"We don't yet know how fast it was going, so we are not yet sure whether it could have been even worse," Karin Köhler from Gotha police in Thuringia told The Local.
There were only four passengers in the bus when it came off the bridge shortly after 5pm on Tuesday.
"The bus initially hit the central barriers of the road, then swerved back in the other direction and smashed through the wall and fell off the bridge into the vegetation," said Köhler.
She said speculation in some German media outlets that the 64-year-old driver had suffered health problems which may have caused the crash were only speculation and should not be taken seriously.
The driver and a 74-year-old woman passenger were so badly injured that they were taken to a nearby hospital by helicopter.
A 36-year-old woman was also seriously hurt, while a 46-year-old man and a 17-year-old girl were less badly injured.
Investigators are examining the bus and the road to determine what caused the accident.