Key concepts for epidemic response managers

A disease caused by an infectious agent or its toxic products is defined as an infectious disease. Infection occurs when the infectious agent establishes itself in the host individual and reproduces itself or some stages of its life cycle.

The transmission of infectious disease is described by a chain of six elements, or the chain of infection, that are needed for infection and disease to occur in an individual. Elimination of only one element of the chain of infection will interrupt disease transmission. This is the core principle behind epidemic prevention and control.

For some infectious agents, there are effective measures to interrupt the chain of transmission at more than one element. An example is yellow fever. Susceptible hosts can be protected with vaccination.  Vector control methods can be used to limit the reservoir and block transmission such as screens and personal protection against mosquito bites.

For other infectious agents, measures towards all of the chain’s elements are less effective. For example, if the reservoir is unknown or it is an animal; the agent can be transmitted by air; or there is no vaccine available. Understanding the chain for specific diseases ensures we can break the cycle of transmission. For each epidemic it is important to know and identify each of the below.

A reservoir of infection is a living organism or material in or on which an infectious agent lives and/or usually multiplies. Reservoirs include humans, animals and the environment. Human reservoirs may or may not show the effects of illness. The reservoir may or may not be the source from which an agent is transferred to a susceptible host. For example: Measles: Humans are the reservoir. The virus passes from an infected person directly to a susceptible host. Lassa fever: Rodents are the reservoir. The virus is present in the rodents’ urine and a human can be infected inhaling the dust containing the urine.

A zoonotic disease or zoonosis is an infectious disease that has jumped from a non-human animal to humans. It represents a major public health problem around the world due to a close relationship with animals in agriculture, as companions and in the natural environment.

A person with asymptomatic infection (a person is infected, but there is no symptoms) who can transmit the agent to others, is called a carrier. They can be:

• Healthy carriers: those who never experience symptoms, despite being infected
• Convalescent carriers: those who have recovered from their illness but remain capable of transmitting to others; or
• Chronic carriers.

There are many ways in which disease can spread. Understanding the source of infection and how it is transmitted is key in being able to prevent and control epidemics. First, the infectious agent leaves the host (exit). There are many routes of transmission that allow diseases to spread. Routes include through the respiratory tract (coughing, sneezing, kissing), genitourinary tract (urine, semen, vaginal secretion), gastrointestinal tract (stool), through skin or the placenta.

Direct or indirect transmission of the infectious agent from its reservoir to a susceptible host can occur:

• Direct transmission includes direct contact (skin-to-skin contact, kissing, sexual intercourse) and droplet spread (aerosols produced by sneezing, coughing, talking). The source is an infected person.
• Indirect transmission may be airborne (infectious agents are carried by dust or are suspended in air), vehicle-borne (food, water, blood and other biological products, objects) and vector-borne (insects). The source of infection can be a person. It can also be contaminated or infected foods, animal vectors or different materials in the environment such as water.

The portal of entry is the way an infectious agent enters a susceptible host. It can be respiratory tract (inhaling germs or kissing), mouth (food or water ingestion), skin (insect bites, cuts), mucous membrane (e.g. contact of eye with dirty hands), the blood (transfusion, needle-sharing) and trans-placental mode of entry.

The table below provides some examples of different reservoirs, portals of exit and entry, and mode of transmission.

Example different reservoirs, portals of exit and entry and mode of transmission

This time from when infection occurs to the onset of symptoms is called the incubation period. It is a range of days and it can be different for each disease. The incubation period duration will vary depending on the:

• Mode of transmission e.g. shorter for foodborne and longer for waterborne,
• Portal of entry e.g. Tetanus or Rabies will have a shorter incubation time if the wound is on the head; and
• Immune status of the susceptible person.

Knowing the incubation period is important in controlling epidemics. It helps identify who else may be infected, when and how control measures should be implemented, and helps in estimating the number of people that could get sick.

Period of infectiousness is the time interval during which an infected person can transmit the infection to other susceptible persons. It varies between diseases and it can include the incubation period, stage of clinical disease and the stage of recovery. During this infectious period, the aim of disease prevention and control is to limit exposure to the source either by using personal protective equipment, infection control measures, quarantine or isolation.

The precondition for an infection is the presence of a susceptible host. Theoretically, if the whole population is immune to an infection due to vaccination, the disease would be eradicated. However newborn babies would be susceptible, some people have contraindications and cannot be vaccinated, and the efficacy of vaccines is not 100%. As an example, one dose of vaccine against Measles is about 93% protective.

Nevertheless, the lower the proportion of susceptible hosts in population, the lower the possibility for occurrence of the infection. This is called herd immunity. A very high percentage of individuals in a population is immune, so the chance an agent will ‘find’ a susceptible host is very low. In other words, those who are immune ‘protect’ those who are not e.g. individuals with contraindications for vaccination

There are two types of immunity:

• Active immunity results when exposure to an agent triggers the immune system to produce antibodies to that disease. It may be natural active immunity (due to infection) and vaccine-induced immunity.
• Passive immunity is provided when a person is given antibodies to a disease rather than producing them through his or her own immune system. It may also be natural (a newborn baby acquires passive immunity from its mother through the placenta). A person can also get passive immunity through antibody-containing blood products such as immune globulin.

Passive immunity provides immediate protection from a specific disease, but it lasts only from a few weeks to a few months. In contrast, active immunity usually takes several weeks to develop, but it lasts much longer, and for some diseases it is a life-long.

Vaccines are biological preparations that improve immunity to a disease. A vaccine typically contains an agent that resembles a disease-causing microorganism. It is often made from a weakened or killed form of the microbe, its toxins or one of its surface proteins. The agent stimulates the body's immune system to recognize the agent as foreign, and subsequently destroy it and "remember" it so it is destroyed on any additional encounters. 

Some of the numerous vaccines available worldwide are part of a routine immunisation programme. The World Health Organisation (WHO) makes recommendations for the routine immunisation and each country decides which vaccines will be included in the national immunisation programmes. This decision is usually made based on a country’s health priorities, epidemiological situation, available health workforce, logistics (including ‘cold chain’) and financial sustainability. The WHO recommendations can be found here: http://www.who.int/immunization/policy/Immunization_routine_table1.pdf

Besides routine vaccination, vaccines can be used as in epidemic control, for international travellers or for some epidemiological (for individuals more exposed to an agent) or clinical reasons (for individuals who suffer from a disease that make them more vulnerable to another disease that can be prevented by vaccination).

Vaccine safety is particularly important as safety failure can cause severe damage to health of individuals and to the entire immunisation programme. Vaccine side effects may be results of the vaccine itself, but also a result of unsafe vaccination practices.

The purpose of the vaccine ‘cold chain’ is to maintain product quality from the time of manufacture until the point of administration. It ensures that vaccines are stored and transported within the WHO-recommended temperature ranges.

A case definition is a set of uniform criteria used to define a disease for public health surveillance. It enables public health officials to classify and count cases consistently. Case definitions can be case-based and syndrome-based.

Case-based definitions are usually based on the individual data of a patient, and epidemiological or laboratory confirmation. The main objective of syndrome-based definitions is to identify illness clusters early, before diagnoses are confirmed and reported to public health agencies. This allows the mobilization of a rapid response, thereby reducing morbidity and mortality.

Case definitions are used for surveillance purpose only - clinical diagnosis and treatment can be based on other criteria. During an epidemic, case definitions used for routine surveillance should be reviewed and modified if needed, to be more specific for the given epidemic and context. CBS is an example of a type of surveillance that uses syndrome-based definitions for detection.

Case definitions can refer to suspected, probable and confirmed cases or they can be clinical, epidemiological and laboratory case definitions, as shown in the table below.

Suspected case - Any person with acute onset of fever, with jaundice appearing within 14 days of onset of the first symptoms.

Probable case - A suspected case; and one of the following:

• presence of yellow fever IgM antibody in the absence of yellow fever immunization within 30 days before onset of illness; or
• positive postmortem liver histopathology; or
• epidemiological link to a confirmed case or an outbreak.

Confirmed case - A probable case;

And absence of yellow fever immunization within 30 days before onset of illness; and one of the following:

• detection of yellow fever-specific* IgM; or
• detection of fourfold increase in yellow fever IgM, or IgG antibody titres between acute and convalescent serum samples, or both; or
• detection of yellow fever-specific* neutralizing antibodies.

or

Absence of yellow fever immunization within 14 days before onset of illness; and one of the following:

• detection of yellow fever virus genome in blood or other organs by PCR; or
• detection of yellow fever antigen in blood, liver or other organs by immunoassay; or
• isolation of yellow fever virus.

An early declaration of an outbreak by health authorities can allow for a faster public health response and boost financial support as relevant sectors activate response actions. A declaration may also change certain procedures to facilitate control. To give an example, the World Health Organization (WHO) recommends that once a cholera outbreak is declared, there is no need to confirm in a laboratory all suspected cases and instead, any person presenting with or dying from acute watery diarrhoea should be registered and reported as a suspected case. The declaration of an outbreak can facilitate importation procedures for necessary medication and equipment or speed the travel of investigators to understand the cause of a disease, if needed.

A declaration of a pandemic is the responsibility of the World Health Organization (WHO) and will normally trigger health authorities around the world to take a range of public health measures. From establishing the use of face masks in public places, for instance, to other more drastic measures such as restrictions on international travel or trade.

Criteria to declare an outbreak, epidemic or pandemic will depend on the type of disease and the authorities (e.g., Ministries of Health) or institutions (e.g., World Health Organization) providing technical guidance for outbreak control. For instance, in the case of cholera the Global Task Force on Cholera Control establishes that if at least one sample tests positive by culture and/or polymerase chain reaction (PCR), an outbreak should be declared, and control measures should immediately follow.

A declaration by a government authority may boost funding at a domestic level, e.g., from the national government to local authorities for control and response measures; or at an international level when funding flows from international organizations and donors. In places with limited access to health services and poor infrastructure, an epidemic declaration may be perceived as an ‘opportunity’ to bring in not only funding, but other much needed infrastructure to address the underlying causes of a disease. Managers should be aware of communities potentially misreading a situation of high risk to the health of the population as a chance for benefits that may be  unrealistic or unfeasible without outbreak declaration. This is often the case of cholera outbreaks that may be perceived as an opportunity to bring in jobs, training and/or water and sanitation infrastructure. While some of these benefits may in fact follow a declaration, they are often temporary. Long-lasting improvements depend on technical skills and supervision over an extended period, continued public support and adequate budgets available. National Societies should be advocates of prevention activities rather than supporting a narrative that is reliant on epidemic crises to enhance change.

On another hand, an important factor to consider is that governments may at times fear that a formal declaration of an outbreak could negatively impact important sectors for the economy such as trade or tourism, or that the declaration may create an image of a country that has failed to provide basic services to its population (e.g., clean water and sanitation in the case of water-borne diseases). Such factors may lead authorities to delay the declaration or to not declare an outbreak altogether, e.g. in 2008 then-president Robert Mugabe declared “there is no cholera” despite an outbreak of over 16,000 cases in Zimbabwe. Other consequences may include sanctions on health workers or the media when reporting about the situation or even when mentioning the name of the epidemic disease. For instance, although cases in a community may meet the case definition for cholera, a government may be reluctant to declare an epidemic and may also unwelcome the use of the word “cholera”. Cholera is a type of Acute Watery Diarrhoea (AWD), and humanitarian actors working in such contexts may have to speak of cases of AWD if health authorities have not confirmed the presence of cholera.

This causes difficulties for the health system to manage caseloads, delays international supply chains of medication, and undermine coordination efforts and information-flow among responding actors, among other obstacles. Working in such circumstances can be particularly challenging for National Societies who are willing to act for the well-being of their communities but also support and adhere to the policies of the governments to which they are auxiliary.

The following are some general guidance pointers for managers. An understanding of the specific context is necessary to assess their relevance:

• Beware of the language that is used to describe the disease to minimize conflict and to maintain the necessary humanitarian space.
• Ensure that volunteers understand the symptoms for the disease of concern and continue supporting the early identification and referral of potential cases to health facilities. Prevention and response actions relevant during a low alert level of surveillance continue to be relevant and therefore volunteers can be able to continue them, albeit on a smaller scale than may be desired.
• Gather information from various reliable sources to understand the epidemiological situation and discuss it with regional IFRC offices in case funds are needed, but which may be more difficult to mobilize without an official declaration.
• Ensure that communities and volunteers understand that a declaration of the beginning (or end) of an outbreak are often politically charged decisions that affect how various actors will respond. Managers should identify key actors in the response sectors, from surveillance to case management, and ensure that information flows in as much as possible.
• Communicate to logistics teams that the importation of certain materials may be a challenge, and this should be considered in procurement timelines.

Defining the end of an outbreak requires an estimation of the risk of cases arising in the future using objective quantitative methods. The end of an outbreak helps health authorities to downgrade the alert level of surveillance efforts, reallocate health care resources to cover other health issues, restore health care workers regular work shifts, and start recovery efforts.

Just as with the declaration of the beginning of an outbreak, criteria to declare its end will depend on the type of disease and the authorities providing technical guidance. In the context of an Ebola Virus Disease (EVD) outbreak, for example, the end is declared following 42 consecutive days (i.e., twice the longest incubation period) of no cases recorded since the outcome of the last case detected. The outcome is defined as the second PCR negative test of blood samples or a safe burial if the person died. Another example of a criterion established during the 2017-18 cholera outbreak in South Sudan was no new cases reported for seven weeks.

The declaration of the end of an outbreak and of the criteria used however, can be problematic for various reasons and managers should be alert for potential issues. For instance, possible case flare ups through less common transmission routes, e.g., sexual transmission for EVD; through underreported cases due to limited or poor surveillance systems; or through asymptomatic cases. A recent study (Bimandra et al., 2021) showed that using the current WHO criteria to declare the end of an EVD outbreak, might be too short and too sensitive to underreporting.

For a wide range of diseases, National Societies can play a key role on supporting community-based surveillance activities during and after the official end of an outbreak to identify potential cases, even after the declaration of the end of an outbreak (See: https://www.cbsrc.org/).