About 50 employees of Essentia Health, an upper-Midwest hospital chain, didn’t go to work Wednesday.

But it wasn’t an early start to the Thanksgiving holiday for them. They were fired for refusing to get flu shots.

It’s part of a growing trend for hospitals to require flu shots for workers. Public health experts say it shouldn’t be surprising.

“It’s a patient safety issue,” said Dr. William Schaffner, an infectious disease expert at Vanderbilt University and a spokesman for the Infectious Diseases Society of America. “It’s so that we do not give flu to our patients.”

Hospital workers can pass the flu virus to some of the most vulnerable people — frail elderly, babies in incubators, patients with immune systems ravaged by cancer treatment. Vaccinating employees protects patients and the employees’ co-workers.

Related: Hospital gets workers vaccinated with in-your-face flu campaign

“Patients are in the hospital because they are sick,” said Dr. Rajesh Prabhu, Infectious Disease and Chief Patient Quality and Safety Officer at Essentia Health. "That puts them at risk of a more severe outcome from influenza. People can die from influenza.”

Each year, influenza virus kills between 4,000 and 50,000 Americans, including children who were perfectly healthy before they caught flu.

Just about everyone is advised to get a flu vaccine. The Centers for Disease Control and Prevention recommends annual flu vaccines for every one of the age of 6 months who doesn’t have a medical reason not to — for instance, an allergy to the vaccine.

RELATED: Bird flu outbreak in France

13 PHOTOS Bird flu outbreak in France See Gallery Bird flu outbreak in France Ducks stand in a livestock farm in Bars, southwestern France. Authorities in south-west France began a cull of hundreds of thousands of ducks January 5 as authorities scramble to contain an outbreak of a virulent strain of bird flu sweeping Europe. The farm birds in France's foie gras heartland are to be slaughtered to stem the spread of an outbreak of the highly pathogenic H5N8 virus. (REMY GABALDA/AFP/Getty Images) Livestock farmer Bernard Dupuy stands near his ducks in the family farm in Bars, southwestern France. Authorities in south-west France began a cull of hundreds of thousands of ducks January 5 as authorities scramble to contain an outbreak of a virulent strain of bird flu sweeping Europe. The farm birds in France's foie gras heartland are to be slaughtered to stem the spread of an outbreak of the highly pathogenic H5N8 virus. (REMY GABALDA/AFP/Getty Images) Duck farmers drive birds out of an enclosure as they prepare to slaughter a portion of the farm's 32,000 ducks, in Belloc-Saint-Clamens, southwestern France, on January 6, 2017, during the first wave of a mass bird slaughter after the detection of bird flu. Authorities in southwest France began a cull of hundreds of thousands of ducks January 5 as the government scrambles to contain an outbreak of a virulent strain of bird flu sweeping Europe. The farm birds in France's foie gras heartland are to be slaughtered to stem the spread of an outbreak of the highly pathogenic H5N8 virus. (REMY GABALDA/AFP/Getty Images) Ducks stand in a livestock farm in Bars, southwestern France. Authorities in south-west France began a cull of hundreds of thousands of ducks January 5 as authorities scramble to contain an outbreak of a virulent strain of bird flu sweeping Europe. The farm birds in France's foie gras heartland are to be slaughtered to stem the spread of an outbreak of the highly pathogenic H5N8 virus. (REMY GABALDA/AFP/Getty Images) Members of the protest group 'Canards en colere' demonstrate in front of the Auch prefecture on January 5, 2017, denouncing what they consider to be an inadequate bio-security required by the Direction generale de l'alimentation (DGAL) and approved by the Comite interprofessionnel des palmipedes a foie gras (CIFOG) and demanding imediate compensation for all the affected farmers as hundreds of thousands of ducks from open farming systems will be slaughtered in a bid contain a new outbreak of bird flu in France. (REMY GABALDA/AFP/Getty Images) Ducks stand in a livestock farm in Bars, southwestern France. Authorities in south-west France began a cull of hundreds of thousands of ducks January 5 as authorities scramble to contain an outbreak of a virulent strain of bird flu sweeping Europe. The farm birds in France's foie gras heartland are to be slaughtered to stem the spread of an outbreak of the highly pathogenic H5N8 virus. (REMY GABALDA/AFP/Getty Images) Duck farmers drive birds out of an enclosure as they prepare to slaughter a portion of the farm's 32,000 ducks, in Belloc-Saint-Clamens, southwestern France, on January 6, 2017, during the first wave of a mass bird slaughter after the detection of bird flu. Authorities in southwest France began a cull of hundreds of thousands of ducks January 5 as the government scrambles to contain an outbreak of a virulent strain of bird flu sweeping Europe. The farm birds in France's foie gras heartland are to be slaughtered to stem the spread of an outbreak of the highly pathogenic H5N8 virus. (REMY GABALDA/AFP/Getty Images) Ducks stand in a livestock farm in Bars, southwestern France. Authorities in south-west France began a cull of hundreds of thousands of ducks January 5 as authorities scramble to contain an outbreak of a virulent strain of bird flu sweeping Europe. The farm birds in France's foie gras heartland are to be slaughtered to stem the spread of an outbreak of the highly pathogenic H5N8 virus. (REMY GABALDA/AFP/Getty Images) An employee of duck farmer Sebastien Pujos drives a tractor with culled birds in Belloc-Saint-Clamens, southwestern France, on January 6, 2017, during the first wave of a mass bird slaughter after the detection of bird flu. Authorities in southwest France began a cull of hundreds of thousands of ducks January 5 as the government scrambles to contain an outbreak of a virulent strain of bird flu sweeping Europe. The farm birds in France's foie gras heartland are to be slaughtered to stem the spread of an outbreak of the highly pathogenic H5N8 virus. (REMY GABALDA/AFP/Getty Images) Ducks stand in a livestock farm in Bars, southwestern France. Authorities in south-west France began a cull of hundreds of thousands of ducks January 5 as authorities scramble to contain an outbreak of a virulent strain of bird flu sweeping Europe. The farm birds in France's foie gras heartland are to be slaughtered to stem the spread of an outbreak of the highly pathogenic H5N8 virus. (REMY GABALDA/AFP/Getty Images) Livestock farmer Bernard Dupuy stands near his ducks in the family farm in Bars, southwestern France. Authorities in south-west France began a cull of hundreds of thousands of ducks January 5 as authorities scramble to contain an outbreak of a virulent strain of bird flu sweeping Europe. The farm birds in France's foie gras heartland are to be slaughtered to stem the spread of an outbreak of the highly pathogenic H5N8 virus. / AFP / REMY GABALDA (Photo credit should read REMY GABALDA/AFP/Getty Images) A picture taken on December 6, 2016 shows chickens at a henhouse near Loon-Plage. France raised its risk level for avian flu to 'high' after new cases of the 'highly pathogenic' H5N8 strain of the virus were detected. (PHILIPPE HUGUEN/AFP/Getty Images) Employees of duck farmer Sebastien Pujos prepare to unload a truck with birds into an enclosure as they prepare to slaughter a portion of his 32,000 ducks, in Belloc-Saint-Clamens, southwestern France, on January 6, 2017, during the first wave of a mass bird slaughter after the detection of bird flu. Authorities in southwest France began a cull of hundreds of thousands of ducks January 5 as the government scrambles to contain an outbreak of a virulent strain of bird flu sweeping Europe. The farm birds in France's foie gras heartland are to be slaughtered to stem the spread of an outbreak of the highly pathogenic H5N8 virus. (REMY GABALDA/AFP/Getty Images) Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

But vaccination rates are low, and even among health care workers, only around 65 percent get flu vaccines every year, the CDC says.

Requiring the vaccine gets those rates up, protects patients and causes no harm, Schaffner said.

When employers require the vaccine, the CDC found, 85 percent of workers get one. Just 43 percent get vaccinated if there is no policy.

Related: Caterpillar-grown flu vaccine works better

Several states, including California, require hospitals to make flu vaccines mandatory and to record and publish their vaccination rates. Maryland has a searchable database telling people a hospital’s vaccination rate.

Minnesota doesn’t require this but Essentia said it started the mandatory policy to protect patients.

“Just like other people, we had a voluntary program really encouraging our health care personnel to get vaccinated,” Prabhu said.

“In 2012-2015 our vaccination rate among health care workers was about 70 percent.” That’s not good enough, he said. “The last flu season, we went to mandatory participation. Everyone who worked at Essentia had to say yes or no.” That got vaccination rates to 82 percent — still not good enough.

Now, Essentia employees must either be vaccinated or go through a process similar to school enrollment requirements: they must apply for a medical or religious/philosophical waiver. The requests are reviewed by expert committees.

RELATED: The Best Drinks to Fight the Flu

5 PHOTOS The Best Drinks to Fight the Flu See Gallery The Best Drinks to Fight the Flu Water This shouldn't be a surprise: the ultimate clear liquid, water is the truest drink to keep you hydrated. Don't like the taste? Add a sugar-free flavor packet, like Crystal Light, to your glass to encourage more guzzling. Credit: iStockphoto/ Thinkstock Ice Pops and Ice Cubes How do you think ice cubes became the 25th most searched recipe in America, after all? Ice pops, or ice cubes if your freezer just happens to be void of summer’s best treat, are two of the best defenses against dehydration. WebMD advises to look for ice pops made with 100 percent fruit juices to get a 2-in-1 fight against the flu – hydration, plus vitamins. Credit: Jupiter Images Black Tea Mother Nature Network notes one Harvard study that shows black tea as the ultimate immunity booster. Drinking five cups of black tea per day for two weeks nearly quadrupled a person’s immunity system – so if tea isn’t a part of your diet already, you germaphobes, you should get on it. Thanks to black tea’s abundance of theanine, flavonoids (antioxidant-like compounds), catechins (compounds that fight free radicals in the body), it’s been proven to fight off the flu. Credit: Flickr/ HinduCindu Ginger Tea Ginger may as well be called a superfood, seeing as how much it can keep the flu at bay. Most people reach for a ginger ale when tummy troubles come about, and we can see why – it’s been a part of Chinese herbal medicine for more than 2,000 years to relieve digestion problems and nausea. Plus, ginger can help detox the body by inducing sweating (not the most fun symptom of the flu) to get rid of the body’s toxins. And ginger’s analgesic and anti-inflammatory properties relieve aches and pains that come with the flu. Convinced? Fortunately, ginger tea is simple to make: the University of Maryland Medical Center advises adding 2 tablespoons of ground ginger or ginger root to a cup of boiling water. (We particularly like this honey, lemon, and ginger flu-fighting concoction from Simple Green Smoothies, too.) Credit: Flickr/ Ali Fayre Chicken Broth Chicken broth may be your best remedy against a cold and the flu. Vitamins and protein? Check. Hydration power? Check. Anti-inflammatory properties? Check. Congestion-fighting properties? Check. The most popular study on chicken broth, from the University of Nebraska, showed that chicken soup helped reduce upper respiratory cold symptoms and helped build up the nose’s protective cilia, hair-like substances in the nose that prevent contagions – like that pesky flu bug – from entering the body. While you may need a few days to feel well enough to eat a hearty chicken soup, the University of Nebraska provides the ultimate chicken noodle soup recipe. A scientifically-backed recipe to feel better? Not even your mom’s homemade soup-made-with-love can top that. Credit: Flickr/ Muffet Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

That’s gotten the compliance rate to 99.5 percent, Essentia spokeswoman Maureen Talarico said. The hospital system does not have a clear vaccination rate yet.

Employees were given extra time to start the review process if they objected to getting a vaccine, but after an extra month, Monday was the cutoff for either getting a vaccine or for starting the opt-out paperwork.

“We were vaccinating on Monday,” Talarico said.

Like many hospital systems, Essentia offered multiple free vaccine clinics, had vaccine carts available to go to workers so they did not have to take the time to line up or seek out a flu shot and made sure every floor or unit had someone certified to administer vaccines.

“You cannot get to a high immunization rate without some kind of mandatory flu vaccination program,” Prabhu said.

“We had a cutoff of November 20. If they did not participate in the process, they would no longer be allowed to work at Essentia Health.”

Out of nearly 14,000 employees, only about 50 took it to that point, Essentia said.

Related: Why flu vaccines aren't so hot

No opt-out except for medical

Influenza is not the only vaccine that’s required for health care workers. Hepatitis B, measles, mumps, rubella, diphtheria, tetanus, chickenpox and whooping cough vaccines are also required, Prabhu said.

Hand hygiene is also enforced, Prabhu added.

The Infectious Diseases Society of America, the Society for Healthcare Epidemiology of America, the Pediatric Infectious Diseases Society and other groups all support mandatory immunization of health care workers — and all say there should be no opt-out except for medical reasons.

This is, in part, because people infected with influenza can pass it to others both before they have symptoms and after they feel better. In addition, people who think they just have the sniffles may, in fact, have flu and can pass it around.

“Obviously, it is also about personal protection,” Schaffner said.

“We want them to be protected because during a flu outbreak we need our healthcare personnel to be vertical rather than horizontal. We need them fit and able to care for all the people who need care,” Schaffner added.

Vanderbilt doesn’t fire people who refuse vaccination but makes them undergo counseling and then wear a mask during flu season.

And they made vaccination into a party they called “Flu-la-palooza”. “It’s a one-day event brilliantly organized to throughput as many people as quickly and easily and appropriately as possible,” Schaffner said.

“This year we did almost 14,000 people.”

In fact, the medical center now holds a Guinness World Record for the event.

Schaffner said it had an added benefit.

“It’s a training process such that if there is a pandemic and we have to vaccinate very large numbers of our people, our personnel and our administrative people are beautifully trained,” Schaffner said.

Why would any health care worker refuse to get a flu vaccine?

Prabhu and Schaffner said health care workers have the same arguments that non-medical people do —they’re afraid of side-effects, even though side-effects are not as serious as people believe they are; they think they don’t need the vaccine because they “never” get flu; or they worry it’s a waste of time because flu vaccines do not always provide perfect protection.

“We all know it has limitations and we all recognize that it is the best that science has to offer us at this point,” Schaffner said.

He quotes an old saying about not letting the goal of perfection undermine doing good.

“We can all do a lot of good with our pretty good vaccine,” he said.

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