The E. coli bacteria outbreak in four Midwest states from an unknown source has affected 47 more people, the U.S. Centers for Disease Control and Prevention said on Thursday, with a majority of the total 84 reported to have sandwiches at Wendy's.

The agency said 52 people of the 62 it interviewed reported to have eaten sandwiches with romaine lettuce at a Wendy's restaurant in the week before they fell ill.

"We are fully cooperating with public health authorities on their ongoing investigation of the regional E. coli outbreak," the burger chain told Reuters in an email.

As a precaution, Wendy's has discarded and replaced the sandwich lettuce at some restaurants in the affected regions, the company confirmed.

"The lettuce that we use in our salads is different, and is not affected by this action," the company said.

The CDC, however, is not advising against eating at Wendy's restaurants or asking people to avoid romaine lettuce as it is yet to confirm the exact cause of the outbreak.

As of Wednesday, 38 people have been hospitalized across Michigan, Ohio, Indiana and Pennsylvania. Although many strains of E. coli are found in the intestines of healthy people and animals and are harmless, certain types can cause severe abdominal cramps, bloody diarrhea and vomiting.

The agency said the actual number of sick people is likely to be higher than reported and the outbreak may not be limited to the four states.