Most deaths occurring early in life are attributable to causes mainly like infections, congenital defects, malignancies and accidents. Very seldom do apparently healthy children die without any evident cause and these are classified as SIDS (Sudden Infant Death Syndrome) and SUD (Sudden Unexpected Deaths) if the death occurs after infancy.
During infancy, a number of vaccines are administered and some events of unexplained deaths (SIDS-SUD) can occur in temporal association but not causatively related with vaccination (purely by chance). It is acknowledged widely that it is difficult to say whether a death soon after immunization is caused by the vaccine or is a coincidental event. Many analyses are done to investigate if the associations of frequent unexplained deaths following a particular vaccine are causatively related. The PSUR does an Observed/Expected analysis of sudden deaths to evaluate whether the number of sudden deaths reported exceeds what can be expected as coincidence.