"What was exciting was the sustained effect over the length of the follow-up," said lead author Dr. William R. Beardslee of the psychiatry department at Boston Children's Hospital.
He and his coauthors had previously found a reduced risk of depression nine months after the cognitive behavioral therapy sessions began. The new results show that risk was still reduced two years after they ended.
The study included 316 teenagers of parents with current or past depressive disorders.
Half were assigned to the therapy program, which involved eight weekly 90-minute group sessions with a trained therapist followed by six monthly sessions, and the other half received standard care. The kids had symptoms of depression, but not diagnosable depressive disorders.
The researchers tracked teens' "depressive episodes" lasting at least two weeks, as reported by the kids and their parents.
During the study and the two-year follow-up period - a total of 33 months - 37 percent of kids assigned to the therapy sessions had at least one depressive episode, versus 48 percent of those in the comparison group.
But that difference was only seen among teens whose parents were not clinically depressed when the study began.
When parents were not depressed at the time of the study, cognitive behavioral therapy prevented one depressive episode for every six kids in the program, the researchers found. However, for kids with currently depressed parents, therapy sessions didn't seem to have an effect, they wrote in JAMA Psychiatry.
"First, we need to understand how current parental depression is related to differential outcomes," Beardslee told Reuters Health. "Then, we need to target these factors to reduce their effects on child outcome."
Image: Teens talking, via Shutterstock