According to the CDC, start dates for when people became ill range from 12 September and 21 October.

The onions were directly distributed in many western and midwestern states, including Colorado, Iowa, and Kansas. Other areas were also affected, such as Oklahoma, Idaho, and New Mexico.

Onion and environmental samples from McDonald's stores and distribution centres have been collected by the FDA, the Colorado Department of Agriculture, and the Colorado Department of Public Health and Environment.

Of those who have become ill, four people developed haemolytic uremic syndrome (HUS) - a rare kidney condition that can damage red blood cells.

The federal agency said that tests on the samples were ongoing, adding that it had completed onsite inspections at an onion grower in the state of Washington and at a Taylor Farms processing centre in Colorado.

In the FDA's update, health officials said it was continuing to work with the food supplier and their customers.

"At this time, there does not appear to be a continued food safety concern related to this outbreak at McDonald’s restaurants," the update said.

It added that it was "unlikely" the recalled onions were "sold to grocery stores or directly to consumers".

McDonald's suspended sales of the Quarter Pounder burger in October in about a fifth of its US restaurants in response to the outbreak.

Some consumers have taken legal action against the firm, including a proposed class-action lawsuit filed in October.

The firm said in October it had stopped working with the supplier of the onions and had removed them from its supply chain.

The fast food giant resumed sales of the burger in all of its restaurants in America after it said samples of its beef patties, taken by the Colorado Department of Agriculture, tested negative for the bacteria.

On 29 October, McDonald's chief executive Chris Kempczinski apologised for the outbreak, saying the firm was "sorry for what our customers experienced".

"We offer our sincere and deepest sympathies and we are committed to making this right," he added.

McDonald's said in a statement that it "identified an alternate supplier" for about 900 restaurants that had "temporarily stopped serving Quarter Pounder burgers with slivered onions".

"Over the past week, these restaurants resumed the sale of Quarter Pounder burgers with slivered onions," it said.

The statement added: "Food safety is something we will never compromise on, and we remain committed to doing the right thing."

The BBC has contacted Taylor Farms for comment.