Australian researchers have found a way to predict whether a person's dose of swine flu is likely to turn life threatening, in a globally significant development.
Melbourne-based Professor Lindsay Grayson said it offered doctors a means to flag those patients who were most likely to experience the worst complications from a swine flu infection.
The discovery also pointed to a potential new treatment and an explanation for the most puzzling aspect of the global A(H1N1) pandemic.
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That was, Prof Grayson said, the virus's ability to be a "pretty mild disease" for the majority while also striking young and apparently healthy people.
"The unusual thing about swine flu is that, if you compare it to seasonal influenza, it mainly affects people aged 15 to 35," said Prof Grayson, who is director of infectious diseases at Austin Health.
"It's very unusual for us to see a 22-year-old about to die with (ordinary seasonal) influenza.
"Our thought is there is something special about swine flu and its interaction with immunoglobulin."
Prof Grayson found those swine flu patients who become the sickest were likely to have a pre-existing deficiency of a specific blood protein (immunoglobulin G2 or IgG2) that is crucial to the proper functioning of the immune system.
It was a chance discovery, he said, sparked by the treatment of a critically ill pregnant woman at the height of Australia's outbreak of swine flu last year.
Doctors ordered a non-routine test on the woman's blood that returned an unsuspected result, and the same test was then conducted on all swine flu patients in the intensive care ward.
"We suddenly saw almost all of them were low, unexpectedly, in IgG2 ... this was the first time anyone had noted this," Prof Grayson said.
"Almost all the patients in the ICU were all short of this protein, their levels were really, really low."
Prof Grayson said up to 20 per cent of the population was known to have low immunoglobulin levels, and those with IgG2 deficiencies appeared to be at heightened risk from swine flu.
Many people with these deficiencies found out after suffering repeated chest and ear infections through their childhood.
"But there's a fairly large chunk of them that would never know," Prof Grayson said.
"What we're hypothesising is that this may be an explanation for the very curious phenomenon that swine flu for the majority of the population is a pretty mild disease.
"But if you happen to be unrecognised and naturally IgG2 deficient, (swine flu) is going to affect you more."
The discovery also suggests a potential new treatment for a swine flu infection, as people low in protein could have their levels topped up from donated blood.
The World Health Organisation was alerted to the finding last year, and a paper detailing the research is published this month in the journal Clinical Infectious Diseases.
