McDonald’s Quarter Pounder burgers have been cleared as the source of an ongoing E. coli outbreak, according to a report from the Colorado Department of Agriculture. Burger patties from the fast-food chain tested negative for the E. coli O157strain, leading McDonald’s to reintroduce the popular sandwiches at select restaurants this week.

The outbreak, which has sickened 75 people across 13 states, initially led McDonald’s to halt the sale of Quarter Pounders in 13 states, representing 20% of its U.S. locations. The most recent case of illness was reported on October 10, with many affected individuals confirming they had eaten at McDonald’s. Of the 42 patients interviewed, all reported consuming food from McDonald’s restaurants.

So far, 22 people have been hospitalized, with two patients developing hemolytic uremic syndrome (HUS), a life-threatening condition that can lead to kidney failure. Sadly, one person has died from the outbreak, with the fatality occurring in Colorado.

Slivered onions used on McDonald’s burgers have emerged as the likely source of the E. coli contamination. These onions were supplied by Taylor Farms of Salinas, CA, which last week recalled whole, peeled, and diced onions distributed to food service providers. However, no test results from Taylor Farms have been made public as of yet.

McDonald’s officials announced that Quarter Pounders will be reintroduced without slivered onions at 900 locations historically supplied by Taylor Farms’ Colorado Springs facility. These restaurants are located in Colorado, Kansas, and Wyoming, as well as parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, Oklahoma, and Utah.

While beef patties were initially suspected, they have since been ruled out, with no contaminated beef remaining in the supply chain. According to the Centers for Disease Control and Prevention (CDC), an estimated 26 additional E. coli cases may be undetected as not all individuals seek medical attention or undergo specific testing for the bacteria.

Public health officials continue to monitor the situation, and McDonald’s has reiterated its commitment to food safety:

“The issue appears to be contained to a particular ingredient and geography, and we are confident that any contaminated product related to this outbreak has been removed from our supply chain and is out of all McDonald’s restaurants,” the company said in a statement.

As awareness of the outbreak grows, more people are likely to seek medical attention, which could result in an increase in confirmed cases.

Consumers are advised to remain vigilant and monitor any symptoms if they have consumed raw onions or products containing them.

Stay tuned for updates on this outbreak and other food safety news.