The long-anticipated fall outbreak of swine flu has begun in Minnesota, with clusters of new cases cropping up at schools and universities, health officials said Monday.

Speaking at a flu pandemic summit that drew more than 600 people, State Epidemiologist Ruth Lynfield said, "We're now experiencing our second wave."

At the same time, Michael Osterholm, a University of Minnesota expert, predicted that the number of cases will peak in the next six to eight weeks, sending absenteeism rates soaring from schools to businesses.

"The bottom line is, it's here," Osterholm told the audience of health, business and government officials gathered in Brooklyn Center. He noted that "none of us can tell you, 12 hours from now, what this virus is going to do." He said he won't be surprised if major sporting events are canceled in the next few months because teams have too many players fighting the flu.

"This train has left the station," he said. "It's moving and gaining steam."

Osterholm also warned that cases of the novel flu strain could peak before enough vaccine arrives, possibly in October.

"I'm afraid too little vaccine is going to get here before the peak hits," said Osterholm, director of the university's Center for Infectious Disease Research and Policy.

Experts have predicted for months that the flu pandemic, which hit Minnesota in April and faded over the summer, would get a second wind once school started.

The vast majority of cases are still relatively mild, Osterholm noted, "but for that other 1 percent, this disease has been hell." In Minnesota alone, 267 people have been hospitalized, and three have died, with the majority of severe cases affecting children and young adults, officials said.

Some advice for employers

Health experts also warned that the fast-spreading virus could pose new challenges for everyone, from parents to church leaders to business owners.

They warned employers, for example, to prepare for a third or more of their employees to call in sick or miss work because of a sick child.

They also urged employers to ease up on strict sick-leave policies this fall in order to slow the spread of the virus. Osterholm noted that the key to battling the flu is to "keep sick people out of the workplace." But many workers are reluctant to stay home even when they're contagious, and that's got to change, he and others told the summit.

Several experts urged companies to drop rules that discourage sick workers from staying home, such as requiring a doctor's note during an illness.

Aggie Leitheiser, who heads the office of emergency preparedness for the Minnesota Department of Health, said some employers penalize workers who take too many sick days, and that could backfire during the current pandemic. "There are many jobs in Minnesota where people don't have sick leave," she added. "What's going to be their plan so they don't come to work sick and infect others?"

The flu outbreak will force patients -- and parents of sick children -- to think twice before rushing to the doctor, said Dr. John Hick, an emergency room physician at Hennepin County Medical Center.

He warned that hospitals and clinics won't be able to care for everyone who gets sick, and that those with milder illnesses will be better off staying home than waiting hours in the ER. "The longer in the waiting room, the more they're going to be exposed," he said.

At the same time, he said, unless they're seriously ill, they're not likely to be tested or receive any special treatment once they get in. Drugs used to treat the flu, such as Tamiflu, are being reserved for those who are severely ill or have high risk of complications, such as kids with asthma.

One minister asked if people should wear masks or gloves while giving communion at church. John Linc Stine, an assistant health commissioner, said guidelines for religious organizations are still under discussion.

Osterholm, however, wondered aloud why some churches still use shared communion cups, where people can easily share germs. "We should ban that," he said.

Many Minnesota colleges have installed extra hand sanitizers in libraries and dorms, and on Monday faculty at the University of St. Thomas were encouraged to remind students to use hand cleansers and wipe down shared keyboards in campus computer labs.

The Associated Press contributed to this report. Maura Lerner • 612-673-7384