How common is unexplained death syndrome in adults?

A news article reporting that police do not know the cause of death does not mean the cause of death is from unexplained death syndrome. In the context of the article, it probably means since there is no sign of foul play, there is no reason for the police to probe any deeper for the cause of death. There are all sorts of circumstances where cause of death cannot be determined. The rates of unexplained deaths depend more on coroner's procedures, skills, budget, and equipment than the actual cause of death. This phenomenon also has a different meaning in the UK than in the rest of the world, due to the way reporting is done in the UK. Docs in the UK lump together sudden cardiac arrest in people with no history of heart problems with all other unexplained deaths, while most other nations consider sudden cardiac arrest different from unexplained deaths even when the cause of cardiac arrest is unknown. In the UK, unexplained death syndrome is about 10-12 in 100,000, and where cardiac arrest is excluded, the rate is about 1-2 in 100,000. In the UK accounting, females and males are affected at similar rates. Elsewhere, the phenomenon affects mostly males, and the rates differ substantially among ethnic groups.

In the mid 1970's Japan raised their vaccination age from two months to two years. Their incidence of Sudden Infant Death Syndrome dropped so dramatically they went from an infant mortality ranking of 17 to first in the world - Japan had the lowest infant death rate when infants were not being immunized.

Never heard of it in the U.S.. The law here is that any unexplained death of a person under the age of sixty requires an autopsy. Sixty is the presumptive age of natural death here, and it's thought that above that age you can go ahead and drop dead if you want to. Under that age, we want to know what happened to you. Unexplained deaths even after autopsy are very rare.