Chipotle Mexican Grill Inc. shares fell 2.6% in early trade Wednesday, after employees at a Boston-area restaurant were reported to have been diagnosed with norovirus.

A restaurant in the town of Billerica, near Boston was closed Tuesday, after four employees were diagnosed with the illness, the Boston Globe reported, citing health officials. Norovirus symptoms include diarrhea, vomiting, nausea and stomach pain as well as fever, headache and body aches.

There were no reports of sick customers, and a Chipotle spokesperson told the newspaper the restaurant would be fully sanitized before reopening.

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The news comes as Chipotle CMG, -0.80% struggles to recover from a series of foodborne illness outbreaks, including an outbreak of norovirus in another Boston outlet that sickened about 140 people in December.

Chief Executive Steve Ellis has apologized for that incident, as well as E. coli outbreaks across the country last year.

The fast-casual restaurant chain has taken a series of measures to improve food safety procedures, including requiring that suppliers test ingredients before shipping them to the company. A new tracing system tracks them as they move to stores. Produce preparation has been centralized and marinade procedures have been changed, among other measures.

The Centers for Disease Control and Prevention said in February that no new cases of E. coli have occurred since December and that the outbreak appeared to be over.

The bad publicity has pummeled Chipotle shares, which have fallen 22% in the last 12 months, while the S&P 500 is down about 5%. The company reported fourth-quarter revenue that represented its first-ever year-over-year decline and a 44% drop in net profit.