I'm with the Minx on this.
You've done SW before, so you've got all the stuff, and you know all the rules. So you don't need to put off starting following the plan, but it really isn't fair to everyone else to go out into a big group of people when you're quite possibly going to be spreading something nasty.
Start on Plan, but leave it a week to join the group. Apart from anything else, there'll be a much smaller crowd next week, once the New Year's Resolution people have done their first week.