The risk is highest among people with primary immunodeficiency diseases (PIDD), chronic genetic disorders that cause the immune system to malfunction.

Although the risk for people with compromised immune systems has been known, and is even included in the package inserts supplied with the vaccine, it was theoretical, say researchers from the Children’s Hospital of Philadelphia, who say they have uncovered “genuine evidence of harm.”

The researchers analysed the health profile of 14 people—four adults and 10 children—who suffered some form of a PIDD. Seven of them still had the rubella virus in their tissues, suggesting that their immune systems were too weak to get rid of the virus in the vaccine. The virus can damage skin cells and cause ulcers, and makes the person more susceptible to the actual rubella virus, the researchers say.

People with a poor immune system already have compromised T-cells—which are responsible for clearing viral infections—and the MMR makes the problem worse.