The CDC announced on Monday that Chipotle Mexican Grill's E. coli outbreak is over.
Two separate outbreaks of the bacteria were investigated by the U.S. Food and Drug Administration, the U.S. Department of Agriculture's Food Safety and Inspection Service and public health officials in several states.
In the first outbreak 55 people were infected by the foodborne illness in 11 states, of which 21 were hospitalized. The second, smaller outbreak, infected five people from three states, of which one was hospitalized.
The CDC determined that 57 percent of people affected by the first outbreak and 80 percent of people affected by the second outbreak were female.
There were no reports of hemolytic uremic syndrome, which can lead to kidney failure, and no deaths in either outbreak.
Chipotle stock was up 4 percent on the news.
"We are pleased that the CDC has concluded its investigation, and we have offered our full cooperation throughout," Chris Arnold, a spokesman for Chipotle, said in a statement. "Over the past few months we have taken significant steps to improve the safety of all of the food we serve, and we are confident that the changes we have made mean that every item on our menu is delicious and safe."
Investigators were not able to identify the ingredient that was responsible for the contamination.
Chipotle, under scrutiny for months over outbreaks of foodborne illness linked to its restaurants across several U.S. states, may have rattled investors, but teen and young adult customers were not deterred from the fast casual chain, according to the NPD Group, which continually tracks consumer's awareness about food safety outbreaks.
"Young adults represent the largest share of Chipotle's overall traffic," Bonnie Riggs, NPD Group's restaurant industry analyst, said in a statement. "Their willingness to overlook any food safety concerns to eat at Chipotle could be a result of unabashed loyalty or lack of awareness."
Consumers that did not remain loyal to the fast casual restaurant were won over by Chick-fil-A, Wendy's, Burger King and McDonald's.
"What our research tells us is that Chipotle has a strong loyal base from which to build its business back up relatively quickly," says Riggs. "To win back the trust of their former customers, Chipotle will need to continually communicate all of the ways in which they are preventing any future outbreaks and prove to them that they able to deliver on their 'food with integrity' promise."
The fast casual chain will be briefly closing all its stores on Feb. 8th.
"We are hosting a national team meeting to thank our employees for their hard work through this difficult time, discuss some of the food safety changes we are implementing, and answer questions from employees," Arnold said.
The company's sales plunged 30 percent in December, following at least six outbreaks tied to Chipotle in the last year, including norovirus, E. coli and salmonella.
At least nine lawsuits have been filed, according to The Chicago Tribune. And more suits are coming, says Bill Marler, a food and safety litigator in Seattle.
"I represent a total of 75 people, but I haven't filed all their lawsuits yet," he told the Tribune.
At an investment conference in Orlando, Florida in January, Chipotle executives said the company is taking measures to reduce the risk of another food scare to "near zero."
The company will start "inviting customers back" to restaurants in February with increased marketing and direct mail offers.
Chipotle Chief Financial Officer Jack Hartung noted the company's recovery would be "messy," with investments in food safety and marketing eating into profit margins.
Chipotle, which is based in Denver and has more than 1,900 locations, said it has already made many changes to tighten its food safety. The steps include moving the chopping of tomatoes and lettuce to a centralized location, and blanching onions to kill germs before they're chopped.
The Associated Press contributed to this report.