The maker of the flu vaccine now implicated in a rash of child sickness across Australia - and possibly one death - denies that a "bad batch" is involved.
A spokesperson for Melbourne-based pharmaceutical company CSL says a check of available batch numbers listed on adverse reaction reports had indicated no single batch could be responsible.
West Australian health authorities have now received more than 250 reports of child sickness linked to the vaccine.
A two-year-old Brisbane girl has died but Queensland's chief health officer says an initial autopsy has shown no sign that the vaccine was to blame for the death.
However further tests are underway in that state.
GPs nationwide have been told to temporarily stop administering the vaccine to children aged under five.
"CSL is continuing to work with the Therapeutic Goods Administration (TGA) and WA Health to urgently investigate the adverse event reports," the CSL spokesperson told AAP on Tuesday.
"What we have been able to analyse so far indicates that it is not batch related ... our evidence shows it was not a bad batch."
The spokesperson said CSL was now working to finalise numbers on how many doses of its FLUVAX vaccine were sent to WA.
WA was the only state to offer a specific program offering the injections for free to healthy children under five, which may account for its higher rate of adverse reactions compared to other states.
The lack of a problematic batch indicates that other factors - such as practices at immunisation clinics - are likely to be drawn into the TGA's investigation.
WA started its pediatric vaccination program on March 19, and related adverse reactions among children have included vomiting and high fever leading to fits or convulsions.
A one-year-old girl remains in a serious but stable condition in Perth's Princess Margaret Hospital after receiving the vaccine.
Queensland's coroner is investigating the tragic case of toddler Ashley Jade Epapara, a two-year-old who was found dead in her cot on April 9 just 12 hours after receiving the seasonal flu vaccine.
All state and territories have been asked to provide data on other possible cases of adverse reactions in children, though a TGA spokesperson said the problem appeared to be centred on WA.
"To date, the pattern and rate of adverse reactions reported by WA have not been seen in other states," the spokesperson told AAP.
"The TGA has contacted the vaccine suppliers to confirm which batches of vaccine were used in WA, and is obtaining samples of the vaccine to test in its laboratories to determine if there are any abnormalities."
The spokesperson said the investigation would take in the content of the vaccine as well as "factors related to the program delivery in WA".
Seasonal flu vaccines are made available ahead of every Australian winter, and their make-up is dictated by the World Health Organisation.
These vaccines are trivalent - they offer protection against three influenza virus strains expected to be in circulation.
This year's seasonal flu vaccine offers protection against the A(H1N1) swine flu, along with an Influenza B strain and an A(H3N2) virus.
CSL is the major distributor and the only Australian-based maker of the vaccine though versions can also be imported into Australia by rival pharmaceutical companies.
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