Shake Shack Killing It?

Shares of the burger chain Shake Shack soared almost 20% after the company beat on first-quarter earnings and revenue. But those gains cannot be justified according to JPMorgan analyst John Ivankoe. He downgraded shares to "underweight" from "neutral."
Ivankoe wrote in a note sent out to clients on Monday, "This is currently a valuation driven exercise and we do not have enough valuation support for a rating other than Underweight following the nearly 20% gain post earnings.