratory viruses were more common, was usually between May and October in WA and the influenza season began in late July and lasted about eight weeks.
Though most flu cases are not diagnosed or confirmed, the WA Health Department said 616 cases had been confirmed this year to Monday, far fewer than the 2271 cases last year when the flu season was relatively intense and early.
Dr Blyth said this was one reason why many more people were aware of influenza this year and getting immunised.
He said the adverse events of 2010 made people understandably fearful of influenza vaccinations and a lot of work was done trying to ensure the vaccines were safe.
Fluvax, which caused the adverse reactions, was banned for children under 10.