By Camilla Wallner, BA
What is epidemiological surveillance?
Epidemiological surveillance is defined as the ongoing systematic collection, analysis, and interpretation of health data that is essential to the planning, implementation, and evaluation of public health practice. The aim is to observe, study and analyze any given infectious disease in order to get a better understanding of its spreading and its impacts on a given population.
It is the foundation for immediate action as well as long-term strategies in order to keep a disease at bay. Epidemiological surveillance falls under the competence of national authorities. However, international agencies and organisations such as the World Health Organization (WHO) play an increasingly important role ever since the SARS breakout in 2003 and even more so in the light of recent events and the outbreak of the Covid-19 pandemic. A rapid communication on an international level, the collection of background data as well as data interpretation of climatic variations are important tools in controlling and predicting the outbreak of diseases including zoonoses. Satellite surveillance of vegetation growth, for instance, is used as an indicator of rising vector numbers, thus enables the prediction of vector-borne diseases. There are different strategies for monitoring diseases, such as passive and active surveillance.
Active surveillance vs. passive surveillance
In order to collect data the different surveillance strategies draw their information directly from the source, notably health care workers. Passive surveillance is the most commonly used surveillance strategy and relies on surveillance forms filled out by health care personnel, which are periodically collected from health care facilities. The aim is to draw an approximate picture in order to assess current trends. However, it is a very shallow method as it is based on minimal data and does not offer many incentives to health care providers to actively engage in the process of data collection. It is common for health care workers to only partially fill out those forms, to neglect certain aspects or to stay vague in the description of symptoms.
One example that illustrates this problem is vaccines. Vaccines are known for being very cost-effective. In other words: the benefits outweigh the risks by far. However, clinical trials for vaccines are very costly, which is why post-licensure evaluations (for vaccines or drugs that are already licensed and allowed on the market) are often based on the results of passive surveillance in order to detect and monitor adverse events. Unfortunately, the results drawn from passive surveillance are often unreliable due to an insufficient proof of causality between the vaccine or drug in question and specific symptoms. However, passive surveillance is still used for the evaluation of contraindications to diphtheria-tetanus-pertussis (DTP) vaccine and also for evaluating the risks of combined or simultaneous vaccinations. In essence, in the case of vaccine adverse events, unwanted symptoms or side-effects of a vaccination, there appears to be a significant trend of underreporting coupled with adverse event reports that lack precision and specification.
In contrast, active surveillance institutions such as healthcare providers, work places, laboratories, nursing homes, or schools etc. are directly contacted by state authorities in order to obtain more detailed information. Both strategies have their advantages as well as their drawbacks. On the upside, passive surveillance can effortlessly be conducted on an on-going basis and reaches a broader population pool. On the downside, passive surveillance is known to provide incomplete data due to underreporting and is therefore suited for routine surveillance tasks. Active surveillance has the benefit that data is collected in a more inquisitive way and therefore requires a more intense mobilization of resources. It therefore provides more accurate data and is primarily used to investigate serious diseases, which display a potential risk to public health.
Other surveillance methods also exist, such as behavioral risk factor surveillance, outbreak surveillance, sentinel surveillance and syndromic surveillance but to name a few. Behavioral risk factor surveillance is based on an ongoing and systematic surveillance of data in order to identify possible risk factors for the outbreak of a disease, whereas outbreak surveillance is used to find the source of the outbreak via active surveillance methods. Sentinel surveillance focuses on identifying disease trends and which groups of a population are at a greater risk of contracting a disease. The aim of syndromic surveillance is to identify illness clusters in their early stages in order to limit a potential outbreak.
Alternative surveillance modi
On top of the classic surveillance methods, countries such as China are using modern technology in the battle against disease. For example, the China Infectious Diseases Automated-Alert and Response System (CIDARS) is an early warning model and a new epidemiological surveillance method. CIDARS is based on a mobile phone network which has a list of all relevant phone numbers of the epidemic staff on all levels from the national level down to the provincial level in order to inform the key personnel involved by SMS providing any given health facility reports a case.
Another big data pool or complementary channel of information is social media. An increasing number of organizations including scientific institutions seek to use data from social media platforms. However, regulations relating to the protection of personal data are at the source of major challenges in this field – which calls upon legal and ethical guidelines establishing a threshold that should not be overstepped.  Putting modern technology at use for epidemiological surveillance appears a pertinent path in the future, although there are a few more practical and ethical obstacles to overcome in order to replace present day surveillance tools.
The WHO – a global response to a global threat
The World Health Organization (WHO) plays a crucial role in epidemiological surveillance and disease prevention worldwide. Its principles for the implementation of mass vaccinations as well as the WHO’s guidelines in the case of a pandemic are an important basis for the prevention and control of illness on an international level. Ever since the outbreak of the covid-19 pandemic, the recommendations of the WHO have gained unprecedented importance, notably in terms of surveillance strategies as well as strategies in combatting the new virus. As the topic of mass vaccinations has turned into an issue of major controversy, the organization recommends carrying out a risk assessment in order to evaluate whether the potential benefits of a mass vaccination outweigh the risks (risk-benefit criteria). Furthermore, the WHO proceeds with recommendations related to the coordination and planning of such mass vaccination campaigns.
More generally, the WHO plays an important role in shaping the actions of major decision-makers, such as governments, independent laboratories, health care institutions or pharmaceutical companies in surveillance. However, the type of disease surveillance used to detect an illness or the type of remedy deployed in order to eradicate a disease are at the discretion of national actors. Public health policies are still a national matter and, even if international organizations play a role in shaping those policies, state sovereignty cannot be violated. The WHO plays a central role in coordinating global health threats – yet its capacities are purely advisory and it has no enforcement power. 
It should also be emphasized that there is a major asymmetry when comparing the scope of action of developed countries to the possibilities of developing and underdeveloped nations. While global surveillance initiatives as well as WHO recommendations may be efficient in developed countries, developing countries struggle from the additional pressure put on their already fragile health systems. It may lead to an undue shift of resources and commitment from already highly prevalent and preexisting diseases, which should however not be left unattended.
The aim of this blog post was to create a better understanding of the existing mechanisms in the field of disease prevention by presenting its central tools and most important actors, such as different types of epidemiological surveillance and the modus operandi of the WHO. Nevertheless, it is also important to outline the limits of the above-mentioned strategies as well as to show new potential analytical tools that are on the rise. To conclude, it is crucial to create awareness of the difficulties developing countries might have in following the WHO’s recommendations when combatting an infectious disease and that many countries lack the trained personnel as well as the medical equipment to meet international health standards.
 Richard J. Wolitski et al., “The Public Health Response to the HIV Epidemic in the U.S.,” in Aids and Other Manifestations of HIV Infection, ed. Gary P. Wormser, 4th ed (Amsterdam: Academic Press, 2004), 983–1000.
 S Rosenthal and R Chen, “The Reporting Sensitivities of Two Passive Surveillance Systems for Vaccine Adverse Events.,” American Journal of Public Health 85, no. 12 (1995): 1706–9.
 Weizhong Yang et al., “Chapter 7 – China Infectious Diseases Automated-Alert and Response System (CIDARS),” in Early Warning for Infectious Disease Outbreak, ed. Weizhong Yang (Amsterdam: Academic Press, 2017), 133–61.
 M. A. Mayer, L. Fernández-Luque, and A. Leis, “Chapter 5 – Big Data For Health Through Social Media,” in Participatory Health Through Social Media, ed. Shabbir Syed-Abdul, Elia Gabarron, and Annie Y. S. Lau (Academic Press, 2016), 67–82.
 WHO, Framework for decision-making: implementation of mass vaccination campaigns in the context of COVID-19. Interim Guidance May 2020 (Geneva: 2020), https://www.who.int/publications/i/item/WHO-2019-nCoV-Framework_Mass_Vaccination-2020.1, accessed February 17 2021.
 Eduardo Lazcano-Ponce, Betania Allen, and Carlos Conde González, “The Contribution of International Agencies to the Control of Communicable Diseases,” Archives of Medical Research 36, no. 6 (November 2005): 731–38.