The FDA and the CDC said that with millions of vaccinations "by chance alone some serious adverse effects and deaths" will occur in the time period following vaccination, but they have nothing to do with the vaccine. More than twenty women who received the Gardasil vaccine have died, but these deaths have not been causally connected to the shot, as correlation does not imply causation. Where information has been available, the cause of death was explained by other factors. Likewise, a small number of cases of Guillain–Barré syndrome (GBS) have been reported following vaccination with Gardasil, though there is no evidence linking GBS to the vaccine. It is unknown why a person develops GBS, or what initiates the disease.