Story highlights A Red Robin worker in Springfield, Missouri, was diagnosed with an infection

The employee last worked on May 16

Shot that offers protection only effective if given within two weeks of infection

People who dined before May 8 may have been exposed but it's too late for them to get a shot

As many as 5,000 people may have passed recently through a Missouri restaurant where an employee with hepatitis A worked while possibly contagious, health officials said Wednesday.

Red Robin said the employee last worked on May 16 and that the restaurant has been deemed safe after an inspection by the Springfield-Greene County Health Department

The New Jersey-based restaurant chain didn't say in what area of the restaurant the employee worked but told diners who ate there between May 8 and 16 to call the health department for information about what to do next.

"It scared me because my husband has been sick," Andrea Hall, a Red Robin customer, told CNN affiliate KOLR . "And a lot of his symptoms of his matched. A red flag just went off and I was like what do I do from here."

Red Robin's website says it opened its first restaurant in 1969 in Seattle.

Hepatitis A is usually transmitted via contaminated food or water, or by someone who's infected, according to the Mayo Clinic . Frequent handwashing is recommended to limit the spread.

The highly contagious infection inflames the liver, limiting its ability to function.

While mild cases don't require treatment and the Mayo Clinic says most of those infected recover completely with no permanent liver damage, severe cases can lead to liver failure and death, according to the World Health Organization

Health officials in Springfield said symptoms include fever, nausea, abdominal pain, dark urine and clay-colored bowel movements.

Vaccination center set up

All workers at the Red Robin have since been innoculated with a immune globulin prophylaxis shot, the restaurant chain said.

The health department will run a two-day vaccination clinic at Remington's, a venue that hosts concerts and trade shows. On its Facebook page, the entertainment complex said 4,000 doses of a vaccine were being shipped there.

According to the Centers for Disease Control and Prevention , the incubation period for hepatitis A is between 15 and 50 days. The CDC says on its website that immune globulin offers protection for about three months for people who get the shot pre- or post-exposure. But the shot needs to be given within two weeks of exposure for maximum protection.

Many people already have been given a two-step vaccine, which was introduced in the United States in 1995. The CDC estimates that 17,000 people contract hepatitis A each year.

CNN affiliate KYTV reported that thousands more customers might have been exposed to the virus because the infected employee worked for Red Robin for several months.

They should watch for symptoms, the station said.

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