If past trends hold, the misery will spread widely over the next six weeks or so, keeping millions of Americans out of school and off the job - including those in nursing homes and hospitals. Some health-care employees will try to work through their illness, putting the most vulnerable at risk.
With tens of thousands of flu-related deaths a year, medical institutions are fighting an uphill battle to persuade workers to get shots.
Some use carrots (win a free iPod!). A few use big sticks (no shot, no job). Penn added "Baby Be Wise - Immunize!," the music video created by an irrepressible nurse and an ER volunteer.
"It's wonderful in my opinion - an engaging and fun way to garner interest on a serious topic while making people feel part of a team," said Gregory A. Poland, director of the Mayo Clinic's vaccine-research group, who saw the video on YouTube.
Nationwide, about 46 percent of health-care workers got flu shots in 2007 - more than in many other countries and an increase over past years, but well below expectations. (HUP's numbers were just a little higher.)
Some doctors say they are too busy or don't think they'll get sick.
Administrative staff might not realize that the hand they sneezed on touched a file that was opened by a nurse who took a cup to a patient, with the virus hitching a ride.
In Philadelphia, all the nursing homes that responded to a survey last year said they vaccinated workers for free, nearly all held educational sessions, and more than a fifth offered incentives to act. Still, just 37 percent of direct-care workers agreed to a shot.
Seattle's sprawling Virginia Mason Medical Center gives loads of carrots, and also a big stick: Annual shots are mandatory for staff and required of volunteers, contractors, and even drug reps. The few workers granted exceptions for medical or religious reasons must wear masks in flu season.
In 2005, the program's first year, 98 percent of the staff was vaccinated. "What really surprised me after that first year was how easy it has become," said Joyce Lammert, chief of medicine at Virginia Mason. "You change how people think about their obligations. You really change the culture."
Amy J. Behrman, an internal-medicine doctor at HUP who oversees health care for the staff, calls mandatory vaccination "a great idea." But it raises legal and ethical issues, she said, particularly for a shot that must be given every year.
Penn has encouraged in-house flu shots through giant "flu fairs," vaccine carts on all wards and at cafeteria entrances, and even signs on lunch trucks.
Employees who decline the free shots are asked to state objections in writing. Most opt-outs last year thought the vaccine would make them sick, was unsafe, or simply didn't work.
"It was disconcerting because it is not true," Behrman said.
So when Mel Kearney, a registered nurse, asked last summer if she could work with Penn junior Ryan A. Leonard to produce a music video encouraging vaccination, Behrman recalled, "I said, 'Sure, great idea.' "
Kearney, 56, is a nurse in Behrman's department - and a fanatic about flu shots. Leonard, 20, is a premed Chinese major, a cappella singer and video producer.
Kearney suggested a flu video. Leonard jumped at the chance.
"I got this idea that we could get the whole hospital staff involved in a musical number," he said. "But that would have been chaos." Lip-synching would be simpler.
The pair started brainstorming lyrics, many picked up - loosely - from federal vaccine guidelines.
But they needed music. Leonard turned to PennYo.
The a cappella group, whose name is a play on péng you, Mandarin for friends, had recorded a catchy Chinese pop song about a guy waiting at the airport for his girlfriend.
"It had sort of a doo-wop feel," Leonard said.
Members of the group recorded solos, and then the nurse and the student roamed the halls of HUP - he with a camera, she with treats and a loud music player. Lyrics were handed out.
"You don't even have to sing," Kearney told them. "You can sway to the beat, click your fingers. Just look like you're having fun."
The video debuted at a flu fair in October and was posted on internal Web sites. Early data show that HUP has vaccinated nearly 20 percent more staff this season.
No one claims a music video made all the difference.
Indeed, appeals to health workers' altruism do not improve vaccination rates, said Jonathan McCullers, an expert on staff vaccination at St. Jude Children's Research Hospital in Memphis.
Still, Neil Fishman, a HUP infection-control specialist, plans to present the video at the Society for Healthcare Epidemiology of America's meeting in San Diego in March.
And it has gotten people talking.
"They say, 'You're on the video, man, that's pretty nice,' " said Ronald Crafton, an ER receptionist, who can be seen lip-synching.
"Then they say: 'Why did you get the flu shot? Didn't it hurt?' It's a way to get into the conversation," said Crafton, who "didn't realize the importance of it" until he got sick and his doctor said his diabetes made him more vulnerable.
He got his first flu shot a year ago, after 34 years on the job.
View the video and find out where to get a flu shot at
Contact staff writer Don Sapatkin at 215-854-2617 or firstname.lastname@example.org.