Smallpox and smallpox vaccination: neurological implications.

Compulsory vaccination was discontinued in the U.S. in 1972; the world was declared free of smallpox infection in 1980. Since that time, no new smallpox infections have been recognized, and only limited numbers of military and laboratory personnel have been vaccinated. As a result, the majority of the U.S. and the world population have no or diminished immunity to smallpox. Widespread vaccination, beginning with the military and health care workers, is now being undertaken. Public health strategies for immunizing the general population include preexposure voluntary vaccination, case surveillance with ring vaccination, and mass vaccination at the time of attack. Cutaneous complications of vaccination occur in immunosuppressed subjects and in those with atopic dermatitis. Among the most serious complications is postvaccinal encephalomyelitis (PVEM). A related condition, postvaccinial encephalopathy (PVE), may be seen in children less than two years of age. There are no markers to predict who will develop PVEM. In the past, mortality was high, ranging from 10 to 50%. The neuropathology of PVEM suggested an immune-mediated attack on the CNS, but the target of the immune response is unknown. Comprehensive programs are needed for surveillance and confirming case definitions for neurologic complications. Multi-institutional controlled trials of antiviral and immune modulating therapy of PVEM should be considered. Neurologists should be actively involved in the planning process for vaccination programs and in the treatment of neurologic complications.