THE US drugs watchdog has slammed Melbourne-based drug manufacturer CSL for mounting an ''inadequate'' investigation of convulsions suffered by children who had used its flu vaccine.
CSL withdrew the vaccine, sold as Fluvax in Australia and Afluria in the US, for use on children under five in June last year after it was linked to convulsions and fevers.
But the company has yet to discover why the dangerous side effects were far more common in young children vaccinated with Fluvax than those who used other brands.
In a letter sent to CSL, the Food and Drug Administration lists 15 ''significant deviations'' from drug manufacturing standards uncovered during an inspection of CSL's Parkville facility in March.
''You failed to thoroughly investigate any unexplained discrepancy, or the failure of a batch [of vaccine] or any of its components to meet any of its specifications,'' the FDA's head of compliance, Mary Malarkey, said in the letter, dated June 15.
Ms Malarkey told CSL that their response so far had failed to take a ''more global approach'', including closer scrutiny of raw materials used during drug making.
CSL spokeswoman Sharon McHale said that the company had 15 days to submit a detailed response.
''We're working on our response now and will meet that deadline,'' she said.
The company will then meet with the FDA, probably in the first half of next month.
''If they say our response is adequate we'll continue to implement our action plan,'' Ms McHale said.
''If they feel as though we haven't complied, then they can take regulatory action.''
Ms McHale said that if the FDA was not comforted by CSL's response, ''it can impact our ability to provide vaccine into the US market''. Flu vaccine forms a relatively small part of CSL's $4.6 billion a year in revenue, with worldwide sales last financial year totalling $124 million, of which $53 million was sold in the US.
Convulsions hit between five in 1000 and seven in 1000 children under three vaccinated with Fluvax last year, according to a study by Australian drug regulator the Therapeutic Goods Administration. The baseline rate, observed in a five-year US study, is less than one in 1000.
A TGA survey of hospital data also found children under five vaccinated with Fluvax were between three and 6.5 times more likely to have a fever than users of competing products.
The TGA said that as a working hypothesis a higher concentration of the enzyme neuraminidase due to the inclusion of a swine flu vaccine might have caused the additional convulsions.
''However, despite extensive analyses the biological basis for the excess cases of fever and febrile convulsions remains unclear,'' the TGA said in its report, issued in October.
CSL no longer sells Fluvax for use on children under five and its use on children aged between five and 10 is now only recommended if no other vaccine is available.