A report published online by The Medical Journal of Australia yesterday said comparing the effect of swine flu to strains of seasonal flu was difficult because much less laboratory testing was done during a normal winter than in a pandemic.

The head of the epidemiology unit at the Victorian Infectious Diseases Reference Laboratory, Heath Kelly, and his co-authors said that while the data suggested the age at which people had become seriously ill or died of pandemic human swine influenza A(H1N1) 09 was younger than in seasonal flu, exact comparisons were difficult.

Eighty-five per cent of deaths from seasonal flu occur in people over 65, many of whom have underlying conditions.

But the estimated total of 18,400 hospital admissions for influenza in Australia each year is derived from modelling rather than laboratory testing of patient' swabs.

"The increased testing associated with the response to the pandemic highlights how little diagnostic testing is done for seasonal influenza, a fact that has ramifications for our understanding of public policy relating to influenza control," the authors wrote.

When it hit in April, swine flu sparked a huge jump in the number of tests, from 16 in the first week of surveillance to 123 in the ninth.

An analysis of the first 244 patients identified as having swine flu found the median age in Western Australia was 22. In Victoria, in the first 135 patients it was 21.

More tests could help to clarify the effectiveness of an influenza vaccine, thought to be 50 per cent but judged by some to be only 5 per cent.