Mandatory vaccination policies may encourage more healthcare workers to get annual flu shots and help prevent the spread of influenza to patients, a study in one Texas health system suggests.

At the University of Texas MD Anderson Cancer Center in Houston, just 56 percent of employees got vaccinated during the 2006-2007 flu season, researchers report in the American Journal of Infection Control.

But after several years of stepped up outreach efforts and on-site vaccination programs that culminated in a mandatory vaccination policy, the study found that 94 percent of employees got inoculated for the 2013-2014 flu season.

"Mandatory flu vaccination programs are the most effective mechanism for increasing health care worker vaccination rates," said lead study author Dr. Elizabeth Frenzel, a researcher in infection control and employee health at MD Anderson.

While unvaccinated health workers in any setting may contract influenza and spread it to patients, the stakes are even higher for people with cancer, Frenzel added by email.

"Cancer patients and other patients with compromised immune systems experience significant mortality rates after flu infection progresses to more serious respiratory infections - so literally their lives may be at stake when care givers, including healthcare workers and family members, aren't vaccinated," Frenzel said.

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Up to 28 percent of cancer patients may die from severe flu-related respiratory infections, the study team notes in its report.

At the start of the study in 2006, Frenzel and colleagues collected data on healthcare worker vaccinations for the 656-bed cancer center with more than 19,000 employees.

That year, they found some workers treating the most vulnerable cancer patients had even lower vaccination rates than the system wide average - 47 percent among people caring for immune-compromised or high-risk patients and 41 percent among inpatient nursing units.

Starting in 2007, the cancer center expanded access to vaccinations with more hours and locations for on-site flu vaccine clinics, initially focused on nurses and workers treating high-risk patients.

In 2009, MD Anderson started a pilot mandatory vaccination program. This required all workers to get the vaccine or sign a waiver indicating they declined to do so for medical or personal reasons.

Later, this policy became system-wide and all workers had to display a sticker on their employee badge proving they got vaccinated or wear a surgical mask when caring for patients during flu season. Failure to comply could result in termination.

Over the first flu season of the study, 2006-2007, the cancer center treated 47 confirmed influenza infections that patients caught outside the hospital and three cases - or 6 percent of the total - that patients contracted during their hospital stay.

By the end of the study, during the 2013-2014 flu season, only 2.3 percent of confirmed influenza cases involved patients who caught the virus while they were hospitalized. That flu season, the cancer center treated 173 patients who caught flu in the community and 4 who contracted it in the hospital.

One shortcoming of the study is that very few patients caught influenza in the hospital during any flu season, the authors note. It's also possible that the lab test used to confirm flu cases might have underestimated the number of infections.

Even so, a growing number of medical centers are moving to mandatory flu vaccination policies as part of a larger infection control effort, said Dr. Tom Talbot, chief hospital epidemiologist at Vanderbilt University Medical Center in Nashville, Tennessee.

To work well, these policies should include a careful review of why workers refuse the vaccine and an effort to communicate the safety and science of inoculations to people who are declining for non-medical reasons, added Talbot, who wasn't involved in the study.

Mandatory vaccination policies should also expand beyond influenza to include other inoculations recommended by the U.S. Centers for Disease Control and Prevention, Talbot added by email.

"Given the rise in measles cases in the U.S. and the risk of transmission in healthcare facilities, the problems with pertussis, and the decreasing rate of immunization of health care workers against hepatitis B, this expansion is essential," Talbot said.