(USA TODAY) -- Efforts to require flu shots for health care workers in order to protect vulnerable patients are being abandoned by some major health systems because of legal challenges and vaccine shortages.

Requiring flu shots is an exception in the health care industry, where 48% of workers were vaccinated against flu last year, according to the Centers for Disease Control and Prevention.

The University of Iowa Hospitals and Clinics, with 6,300 workers, dropped a mandate requiring seasonal and swine flu vaccinations this week after the Service Employees International Union (SEIU) won an arbitration ruling. The union supports voluntary vaccination.

A voluntary program immunized 86% of University of Iowa health workers last year, spokesman Tom Moore said.

"We see the sickest of the sick ... patients with compromised immune systems, people with multiple chronic health conditions who are especially vulnerable to influenza," he said.

Last month, New York rescinded its mandate that health workers statewide be vaccinated against seasonal and swine flu, citing shortages of H1N1 vaccine. Unions also had challenged the mandate.

"I think it's pathetic and highly disappointing when health care workers go to court to put their own self-interest ahead of those they care for," said Arthur Caplan, director of the University of Pennsylvania Center for Bioethics. Any vaccine risk is small, he said, but a flu-infected worker can be deadly to high-risk patients. "I think they have a duty to get these shots."

Union officials say mandates are wrongheaded. "When you educate health care workers they will get the flu vaccine. The problem is it's not being done," said Bill Borwegen, SEIU's occupational health and safety director.

The Infectious Diseases Society of America this fall called for mandatory flu shots for health workers, citing studies indicating worker vaccination protects patients from catching the virus.

Three workers in the Greensboro, N.C.-based Moses Cone Health System resigned this week after refusing seasonal flu vaccination by Oct. 31. That's out of 8,500 workers and volunteers covered by the mandate. The system also requires H1N1 vaccinations but can't set a deadline because of shortages.

Among other mandatory programs hampered by vaccine shortages:

*Chicago's Cook County Health and Hospitals System mandated seasonal and swine flu shots for its 11,000 workers, volunteers and contractors on Oct. 13. It delayed its deadline for H1N1 vaccination to at least Dec. 1.

*HCA, the Nashville-based health care company, delayed by a month its Oct. 1 deadline for 120,000 workers to get seasonal flu shots due to vaccine delays. "The vast majority are entirely enthusiastic," said Jonathan Perlin, chief medical officer.

An estimated 41% of health workers nationwide had seasonal flu vaccinations as of mid-October -- more than usual for the time of year, said the CDC's Jim Singleton. It's too early to know how many have received the H1N1 vaccine.

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