Public health surveillance
Observance of a single case of an infectious disease can signal the start of an outbreak or cluster of cases of the disease. This may be an unusual occurrence of an epidemic prone disease or a localised outbreak within the context of an ongoing epidemic or pandemic like COVID-19. Either way, public health authorities constantly watch out for such events in order to take timely and appropriate action.
Some diseases, like malaria, present an ongoing problem in areas of the world where they occur. But, under certain conditions, they take on epidemic status and create a public health emergency. Epidemics of some diseases, such as cholera, occur spasmodically when an environment becomes favourable to the growth of its bacteria or virus. Individual cases of diseases that have been brought under control, like smallpox or measles, alert the authorities to an outbreak that could become an epidemic if not addressed.
Sudden outbreaks of dangerous known viruses like Ebola and MERS that have the potential to spread across borders become international health emergencies and become a pandemic. Public health authorities, physicians and other scientists constantly watch not only for these occurrences but for the emergence of new viruses like COVID-19.
All countries have systems of surveillance that record data about prevalent and newly emerging diseases or public health events and the international community communicates through various networks. WHO Member States have committed to the International Health Regulations (2005), agreeing to build their capacities to detect, assess and report infectious diseases and other public health events.
The public health surveillance page of this website describes how surveillance systems work. These systems use all available data to detect and control outbreaks at the local level in order to prevent them spreading as epidemics within a country or as international emergencies or even pandemics across countries.
Effective disease surveillance depends on the existence of a functioning health information system as described on the country production and use of data page of this website. Data to monitor COVID-19, for example, depend on: maintenance of patient records by health facilities; and civil registration of deaths and cause of death
Counting COVID-19 cases
Cases that present to the health system
National governments report confirmed cases of COVID-19 to the World Health Organization which issues a daily situation report that summarises all known new cases by country around the world. WHO feeds this information into its COVID-19 dashboard.
The WHO provides this guidance to member states and requests them to either:
- Use case-based reporting when numbers are few: Report each probable and confirmed case of COVID-19 infection within 48 hours of identification either on this form or as a line on a pre-formatted excel template using this data dictionary; or
- Use aggregated reporting as numbers increase: Report weekly the aggregated numbers of new confirmed cases together with other information as detailed in this excel sheet;
In order to produce these reports, the coordinating body of each government collates all cases that have tested positive for COVID-19 and have been reported to health facilities across the country. The number reported depends on cases being identified and being tested. Hence the numbers reported depend on the surveillance strategies in place in the country and the availability of suitable testing.
These reports are of cases confirmed by laboratory tests and hence only represent a proportion of the total number of cases in the community. The proportion will depend on whether cases reach health facilities and the availability of reliable testing at those facilities.
Seeking out cases in the community
When testing is available, public health authorities can test all health workers, follow up and test contacts of known cases, test self-reported mild/moderate cases reported in the community, and proactively visit care homes, prisons and other potential local epicentres of the virus.
Identifying past infections in the community
With a reliable antibody test, public health authorities can undertake sample surveys to assess populations for evidence of past infection. This will provide information of the extent of past infection – and potentially the degree of herd immunity – and help identify the population sub-groups that are more exposed to the virus.
Counting COVID-19 deaths
The accuracy of death and cause of death reporting depends on the state of the civil registration and vital statistics system in a country. We explain how civil registration and vital statistics systems operate on this page of the website and describe the varying qualities of these systems around the world. In high-income countries , every death is recorded with a certification of its direct and underlying causes. This is not the case in many low- and middle-income countries where not all deaths are reported to a central system and causes of death may not be provided at all.
Countries that have functioning civil registration and vital statistics systems publish weekly mortality reports and provide regular breakdowns by cause of death. Since the start of the COVID-19 virus, countries have been reporting the numbers of deaths from all causes and the number of deaths associated with COVID-19 (using varying definitions).
The total number of deaths from all causes is an important indicator of the mortality consequences of COVID-19. It is possible to compare this figure to a baseline for a previous year, for example, and calculate the number of excess deaths in a particular time period, as in the example from the UK below. The number of excess deaths, over and above those classified as COVID-19, may include COVID-19 deaths that were not certified as such and deaths resulting indirectly from COVID-19, for example because of insufficient health system capacity or people’s reluctance to use the system.
In the future, demographers will make retrospective estimates of the excess numbers of deaths during the COVID-19 pandemic, even for countries which do not record every death – using indirect demographic techniques.
Deaths in England and Wales in 2020 compared with the average between 2015 and 2019, by whether or not they involve COVID-19
World Health Organization Global Surveillance of Epidemic-prone Infectious Diseases.
The International Health Regulations (2005) are central to global surveillance.
The World Health Organization has issued these International guidelines for certification and classification (Coding) of COVID-19 as cause of death.
The Health Foundation explains how to interpret excess deaths: Understanding excess mortality. What is the fairest way to compare COVID-19 deaths internationally?
The United Nations Statistics Division provides an overview of how each country’s registration system is dealing with the COVID situation. Impact of COVID-19: Maintaining Civil Registration and Vital Statistics during the COVID-19 pandemic.
The United Nations has also issued these recommendations Maintaining Civil Registration and Vital Statistics during the COVID-19 pandemic.
The Ministry of Public Health, Thailand published Thailand s experience in the COVID-19 response. This book aims to provide policy lessons for other countries to overcome the disease.
This article in OurWorldInData.org describes issues of counting COVID-19 cases and deaths