A growing number of U.S. hospitals now compel health care workers to get vaccinated against the flu and other infectious diseases to protect patients from communicable diseases. In the case of the flu, the need is obvious: hospitalized patients who become infected have an elevated risk of developing complications or dying.

Yet the vaccination requirement is not universal across all hospitals, and some health care worker unions object to such demands. A range of findings, however, suggest that the benefits of compulsory vaccination outweigh the downsides.

In my first investigation of disease outbreak, when I worked at the Centers for Disease Control, five newborns contracted whooping cough in a neonatal intensive care unit (NICU) where coughing health care workers who had not been vaccinated against the disease had exposed 40 babies to the illness. The study could not prove that the health workers caused the disorder in the babies—visitors or others could have been the vector. But a link seemed highly likely.

Other disease outbreaks have been traced more firmly to unvaccinated health care workers, particularly in the case of influenza. In 1998, two patients died and 25 were sickened with flu at Memorial Sloan-Kettering Cancer Center in New York. The deaths occurred in the hospital’s bone marrow unit. Only 12 percent of health care workers in the unit at the time had been vaccinated against the flu. In another fatal flu outbreak, one baby died and 19 were sickened in a Canadian neonatal intensive care unit. Only 15 percent of health care workers were vaccinated.

These unvaccinated health care workers weren’t breaking any laws. Indeed, many states have no laws requiring hospital employees, even those working in a NICU, to be vaccinated against whooping cough or other infectious diseases. Ten states require hospital employees to get the vaccine that protects against measles, mumps and rubella. Only three states mandate flu shots for hospital workers.

The CDC has recommended since 1984 that health care workers receive the flu vaccine, but less than three quarters of healthcare workers got a flu shot in 2011. Now more hospitals are taking the matter into their own hands and establishing mandatory flu vaccination policies. That approach may be the more effective way to go. A recent study shows that hospital policy mandating vaccination gets better results than state policy does. Researchers at the University of Pittsburgh found hospital mandates were associated with a 3–12 percent increase in health care worker vaccinations whereas state laws did not result in a significant increase.

The Children’s Hospital of Philadelphia mandated the flu vaccine for its employees in 2009. They saw health care worker vaccination coverage rise from 90 percent to nearly 100 percent. The hospital administration feared a mass exodus of staffers and was reassured to see very few leave.

“We’re a big institution,” says Susan Coffin, associated director at the center for pediatric clinical effectiveness who helped devise the policy. “We employ around 10,000 people and in the first year nine people declined the vaccine.”

Loyola University Medical Center in Illinois was also an early adopter of mandatory flu vaccinations. When it implemented the policy in 2009, one employee out of nearly 8,000 declined the vaccine and left the hospital. Loyola has boasted employee flu vaccination rates of 99 percent ever since.

Johns Hopkins Health System in Maryland employs around 25,000 people. Less than one percent declined the flu vaccine after they implemented a policy mandating the flu vaccine last year. Many medical centers in Baltimore have followed their example.

“When we looked at the literature there was evidence that with the type of work we were doing and the type of patients we’re looking after, it was our ethical duty to do this,” says Trish Perl, senior epidemiologist at Johns Hopkins Health System. Some hospitals have seen patient mortality decrease by up to 40 percent following the mandatory vaccination of health care workers.

Still, some health care worker unions are taking legal action to block such policies.

The National Union of Nurses and Hospital Employees stalled a University of New Mexico mandatory vaccine policy last month. The complaint prevents the university from firing employees who refuse the vaccine.

When New York State mandated flu vaccinations for health care workers in 2009, unions obtained a court injunction that stopped the mandate. This year, New York responded by requiring unvaccinated health care workers to wear masks during flu season.

The New York State Nurses Association opposed the 2009 mandate and disagrees with the mask rule. It argues that nurses who choose not to be vaccinated should instead receive paid sick leave when they are ill.

“The best way to protect patients and stop the spread of flu is to let nurses stay home when we are sick,” said Carol Lynn Esposito, director of Nursing Practice, in an emailed statement.

But some studies show that viral shedding can occur before a person develops flu symptoms. A report by the Children’s Hospital of Philadelphia (pdf) states that a person can transmit the influenza virus 24 hours before they feel ill.

For now, patients will have to rely on contacting hospitals directly to learn about specific health care worker vaccination policies. The CDC and Health Resources and Services Administration have yet to collect these data.

Until that information is available, “Have you been vaccinated against measles, mumps, rubella, influenza and whooping cough?” should be every patient’s first question.