BRUSSELS—Omicron is expected to be the dominant coronavirus variant in the European Union’s 27 nations by mid-January, the bloc’s top official said Wednesday amid concerns that a dramatic rise in infections will leave Europe shrouded in gloom during the holiday season. European Commission President Ursula von der Leyen said the EU is well prepared to fight omicron with 66.6 percent of its population fully vaccinated. She expressed disappointment that the pandemic will again disrupt year-end celebrations but said she was confident the EU has the “strength” and “means” to overcome COVID-19. “Like many of you, I’m sad that once again this Christmas will be overshadowed by the pandemic,” she said. That EU figure for vaccinations obscures the fact that some EU nations like Portugal and Spain have very high vaccination rates while others are lagging way behind. Bulgaria has just 26.6 percent of its people fully vaccinated, according to the …