A thorough investigation into CSL Ltd's seasonal flu vaccine that includes the H1N1 pandemic strain is now being performed by Australia's drug regulator, after the flu shots were linked with adverse reactions in children.

Kay McNiece, a spokeswoman of the drug regulator said the Therapeutic Goods Administration (TGA) have asked all states and territories to report all cases of fevers among children under five, even ones that do not directly involve flu vaccinations to widen the scope of investigation, in order to find patterns that might explain the incidence.

The regulator said children under five should stop receiving CSL's Fluvax and the company said it had stopped the distribution of the product after complaints of fits and fevers among children.

Alan Hampson, chairman of the Australian Influenza Specialist Group said the reports of possible side effects may deter parents from vaccinating their children even if the shot is proven to be safe.

There'll be collateral damage, said Hampson.

It's a bit of a disaster not only here, but globally in casting apparent doubts on the vaccine, which is potentially quite valuable in young children.

Western Australia's health department had reported 55 children under five have experienced convulsions after being inoculated by the vaccine and an additional 196 has suffered from less severe reactions such as fever, vomiting and inflammation at the site of injection.

A study of 298 children ages 6 months to 8 years old who were vaccinated with Fluvax last year found that 3.4 per cent experienced severe fever and one child had convulsions.

The risk of whether or not children have a convulsion with a fever is usually age-related, said Jodie McVernon, senior researcher at the University of Melbourne who was involved in the study.

The younger you are, the more likely you are to have a fit with a fever, and it's something that you grow out of.

Australia Medicine