TORONTO - The new swine flu virus spreading around the globe remains a source of significant global public health concern, the World Health Organization's top flu scientist said Thursday.

To date, the virus hasn't ignited the type of spread outside of North America that would prompt the WHO to declare a pandemic. Interest in the issue is sinking like a stone in the news media. The website Google Trends shows searches for the topic have plummeted. Swine flu is no longer a top-10 search term on the social media site Twitter.

But the virus hasn't gone away and concern about its potential to cause serious illness and death remains unabated among public health officials, suggested Dr. Keiji Fukuda, the WHO's acting assistant director general for health security and environment.

"In terms of the level of worry at WHO, I think that we are very similar to where we were at the beginning," Fukuda said during a briefing from Geneva.

"We know what past pandemics have done. We know that this is a new influenza virus. And we know that we are a few weeks into this situation and we don't quite see what the future is going to bring. ..."

"So I think our message to everybody is of course don't over-worry about these things. It's important to know. It's serious to monitor. And we are watching it very closely as the countries are out there. And so it's something which requires following closely."

As of Thursday morning the WHO reported nearly 6,500 laboratory confirmed cases in 33 countries. As most cases of flu are never confirmed -- especially if the illness is mild -- no one expects that this figure represents the true case count or the real scope of spread.

But as the level of public concern has been dialled down, investigations are clearing up the picture of what is actually occurring or did occur in places where the virus has spread.

Fukuda said for the most part disease has been mild. But he balked at the characterization of the wave of illness being similar to seasonal flu, saying that is misleading.

"There are some features about this current phenomenon which are very different than what we see with seasonal influenza," he suggested.

Seasonal flu viruses infect all age groups, but the bulk of complications -- hospitalizations and deaths -- occur in people aged 65 and older and in those with chronic health conditions that impede their ability to fight off the infection.

In this case, the average age of cases has been in the mid-20s and about half of the people who have died were previously healthy. Most of those deaths have occurred in Mexico.

"It's highly unusual for young, healthy people to die from influenza. So this is a pattern that's different from what we see with normal influenza," Fukuda said.

Different patterns of infections always raise the antennae of flu experts. Of the three pandemics of the 20th century -- 1918, 1957 and 1968 -- deaths in younger, healthier people were disproportionately high.

"So this appears to be perhaps one of the features of new influenza viruses as they emerge and they sweep around. The pattern of severity, who they affect, is a little bit different than what we see with seasonal influenza," Fukuda said.

"So these are important differences and these are part of the reason why we take this so seriously. We do see significant and basic differences with how this virus is impacting people."