Tufts Health Plan warned subscribers Thursday that personal information for about 8,830 seniors enrolled in the insurer's Medicare plan was stolen.   The Watertown-based organization said the theft included the names, birth dates and Social Security numbers of current and former subscribers of the Tufts Medicare Preferred plan. Medical and health information was not included in the theft, Tufts reported.   "As soon as we learned of this situation we took immediate action, conducting our own investigations while working very closely and in full cooperation with federal investigators," Tufts said in a statement.   It was not immediately clear how or when the information was stolen. The insurer said in a news release that the theft was brought to Tufts' attention by federal officials during an investigation that continues. Tufts reported the theft was not caused by an electronic breach, hacking or information technology system vulnerability.   A Tufts spokeswoman declined to provide more details on the theft. A spokesman for the Office of the Inspector General for Social Security did not immediately return a phone call late Thursday afternoon.   The insurer said it would offer credit monitoring and identity theft protection to people impacted by the theft and to Tufts Medicare Preferred members whose information was not stolen.   "We cannot convey strongly enough our regret that this incident occurred," Tufts said in a statement.  ;