Surveillance is the process or system for tracking cases of risk factors, medical conditions, disease cases, adverse events, etc. It’s often used to track incidence of a disease or side effects from drugs or vaccines. The two basic types of surveillance are active and passive. Passive surveillance is collection of data from those voluntarily reporting it, such as hospitals, health care providers, parents or health departments. Active surveillance involves actively looking for cases either with a reporting system or using a systematic protocol, such as calling every health department in a region during a disease outbreak.
Deeper dive
With passive surveillance, no one is actively systematically looking for specific cases. Passive surveillance is useful for looking for patterns or “signals,” such as a cluster of disease cases or a higher-than-expected report of side effects with a pharmaceutical. The biggest limitation of passive surveillance is not knowing the denominator, the total number of encounters or people that the cases are reported from, since reporting is voluntary and self-selecting. Active surveillance, on the other hand, has a clear denominator, the total number of individuals, clinics, hospitals, populations, etc. that are assessed. With active surveillance, you can establish incidence or prevalence (the numerator) as a fraction of the population assessed (the denominator).
The Vaccine Adverse Event Reporting System (VAERS) and FDA Adverse Event Reporting System (FAERS) are both passive surveillance systems: adverse events are reported by providers or patients, but the existence of a report does not mean the event resulted from the vaccine or drug. The report may or may not be true, the event may or may not have been caused by the drug, and there’s no way to know how many people did NOT report the same event (or did not experience it at all).
Vaccine Safety Datalink (VSD) is an active surveillance program which researchers can use to search for a specific adverse event among an established number of patients who received a vaccine (the denominator).