ATLANTA - THE swine flu outbreak that has alarmed the world for a week now appears less ominous, with the virus showing little staying power in the hardest-hit cities and scientists suggesting it lacks the genetic fortitude of past killer bugs.

A flu expert said he sees no reason to believe the virus is particularly lethal. And a US federal scientist said the germ's genetic makeup lacks some traits seen in the deadly 1918 flu pandemic strain and the more recent killer bird flu.

POSSIBLE SCENARIOS WASHINGTON - So far the swine flu virus has behaved like seasonal flu as it has spread to 14 countries, carried mostly by travelers from Mexico. The World Health Organization says it cannot be stopped, but has no immediate plans to declare a pandemic - a global outbreak of a new and serious disease. ... more

Still, it was too soon to be certain what the swine flu virus will do. Experts say the only wise course is to prepare for the worst. But in a world that's been rattled by the spectre of a global pandemic, glimmers of hope were more than welcome on Friday.

In New York City, which has the most confirmed swine flu cases in the US with 49, swine flu has not spread far beyond cases linked to one Catholic school. In Mexico, the epicentre of the outbreak, very few relatives of flu victims seem to have caught it.

Officials in Mexico have voiced optimism for two days that the worst may be over. But Dr Scott F. Dowell of the US Centres for Disease Control and Prevention said it's hard to know whether the outbreak is easing up in Mexico. 'They're still seeing plenty of cases,' Dr Dowell said.

He said outbreaks in any given area might be relatively brief, so that they may seem to be ending in some areas that had a lot of illness a few weeks ago. But cases are occurring elsewhere, and national numbers in Mexico are not abating, he said.

Worldwide, the total confirmed cases neared 600, although that number is also believed to be much larger. Besides the U.S. and Mexico, the virus has been detected in Canada, New Zealand, China, Israel and eight European nations.

Scientists looking closely at the H1N1 virus itself have found some encouraging news, said Nancy Cox, flu chief at the federal Centres for Disease Control and Prevention. Its genetic makeup doesn't show specific traits that showed up in the 1918 pandemic virus, which killed about 40 million to 50 million people worldwide.

'However, we know that there is a great deal that we do not understand about the virulence of the 1918 virus or other influenza viruses' that caused serious illnesses, Ms Cox said. 'So we are continuing to learn.'

She told The Associated Press that the swine flu virus also lacked genetic traits associated with the virulence of the bird flu virus, which grabbed headlines a few years ago and has killed 250 people, mostly in Asia. -- AP