Postvaccinal encephalitis

Did You Know?

• Symptoms of PVE appear between the 8th and 14th day after vaccination.
• Initial symptoms include fever, headache, confusion, and nausea.
• Vaccinia immunoglobulin showed significant effects only if given before PVE developed.

Histology

Inflammatory extra-adventitial lesions occur in both the brain and spinal cord. Lesions may be uniform in the acute phase or disseminated in the subacute phase. The main damage is in white brain matter, unlike encephalitis lethargica. Meninges show infiltration by T cells, plasmatic cells, and phagocytic cells; polymorphonuclear cells appear only in severe lesions. Perivascular cellular infiltrate is accompanied by tissue rarefaction near damaged blood vessels, where accumulated small nuclei are found. Strong demyelination with rapid clearance of degraded myelin is observed, and tissue damage ultimately leads to necrosis.

Incidence

Deaths directly linked to PVE also differed, with 11 deaths per million for the Bern strain and 1.2 per million for the NYCBH strain. Age influenced incidence, leading to exclusion of children up to one year of age in the US and up to three years in Europe from vaccination.

History

Central nervous system complications after smallpox vaccination were observed soon after vaccination began, with the first diagnosed case of PVE in 1905. During the smallpox eradication campaign, tools to identify the immune mechanism behind PVE were unavailable. Modern smallpox vaccines are safer and only selected personnel are vaccinated, so PVE is no longer a central concern. Due to its similarity with acute disseminated encephalomyelitis (ADEM), another postvaccinal adverse reaction, PVE is considered autoimmune in nature, though direct proof of vaccine virus replication in neural tissues causing PVE is lacking.