The Australian reported that CSL is blaming a world-first "blending" of influenza virus strains as the likely cause of convulsions in children immunised with its Fluvax vaccine last year.
A CSL spokeswoman said "our scientific studies indicate that the interaction between the particular virus strains used in the 2010...vaccine contributed to the reactions, but we are still working to understand the how and why."
She added the company "completed comprehensive investigations into our manufacturing operations (which) have not identified any change or deviation in our standard registered manufacturing process that could have contributed to the increased reactions."
An FDA spokeswoman said the agency did not "discuss the details of ongoing investigations or speculate on additional action."
However, Ian Barr, the deputy director of the World Health Organization's influenza research laboratory in Melbourne, which is involved in selecting the infleunza strains for worldwide vaccine production each year, denied the choice of viruses was to blame.
"None of the other manufacturers have seen the same sort of effects with these strains," Barr said. He ruled out testing the WHO's infleunza vaccine before new vaccines were used on people. "It's just not possible because this is more or less a just-in-time vaccine," he said.