EXPOSURE to last year's swine flu outbreak may have predisposed children to the severe reactions they are now showing to influenza vaccinations.

That is one of the theories as mystery continues to cloak the reason why more than 250 children in Western Australia have been reported to have become sick after receiving the seasonal flu vaccine. The death of a Brisbane infant girl has also been linked to the vaccination.

As of yesterday, 55 children had suffered febrile convulsions after receiving the vaccine and a further 196 had experienced less serious reactions such as fever, vomiting and "grizzliness".

Professor Peter Collignon, an infectious diseases expert at the Australian National University, said a possible explanation was that the children's brush with swine flu last year could have made their bodies more likely to react aggressively to an injected dose of the vaccine, which includes the swine flu strain.

But Professor Terry Nolan of the University of Melbourne, who led research into children's reactions to swine flu vaccine last year, said that explanation seemed unlikely given that the children did not exhibit more severe reactions to the second of the two injections they were given last year.