Sudden adult death syndrome, also known as SADS, occurs when a person under the age of 40 years old suffers a sudden death without a known cause after an autopsy and toxicology screen (drug test). SADS causes a cardiac arrest when the heart rapidly stomps pumping, and the body loses heart function, breathing, and consciousness.

SADS is often referred to as sudden arrhythmic death syndrome. It is also known as sudden death syndrome, though the events themselves are not always the same. 

Most of the time, SADS occurs when the heart develops an abnormal rhythm, otherwise known as an arrhythmia. This can usually cause the heart to beat at an irregular rhythm that is too fast or too slow. If the arrhythmia is not diagnosed and treated, it can lead to cardiac arrest. 

Most scientists think SADS is caused by a heart condition that interferes with the heart’s electrical system. SADS is not the same as a heart attack (also called a myocardial infarction) as it is not caused by a blockage but an arrhythmia.

"Sudden and unexpected deaths during sleep of young adults" were first noted in medical literature in 1917. However, it is extremely likely this event has been happening in adults for centuries.

The biggest risk factors for SADS are a previous heart attack or coronary heart disease given 75% and 80% of SADS cases are linked with these conditions, respectively.

Other potential risk factors include:
- Heart issues including ejection fraction of less than 40 percent, combined with ventricular tachycardia; prior cardiac arrest; abnormal heart rhythms; Wolff-Parkinson-White syndrome; fainting episode with no known cause; extremely fast or low heart rates; heart blocks; former or current congenital heart failure; cardiomyopathy (dilated and hypertrophic); changes in potassium and magnesium in the blood; thickened heart muscles; and ventricular tachycardia or fibrillation after a heart attack
- Family history
- Genetics
- Obesity 
- Diabetes 
- Use of drugs

Finally, young people with undiagnosed congenital heart effects in younger populations or people over 35 with undiagnosed coronary heart disease may be at an increased risk for SADS due to these conditions being potentially associated with the event. Some of these undiagnosed and thus untreated conditions include long QT syndrome. Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia, and progressive cardiac conduction defect.

Vaccines have not been associated or linked to sudden death in adults or children in general. Myocarditis, inflammation of the heart muscle, has been associated in extremely rare cases following some vaccination efforts but cardiac issues following a COVID-19 diagnosis are common.

The incidence of COVID-19 in communities is closely linked with higher numbers of sudden cardiac arrest incidence and lower survival rates. In fact, the Sudden Arrhythmia Death Syndromes Foundation recommends that all SADS patients receive a COVID-19 vaccination.

A recent article in a United Kingdom newspaper has influenced several social media accounts into discussing the potential link between SADS, COVID-19, and COVID-19 vaccines. At this point in time, there is no direct link between SADS and COVID-19 vaccines. 

Myocarditis and other heart issues following COVID-19 vaccinations are currently being studies as these events have occurred in people who received vaccinations immediately before developing the condition.

However, several heart conditions are now linked to COVID-19 diagnoses.