This season’s flu vaccine is expected to be more effective than last year’s, when the vaccine proved to be a poor match for circulating strains of the virus, health officials said yesterday. This year, the Centers for Disease Control and Prevention is recommending that more Americans than ever before get vaccinated against the flu. Children and adolescents ages 6 months to 18 years old should get the vaccine, as should those with chronic health conditions, those who will probably interact with people who have the flu (like healthcare workers), pregnant women, and adults older than age 50.