In its evolutionary fight for survival, the COVID virus is switching strategies: It’s becoming a master at slipping past our immune systems. And that, say experts, is largely why we’re dealing with an unexpected surge. Powered by two mutations, new lineages of the omicron variant — called BA.2 and its more recent descendants BA.2.12.1, BA.4 and BA.5 — are increasing rates of vaccine breakthrough and reinfection, according to an analysis published Saturday by Trevor Bedford, a virologist at the Fred Hutchinson Cancer Research Center in Seattle, who studies the evolution of viruses. These latest…