Chipotle Mexican Grill Inc. (CMG.N) shares took a hit after its latest run-in with sick customers.

The Mexican chain, still recovering from a food-safety crisis that battered its brand, shuttered a restaurant in Powell, Ohio, on Monday after reports of customer illnesses. The shares slumped as much as 4.4 per cent on Tuesday, the biggest intraday slide in more than a month.

“Our protocols identified a handful of illness reports at one restaurant in Powell,” the company said in an emailed statement. “We acted quickly and closed this single restaurant out of an abundance of caution and we are working with health officials to reopen this restaurant as soon as possible.

A spokeswoman for the company didn’t immediately reply to a request for comment on Tuesday.

The latest round of negative headlines comes amid renewed optimism on Wall Street that the chain can mount a comeback under Chief Executive Officer Brian Niccol, the Taco Bell veteran who took over in March. Chipotle recently posted same-stores sales that beat estimates for the second quarter as Niccol starts to reshape the company, with new menu items, increased marketing, a delivery push and store remodels.

The chain was upgraded to buy from hold by Andy Barish, an analyst at Jefferies, in a research note sent to clients early Tuesday. He cited the shift to digital sales and operational improvements that should help boost sales. Barish said Tuesday in an email that he was not changing his view on the company in light of the incident.

This “appears to be an isolated incidence in Ohio,” he said, calling it “obviously unfortunate but not indicative” of the changes and training improvements the chain has put in place.

The shares had gained 61 per cent this year through the close of regular trading on Monday, mounting a recovery after dropping each of the last three years amid the fallout from multiple outbreaks of foodborne illnesses that began in 2015.