Shares of Chipotle Mexican Grill (CMG - Get Report)  were falling 1.4% in trading Thursday to $708.76 after the burrito chain was downgraded to hold from buy at Jefferies on concerns that it has reached full valuation after a nearly 70% run year to date. 

"Although we believe that there are strong fundamental drivers still to unfold for CMG ... earlier than expected loyalty program roll-out and good [same store sales] checks have propelled the stock," Jefferies analyst Andy Barish wrote in a note to investors.

The firm raises its price target on Chipotle to $700 from $600.

"Although the company reached an impressive 1 million rewards members in barely 10 days, we think this key driver, along with the company's powerful digital drivers are now being better understood and discounted in the stock," Barish added.