Surveillance refers to how researchers and public health officials identify, locate, count, and track a particular disease or other condition. Surveillance is used to understand a disease’s epidemiology, including its spread, its risk factors and groups with the highest risk. It’s also used as a tool for identifying and implementing preventive or treatment-related interventions and, as the WHO put it, documenting “the impact of an intervention or track progress towards specified goals.” For example, worldwide polio surveillance is vital to the goal of eradicating polio through vaccination.
Deeper Dive
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.
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).