A two-alarm blaze that began at 96 Chelsea St. in East Boston sent two firefighters to the hospital Thursday evening, fire officials said. One firefighter sustained a shoulder injury, the other a head injury, the Boston Fire Department wrote on its official Twitter account. Both were taken to Massachusetts General Hospital and were expected to be released Thursday night. Fire officials said they received a call at 5:36 p.m. for a fire on the top floor of the three-story wood frame duplex. The fire, which caused an estimated $200,000 in damage, spread to 98 Chelsea St. before it was extinguished shortly after 6:30. Five of the building’s six apartments were vacant because the building was being renovated, fire officials said.