Getting a flu shot may protect you not only from flu, but also from pneumonia, the leading cause of flu-related hospitalizations and deaths, a new study suggests.

Most children and adults hospitalized for flu-related pneumonia haven't had a flu shot, the researchers said.

"Influenza vaccine can substantially reduce the risk of hospitalizations for influenza pneumonia, a serious complication of influenza infections," said lead researcher Dr. Carlos Grijalva, an associate professor of health policy at Vanderbilt University School of Medicine in Nashville.

"We estimate that approximately 57 percent of hospitalizations due to influenza pneumonia could be prevented by influenza vaccination," he said.

The report was published online Oct. 5 in the Journal of the American Medical Association.

For the study, Grijalva and his colleagues collected data on nearly 2,800 patients hospitalized for pneumonia in four U.S. hospitals from January 2010 through June 2012.

Approximately 6 percent of these patients had flu-related pneumonia, while other patients were hospitalized for pneumonia that was not caused by influenza, Grijalva explained.

"We compared the history of influenza vaccination between these patients. We found that influenza vaccination was associated with a reduced risk of influenza pneumonia that required hospitalization," he said.

In the United States, annual flu epidemics send more than 200,000 people to the hospital and kill as many as 49,000, according to the U.S. Centers for Disease Control and Prevention.

The leading cause of these hospitalizations and deaths is not flu itself, but pneumonia, which is a common complication of flu, according to Dr. Marc Siegel, an associate professor of medicine at NYU Langone Medical Center in New York City.

"The flu is a great enabler," Siegel said. "It enables heart disease, pneumonia, appendicitis, sore throats and earaches."

And pneumonia is the number one cause of hospitalization and death, he added.

Siegel pointed out that the study didn't prove that getting a flu shot also prevents pneumonia, but only shows that most people who had flu-related pneumonia hadn't been vaccinated.

To prove a cause-and-effect relationship, researchers would have to assign one group of people to receive flu vaccines and another group to not receive flu vaccines. The rates of hospitalization for flu-related pneumonia in both groups would then have to be compared, he explained.

"This study raises the prospect that the flu shot decreases the risk of getting pneumonia," Siegel said. "I'm buying that -- but it's not proof."

But it is possible that the flu shot offers protection from pneumonia, Siegel added.

"People may get some protection they are not even counting on," he said. "You may get added protection against the real killer -- pneumonia."

All Americans over the age of 6 months should be vaccinated each year against the flu, the CDC recommends.