The World Health Organization made it official Thursday, declaring that swine flu is now a global pandemic.

At the same time, Minnesota authorities reported that the number of confirmed cases in the state has more than doubled in a week, raising concerns that people have let down their guard too soon.

This week, an average of 30 new cases a day have been reported in Minnesota, according to the Minnesota Department of Health.

"I am concerned that people are not taking this seriously," said Dr. Ruth Lynfield, the Minnesota state epidemiologist. "This story," she added, "hasn't played out yet."

Ironically, Lynfield and other experts worry that Thursday's announcement -- of the first worldwide flu pandemic in 41 years -- could lull the public into complacency. This is the fourth pandemic in the last century, and so far, the least deadly.

"People are going to say, 'Geez, pandemics don't amount to much'," said Dr. Allan Kind, a retired Park Nicollet infectious disease specialist who now serves on a pandemic advisory panel for the state. "That," he said, "is going to be the problem."

As of Thursday, a total of 221 cases have been confirmed in Minnesota, according to Lynfield, and that represents only a fraction of the total, because the state relies on a sample survey from hospitals and 26 "sentinel" clinics.

"I am concerned that it may be that there's a bit more transmission because people aren't staying home when they're sick," Lynfield said.

One big concern is that the new flu is hitting children and young adults harder than any other group, Lynfield said. The median age is just 13, and many of those hospitalized have been even younger.

The wrong message?

For years, health experts have been warning that a new pandemic was inevitable. Now that it's here, some fear that people may get the wrong message.

Technically, a pandemic doesn't mean that an outbreak is especially deadly. The World Health Organization (WHO) defines it as a new virus that spreads easily from person to person and causes widespread illness around the world. As of Thursday, 74 countries had reported a total of 28,774 cases -- nearly half of them in the United States.

The last pandemic, in 1968, claimed about a million lives worldwide, but it didn't get the kind of media attention as this year's swine flu, said Michael Osterholm, a pandemic expert at the University of Minnesota. "This was right at the heart of Vietnam," he said, and other events --riots, assassinations and war -- dominated the news.

Dr. Gregory Poland, a pandemic expert at the Mayo Clinic, was only 13 then, and remembers getting his first flu shot that year. "There was stuff on the news about it," he recalls, "so I knew that, OK, this must be something important."

An earlier global pandemic, in 1957, was blamed for up to 4 million deaths, including 70,000 Americans.

But it's the specter of the 1918-1919 Spanish flu -- the one that killed up to 50 million people -- that haunts the health experts. Particularly because it started as a relatively mild illness in the spring and summer of 1918. By the fall, said Poland, it "had changed to become the most lethal influenza virus the world had seen."

The problem now is that no one knows what will happen next.

"All we can hope is that this is as bad as it's going to get," Osterholm said. But "we just don't know."

Osterholm blames the news media for jumping on the swine-flu story with a "sky is falling" mind-set earlier this year, and then dropping it when the outbreak turned out to be less deadly than feared.

Lynfield, the state epidemiologist, worries that people may have stopped paying attention. "It is, I think, a hard message for people to be vigilant."

Poland, of the Mayo Clinic, agrees. "To me, the thing to be cautious of is panic or complacency," he said. "Both are equally dangerous."

Maura Lerner • 612-673-7384