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It's a mantra public health officials keep repeating: H1N1/swine flu seems to behave very similarly to regular, seasonal flu in terms of duration and severity of illness.

But some infectious disease experts say New Yorkers should not get too comfortable with that comparison.

Over 30 years ago, Dr. Edwin Kilbourne created the first genetically-engineered vaccine, a method still used for producing flu vaccines. Kilbourne, a professor emeritus at New York Medical College, points out an obvious difference between seasonal flu and the new H1N1 strain.

"I think it is very different from regular seasonal flu in one respect. It's not seasonal," he says. "I mean, it is the wrong time of year to be having this much, particularly in schools. So I think that it is a relatively scary situation."

He's not saying that to be an alarmist. But Kilbourne, along with some of the nation's other top experts, reminds New Yorkers that while the strain has only triggered mostly mild disease so far, it is still a pandemic.

"Influenza viruses, when they are highly transmissible, can effect the entire population," says Dr. Kanta Subbarao of the National Institutes of Health. "And the WHO, the World Health Organization, has these phases of a pandemic alert. And that's essentially what they are calling it now."

Public health experts say there are other differences that concern them about the new strain, including some of the symptoms. For example, nausea and vomiting seems to be a problem, especially for the young, which is unusual for most flu.

Many older adults are showing stronger respiratory symptoms like shortness of breath and coughing. Usually those symptoms run across all age groups.

Another difference, many patients are not initially showing fevers.

"In a classic influenza case, a person can usually tell you within a half hour of when they hit. It causes a sudden spike in fever," explains Dr. Michael Shaw of the Centers for Disease Control. "Often we are not seeing that in this case, which is probably why people are spreading it, because they probably don't know they have it at first."

Unlike regular flu, the new bug also seems to be mostly impacting the young, although there's a theory that many older adults may have some built-in immunity because of exposure to a similar strain at some point in their lifetime.

Epidemiologists say pointing out these differences are not meant to frighten citizens, but to emphasize there is a lot that's unknown about the new virus and how it ultimately will behave.

And as they race to find answers and, they hope, a vaccine, they say while there's no need for public panic, it still needs to be recognized as a potentially serious problem.