Boston College said on Tuesday eighty students fell sick after eating at a Chipotle Mexican Grill restaurant this weekend, but the company said the outbreak appeared to be limited to one outlet and not caused by E. coli.
Boston College had said on Monday that thirty students had taken ill after eating at the restaurant in Boston's Cleveland Circle and Chipotle said it had temporarily shut the outlet.
Chipotle, which is grappling with a states-wide E. coli outbreak linked to its burrito chain, said the Boston illnesses were likely caused by norovirus, not E. coli, and that it would not shutter any other outlets in the city.
"There are no reports of illness from any other restaurants, and the pattern here looks like norovirus isolated to one restaurant," spokesman Chris Arnold told Reuters on Tuesday.
Chipotle has come under scrutiny since November after it was linked to an E. coli outbreak in nine U.S. states, raising concerns about the fast-growing company's continued success and the damage to its brand.
Norovirus is a common cause of food-borne illnesses and has symptoms similar to E. coli, which are bacteria commonly found in the intestines, some strains of which can cause severe food poisoning. Norovirus can be contracted from an infected person or from contaminated food, water or surfaces.
Boston College spokesman Jack Dunn said on the Tuesday that affected students have been tested for E. coli and norovirus, and that the results are not expected for at least two days.
If the incident in Boston does turn out to be linked to norovirus, then that would not be as serious as E. coli, Maxim Group analyst Stephen Anderson told Reuters.
The Boston incident is a "near-term negative reinforcement", when viewed along with the E. coli incidents in Chipotle's restaurants and other health-related issues the company has dealt with since the summer, Anderson said.
Chipotle's shares fell as much as 5.4 percent in early trading on Tuesday, but has since recovered some of their losses to trade down 1.8 percent at $542 in the afternoon session.
The stock had fallen more than $100, or 14 percent, since November when reports of an outbreak in Washington and Oregon first emerged.
Since then cases have been reported in Illinois, Maryland, Pennsylvania, California, Minnesota, New York and Ohio.
Chipotle warned last week that the outbreak would likely cause same-restaurant sales to fall for the first time in company's history in the current quarter.
The company also said sales could be battered by additional reports of illness.