Omicron is the unwelcome party guest at everyone’s holiday gatherings this year.Scientists are still trying to learn about the new Covid variant — including how transmissible it is, how well current vaccines work against it and where its biggest hotspots are. Many of those answers won’t be known for at least a couple weeks, just in time for Christmas. Without those answers, you may need to ask yourself a tough question: Is it safe to attend a holiday party? If everyone at your party is vaccinatedIf you’re vaccinated and gathering with other vaccinated people, the answer is a simple “yes,”...