common cause of death of infants between ages 2 weeks and 1 year, accounting for one-third of all deaths in this age group.

Thrombocytopenia.

A severe decrease in the number of platelets, the cells involved in clotting. Thrombocytopenia may stem from failure of platelet production, splenic sequestration of platelets, increased platelet destruction, increased platelet utilization, or dilution of platelets.

Thrombocytopenic purpura.

Severe thrombocytopenia characterized by mucosal bleeding and bleeding into the skin in the form of multiple petechiae, most often evident on the lower legs, and scattered small ecchymoses (bruises) at sites of minor trauma. In children, idiopathic thrombocytopenic purpura is usually self-limited and follows a viral infection.

Transverse myelitis.

A clinical syndrome characterized by the acute onset of signs of spinal cord disease, usually involving the descending motor tracts and the ascending sensory fibers, suggesting a lesion at one level of the spinal cord. It can occur in isolation or as part of a multifocal demyelinating disease such as multiple sclerosis or acute disseminated encephalomyelitis. See Chapter 3.

Vaccination.

Inoculation with a vaccine for the purposes of inducing immunity. In this report the term has been accepted to be synonymous with immunization. See Immunization.

Vaccine.

A material containing live attenuated or killed microorganisms, or constituents of microorganisms, capable of eliciting protection against infection.

Vaccine Adverse Event Reporting System (VAERS).

A passive surveillance system intended to collect reports of reactions to vaccines that is under the aegis of the Centers for Disease Control and Prevention and the U.S. Food and Drug Administration. This system replaces the Monitoring System for Adverse Events Following Immunization (MSAEFI) and a similar system formerly run by the Food and Drug Administration. See Chapter 10.

VAERS.

See Vaccine Adverse Event Reporting System.