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RENEE MONTAGNE, host:

An update now on the swine flu. It looks like the second wave of the flu pandemic of 2009 has peaked in the U.S. Twenty-five states report widespread flu. That's down from 48 states a month ago. Hospital admissions and deaths from flu are declining steadily. So are the number of people going to doctors complaining of flu symptoms. And there's new data from the Centers for Disease Control and Prevention, as well as outside researchers, that indicates this pandemic is much milder than expected. Meanwhile, this week, the federal government is launching a big ad campaign to convince more Americans to get flu shots. NPR's Richard Knox has the story.

RICHARD KNOX: Soon, you'll be hearing government-sponsored public service announcements like this one.

(Soundbite of public service announcement)

Unidentified Woman: This isn't going to be just any flu season, and children are specially at risk with the H1N1 flu virus.

KNOX: Well, it certainly isn't just any flu season, that's for sure. But the new numbers show the pandemic is far milder than officials expected, or have let on so far.

Dr. MARC LIPSITCH (Harvard University): It is probably going to be the mildest pandemic on record compared to the three that happened in the 20th century.

KNOX: That's Dr. March Lipsitch of Harvard. He's one of the authors of a new analysis. It's in the journal called Public Library of Science.

There are a number of ways to measure how severe or mild a flu pandemic is. One is what proportion of the population gets sick. So far, it's been less than 8 percent. Of course, it's not over. Lipsitch says the numbers are uncertain.

Dr. LIPSITCH: But it's reasonable to expect that something between 10 and 20 percent of our population will become ill with this virus, assuming that the virus doesn't change.

KNOX: Ten to 20 percent - that sounds like a typical flu season.

Dr. LIPSITCH: It's toward the upper end of a typical flu season, and that's one reason why this virus is turning out to be less severe than was anticipated.

KNOX: Another measure is the number who get so sick they need hospital care. Lipsitch says if 15 percent of Americans get this flu�

Dr. LIPSITCH: �then we would expect anywhere from about 70,000 up to over 600,000 hospitalizations.

KNOX: He says we'll probably end up with something in the middle, about where we'd be in a typical flu season.

Of course, the most important measure is how many people die. Last spring, experts thought it was entirely possible swine flu would kill one out of every 100 people who got the virus.

Dr. LIPSITCH: We now know that's at least 20-fold too high, and probably more than 20-fold too high.

KNOX: In fact, the death rate from swine flu so far has actually been less than the average flu season. It's around one out of every 2,000 who've gotten sick, maybe less. The big difference this year is that most of those deaths have been among children, teenagers and adults under age 50. Typically, flu mostly kills people over 65. But that's not because this flu is more severe among children and young adults, as many think. It's simply because many more young people are getting the flu than usual.

Mr. PETER SANDMAN (Expert in Risk Communication): The pandemic is making more kids sick. But it's killing a smaller percentage of the kids it makes sick than it is of the adults and seniors it makes sick.

KNOX: That's Peter Sandman, an expert in risk communication. He says the CDC has been reluctant to acknowledge that swine flu has been much milder than expected.

Mr. SANDMAN: The CDC may be thinking, you know, there are already millions of people who plan not to get vaccinated because they think the pandemic is mild, and if we announce as the official health agency of the U.S. government that the pandemic is mild, even fewer people will get vaccinated and some of those people will die.

KNOX: Not so, says Dr. Thomas Frieden, who's head of the CDC.

Dr. THOMAS FRIEDEN (Director, Centers for Disease Control): I think we've been completely transparent with what we think is happening. I think we have a difference of opinion on whether that is mild or severe.

KNOX: He points out that the CDC has counted more than 250 deaths among children.

Dr. FRIEDEN: Any flu season that kills at least three times more children than a usual flu season, I think it would be very misleading to describe that as mild.

KNOX: But Frieden agrees perception is what matters. The more that people think the pandemic threat is over, the fewer who will get vaccinated. Experts worry that could increase the chance of a third wave of swine flu early next year.

Richard Knox, NPR News.

(Soundbite of music)

MONTAGNE: You can follow all the latest health news at our science blog. It's called Shots. Just go to npr.org.

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