Do I have to have a launch party, or can I just send out announcements?
No. You don't have to have a launch party.
However, book sales are greatly benefited by the human connection—the more you interact with readers on a personal level where they can meet and greet and get to know you, the better your sales. A launch party is a fun way to do this.
If you just can't do a face-to-face launch party, then get on Facebook and make friends. Blog. Have a virtual launch party with activities and prizes. Do something to let people know you and like you.