3.1 Passive surveillance

As the term suggests, passive surveillance is defined by the absence of an ‘active’ search for cases. It involves using notifications of disease cases by health facility workers who are not employed specifically to conduct surveillance or the use of routinely collected data generated by organisations as part of their normal day-to-day activities. For example, patients admitted to hospital with fever may have cultures taken from blood or urine to try and find the cause of their fever. The results of these tests, as well as being used for patient care, may also be reported to a passive surveillance programme looking at the incidence of particular bacterial infections. In a passive surveillance system, there is no deliberate stimulation of reporting by reminding health facilities to report on disease events. Such surveillance may be relatively cheap to conduct if it uses existing facilities and systems and so can be implemented across large areas, for example, to include all hospitals in a country. The downside to this passive surveillance method is that the quality, comprehensiveness and timeliness of data collected is difficult to control. Most mandatory reporting requirements fall into this category of surveillance.

One form of passive surveillance is laboratory-based surveillance, where routinely collected laboratory data is reported to the surveillance system to give information on the occurrence of specific diseases.

Example 1: The National Respiratory and Enteric Virus Surveillance System (NREVSS) in the United States is an example of a passive, laboratory-based surveillance system. Its purpose is to support preparedness and response to influenza, monitor influenza epidemiology, and serve as an alert system for novel influenza and other viruses. Human influenza virus and other viral infections place a significant burden on healthcare systems, including in the USA. These viruses are constantly changing and require annual updates to vaccines administered to prevent their spread. The NREVSS gathers data from the tests being routinely performed in over 400 laboratories. Data reported weekly from these laboratories include total number of respiratory samples tested for influenza and number of laboratory-confirmed influenza positive samples. These samples are further broken down by type of influenza virus, alongside demographic data, for example, the age of patients testing positive. If you have time, see more information about this system [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] (CDC, 2020).

Example 2: RESAVIP (Reseau national de Surveillance des Virus Influenza chez le Porc) in France is a passive, event-driven, surveillance of the clinical suspicion of Influenza A in pigs. Influenza A can have severe health and economic impacts for both human and animal public health. These viruses can be zoonotic (that is they are spread between animals and humans) and have an epidemic, or pandemic, potential. Furthermore, pigs are considered reservoirs of human Influenza A viruses and can be an intermediary host for avian influenza viruses. For all these reasons, a system that tracks, for example, the circulation of influenza viruses in pigs can be most useful in the prevention of disease outbreaks. The system described here is not meant to generate prevalence or incidence rates, nor to effectively detect new viral strains, but it does allows veterinary public health professionals to have a good idea of the viral strains circulating in the country and their geographic spread. In practice, veterinarians (note, in France, clinical veterinary services are exclusively provided by private veterinarians) are given specific kits to take samples on farms wherever flu is suspected. Farmers participate voluntarily and anonymously. If Influenza A is detected in the sample by the regional laboratory, it is sent to the reference laboratory for further genetic analysis. The reference laboratory is also in charge of data management, analysis and interpretation of the results. If you have time, see futher information on RESAVIP (ESA, 2015).