But there is insufficient evidence to know whether programs that focus on encouraging teens to remain sexually abstinent until marriage reduce the chances adolescents will engage in sexual behavior, be less likely to become pregnant or less likely to get a sexually transmitted disease, the panel concluded.

"There is sufficient evidence that comprehensive risk reduction efforts are effective," said Randy Elder of the Centers for Disease Control and Prevention, who works with the Task Force on Community Preventive Services, an independent panel that issues public health recommendations.

"As regards abstinence education, after a similar look the task force determined that based on a number of problems with the studies presented to them there was insufficient evidence to determine their effectiveness," Elder said.

The conclusions came after the task force reviewed an analysis of 83 studies of sexual education programs run between 1980 and 2007. The analysis was conducted by a 19-member team of experts assembled by the CDC to examine the politically sensitive question.

The findings were immediately welcomed by advocates of programs that reduce teen pregnancy and critics of abstinence programs.

"At long last, evidence and common sense have returned to public health policy," said James Wagoner, of Advocates for Youth in Washington, D.C.

But the findings were condemned by advocates of abstinence programs.

"If you compare the statistical significance of outcome measures related to sexual activity and sexual initiation the data is better for the abstinence education program than the comprehensive program," said Valerie Huber of the National Abstinence Education Association.