THE swine influenza virus could fizzle out, trigger a "mild" pandemic still capable of claiming many lives or become a historic killer, a major medical conference has been told.

Renowned European virologist Albert Osterhaus urged watchdogs to keep up their guard, even if the new strain of H1N1 virus had turned out to be less worrisome than thought a few weeks ago.

"The current H1N1 threat is serious," Professor Osterhaus said in a briefing to the European Congress of Clinical Microbiology and Infectious Diseases (ECCMID) in Helsinki, the first big medical forum to be held since the crisis erupted on April 24.

"The virus has three options," Professor Osterhaus of the Erasmus Medical Centre at the University of Rotterdam in the Netherlands said.

"First, it could disappear spontaneously, but I'm not convinced that this will be the case.

"It could also cause a mild pandemic, like Asian flu," he said, referring to a 1957-8 outbreak that killed between one million and four million people.

By comparison, normal - so-called seasonal - flu kills between a quarter to half a million people each year, according to the World Health Organization.

"In a doom(sday) scenario, we could have a severe pandemic, similar to the Spanish flu, and that could arise out of a mutation of the virus," he said, referring to the 1918-19 pandemic believed to have killed about 50 million people.

He cautioned about the grimmest ideas: "I'm not predicting that any of these things is going to happen but even if there's only a 10-per-cent chance that you would have of a scenario like that, we had better be prepared."