I don't know if anyone has ever gotten Rubella after receiving a MMR, but what I can tell you is the following: It's not possible to say whether any vaccine, including MMR, is absolutely safe. Some children do get side-effects. However, these are very rare after the first dose and even less likely after the second. Vaccines are very safe, and it is much safer to get the vaccine than to get measles, mumps, or rubella. As you have probably seen on the news recently, measles are unfortunately starting to make a comeback again in the US, and I would much rather have my child vaccinated then take the chance of getting rubella.