Roughly half of health workers skip the immunizations, raising two concerns: If doctors and nurses get sick, who will treat what could be millions of Americans reeling from seasonal or swine flu? And could infected health workers make things worse by spreading flu to patients?

New York, the first state to be hard-hit by swine flu, is requiring all health workers to get immunized against both types of flu. Other states are weighing whether to follow suit.

But shots for all health workers may not be an easy sell.

Fewer than half of them got flu vaccinations last year, according to a Centers for Disease Control and Prevention survey of about 1,000 workers. That includes physicians in clinics, lab technicians, respiratory therapists and home health aides. Rates are highest among doctors and nurses in hospitals — 70 to 80 percent, but the overall rate shows many still shun the shots.

Why? The reasons vary from safety concerns to skepticism over vaccine effectiveness.

Sandra Morales, a labor and delivery nurse in New York City, had her last flu shot 16 years ago. She says she got the flu anyway.

She objects to New York's new law, saying it infringes on free-choice rights. "It's crossing the line, and I'm opposed to that."

Hospital workers "are at risk for being exposed to many, many diseases," she said. "Imagine if we had to take a vaccine for everything that comes in the door."

Morales worries she might lose her job if she refuses — it will be up to individual clinics and health centers to decide how to enforce the law. She has until Nov. 30 to get her shots. Both the seasonal flu vaccine (available this month) and swine flu vaccine (expected in October) are required for workers in hospitals, treatment centers and in home care.

That may mean three separate shots, if the swine flu vaccine requires two doses to be effective. Testing in the U.S. is still under way to determine the dose.

The uncertainty about the new swine flu vaccine has added to the challenge.

"If health care workers have concerns about the safety and efficacy of a vaccine that has been around for decades, I'm sure they're going to have those same concerns about a vaccine that we've never used before," said Dr. Gregory Poland, a Mayo Clinic vaccine specialist.

He says health workers are ethically obligated to get vaccinated for both kinds of flu. He supports requiring them. New York, which had the first big surge of swine flu cases in the spring, is the only state doing that, although some states are considering the issue.

The theory that health care workers could spread the infection is supported by only isolated evidence, but the fear persists.