As federal health agencies launch a big campaign to convince more Americans to get flu shots, new data from the Centers for Disease Control and Prevention and outside analysts indicate this pandemic is much milder than officials expected or have let on so far.
"It is probably going to be the mildest pandemic on record — compared to the three that happened in the 20th century," says Dr. Marc Lipsitch, a professor of epidemiology at the Harvard School of Public Health and co-author of a new analysis in the journal Public Library of Science.
There are a number of ways to measure how severe — or mild — a flu pandemic is. One way is to look at the proportion of the population that gets sick. The season isn't over yet, but so far it's been less than 8 percent.
Lipsitch says assuming the virus doesn't change, it's reasonable to expect that between 10 percent and 20 percent of the population will become sick from it.
"[That's] toward the upper end of a typical flu season," he says, but still not as bad as was anticipated.
Another measure is the number of people who get sick enough to require hospital care. Lipsitch says if 15 percent of Americans get this flu, "then we would expect anywhere from about 70,000 up to over 600,000 hospitalizations."
He says the figure will probably end up somewhere in the middle, about where we'd be in a typical flu season.
The most important measure is how many people die. Last spring, experts thought it was entirely possible swine flu would kill 1 out of every 100 people who got the virus.
"We now know that's at least twentyfold too high and probably more than twentyfold too high," he says.
In fact, the death rate from swine flu so far has actually been less than the death rate during the average flu season. It's around 1 out of every 2,000 who've gotten sick, perhaps fewer.
The big difference this year is that most of those deaths have been among children, teenagers and adults under age 50. Flu typically kills mostly people older than 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.
"And what you find is that 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," says 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.
"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, then even fewer people will get vaccinated and some of those people will die,' " he says.
Not so, says the head of the CDC, Dr. Thomas Frieden.
"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," he says.
He points out that the CDC has counted more than 250 deaths among children.
"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," he says.
But Frieden agrees that perception is what matters. The more 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.