When Matthew applies to the conception of Jesus by the Holy Spirit the words of Isaiah that “the virgin shall conceive and bear a son, and they shall call his name Immanuel”, he is not saying that Jesus is God incarnate. The meaning of the allusion derives from the story that is being told in Isaiah 7-8. Matthew’s point, I think, is that the manner of Jesus’ conception is a sign to Israel at a time of crisis that God is with his people both to judge and to preserve.
John Doyle, however, asks about the relation of the boy Immanuel to the boy described in Isaiah 9:6-17—a passage very familiar to us from Christmas readings but, oddly, nowhere cited in the New Testament:
For to us a child is born, to us a son is given; and the government shall be upon his shoulder, and his name shall be called Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace. Of the increase of his government and of peace there will be no end, on the throne of David and over his kingdom, to establish it and to uphold it with justice and with righteousness from this time forth and forevermore. The zeal of the LORD of hosts will do this.