Close to 50 percent of all adults nationwide who have been hospitalized for the flu are obese, USA Today reported.

According to the Centers for Disease Control and Prevention, 46 percent of adults hospitalized for the sometimes deadly virus were obese as of Jan. 4. This is an "unusually high" for the flu season.

"I think we're seeing the same sort of pattern emerge we saw in 2009," Joseph Bresee, a CDC influenza expert, told USA Today.

In previous years, the percentage of obese patients hospitalized for the flu is around 20 to 30 percent, Zack Moore, a medical epidemiologist with the North Carolina Division of Public Health in Raleigh, N.C., told USA Today.

This flu season the majority of cases have been caused by the H1N1 strain, which caused a global pandemic during the 2009 - 2010 flu season.

The CDC also found that more pregnant woman than normal has been hospitalized with the flu this season. So far, 22 percent of expecting woman was hospitalized for the virus. Typically, the percentage of pregnant women who went to the hospital because of the virus is around 4.6 percent.

Researchers are reportedly not sure why these groups are at an increased risk for catching the flu than in previous years, but Bresee said the trend could be linked to immunological effects. Obesity and pregnancy alters the immune system and its response to viruses. They both usually result in respiratory restrictions.

"The most common complication that sends people with the flu to the hospital is pneumonia," Lisa Winson, infectious disease physician with San Francisco General Hospital, told USA Today. She added that some of her patients infected with the virus needed "to go to the intensive care unit and be on a ventilator."