Yet another panel of scientists has found no evidence that a popular vaccine causes autism. But despite the scientists’ best efforts, their report is unlikely to have any impact on the frustrating debate about the safety of these crucial medicines.

“The M.M.R. vaccine doesn’t cause autism, and the evidence is overwhelming that it doesn’t,” Dr. Ellen Wright Clayton, the chairwoman of the panel, assembled by the Institute of Medicine, said in an interview. She was referring to a combination against measles, mumps and rubella that has long been a focus of concern from some parents’ groups.

The panel did conclude, however, that there are risks to getting the chickenpox vaccine that can arise years after vaccination. People who have had the vaccine can develop pneumonia, meningitis or hepatitis years later if the virus used in the vaccine reawakens because an unrelated health problem, like cancer, has compromised their immune systems.

The same problems are far more likely in patients who are infected naturally at some point in their lives with chickenpox, since varicella zoster, the virus that causes chickenpox, can live dormant in nerve cells for decades. Shingles, a painful eruption of skin blisters that usually affects the aged, is generally caused by this Lazarus-like ability of varicella zoster.