Most of the side effects are mild and transient. About 10 to 15 percent of recipients develop a fever, and fewer recipients develop a rash, swollen glands, or joint pains. A scarier side effect is the one in three thousand risk of a seizure. Fortunately, the seizure is also transient and not likely to lead to long-term consequences like epilepsy.
Another rare complication of the vaccine is a temporary low platelet count. Platelets are cells that help prevent bleeding, so it is possible to have problems stopping bleeding when the platelets are low. This complication occurs in about one out of thirty thousand doses.
There have been some cases of encephalitis after the MMR vaccine, but it is unclear if they were due to the vaccine or not. There is a natural background rate of encephalitis in the general population, even when no vaccines are given and it is not clear that the vaccine added to the background rate. In other words, the cases of encephalitis may have been coincidental and not caused by the vaccine. However, the MMR product insert says that the “data suggest the possibility that some of these cases may have been caused by measles vaccines.” Even if this is true, though, the estimated risk of one in a million is much less than the risk of encephalitis from the disease (one in a thousand).
There has been much fanfare about the MMR vaccine increasing the risk of autism. In brief, many well-respected studies find no link between the MMR vaccine and autism when looked at on a population basis. Whether certain individuals are particularly susceptible is another question, but on a statistical basis, we am comfortable that there is no increased risk.