Chipotle Mexican Grill can't seem to put its food-safety woes behind it.
Shares of the burrito chain fell more than 4 percent on Thursday to below $311 after UBS analyst Dennis Geiger said the brand's food-safety reputation was still tarnished.
Geiger downgraded the brand to sell from neutral and slashed his price target to $290 from $345.
In partnership with the UBS Evidence Lab, Geiger analyzed online customer review scores and found that they have continued to decline and are now below what they were during Chipotle's food safety crisis in 2015.
"Despite aggressive efforts to improve brand perceptions through a new national advertising campaign and the launch of new products including queso recently, customer review scores have not shown any signs of improvement," he wrote in a research note.
"Chipotle's brand perceptions remain challenged and headline risk from potential future incidents remains elevated," Geiger wrote. "As a result, it remains unclear how long it will take for Chipotle brand perception to reach previous levels."
Earlier this week, shares of the burrito chain plummeted on anecdotal food-poisoning reports that surfaced on Twitter and the iwaspoisoned.com website, where people can report food poisoning incidents. At the time, representatives for Chipotle told CNBC they had not been informed of any widespread food-safety incidents.
According to Geigor, 37 percent of the more than 1,600 customers polled online by UBS said they ate Chipotle less frequently because of food safety concerns.
"Chipotle is a case study in what happens when a brand loses the trust of its consumers," David Henkes, principal at Technomic, told CNBC via email. "The market is too competitive right now, and consumers have so many choices, that once that bond is broken, it's hard to get consumers back."
Chipotle is set to report fourth-quarter 2017 earnings after the bell on Feb. 6.