Summary of study module (EC08)

Module: EPM301 Epidemiology of Communicable Diseases

Course: PG Diploma/ MSc Epidemiology

This document contains a copy of the study material located within the computer assisted learning (CAL) session. The first three columns designate which page, card and screen position the text refers to. If you have any questions regarding this document or your course, please contact DLsupport via DLsupport@lshtm.ac.uk. Important note: this document does not replace the CAL material found on your module CDROM. When studying this session, please ensure you work through the CDROM material first. This document can then be used for revision purposes to refer back to specific sessions. These study materials have been prepared by the London School of Hygiene & Tropical Medicine as part of the PG Diploma/MSc Epidemiology distance learning course. This material is not licensed either for resale or further copying.

London School of Hygiene & Tropical Medicine September 2013 v1.0

Section 1: EC08 Summary of study module Aim

To review the topics covered in this unit. Objectives By the end of this session, you should be able to:

Summarise the topics in this unit Recall the basic concepts of infectious disease epidemiology

This session should take you between 30 and 45 minutes to complete. Section 2: Introduction During this study module, you have been introduced to some of the unique characteristics of infectious disease epidemiology.

While many of the basic epidemiological skills you have been learning apply equally to infectious and non-infectious disease epidemiology, there are specific issues that need to be considered when dealing with infectious diseases. The ability of an infectious agent to be transmitted between individuals, and the characteristic of acquired immunity, require special methods of measurement and analysis.

In this session you will review the topics covered during the unit. You will be directed to references in your reader that combine some of these topics, to consider the issues more comprehensively. Section 3: Characteristics of infectious diseases In session EC01 you were introduced to some of the characteristics of infectious diseases that influence their epidemiology.

You learned that epidemiologists approach the quantification of microparasites and macroparasites differently. While prevalence of infection is used to measure the burden of infection in a population, intensity of infection can also be used to distinguish between the relative burden among individuals of macroparasites and some protozoan microparasites. The variety of patterns of infection showed how infectious agents can differ in their persistence within a host, and how this might relate to the burden of disease they impose on their host. Some infections are acute, others are chronic, and others are latent, being able to relapse and cause disease after years of no apparent infection. Section 4: Transmissibility

Infections can be transmitted in different ways, either directly or indirectly. The mode of transmission defines the effective contact necessary between a susceptible and infectious individual. This in turn influences the duration of infectiousness.

Many infections can be transmitted in more than one way - better ensuring their persistence in different settings. The mode of transmission determines who gets infected and when and where the infection occurs. These epidemiological characteristics were described in session EC02, as indicators that a disease has an infectious cause.

In addition, determining the people, places and times at greatest risk of disease, can help to identify the cause of the disease and the mode of transmission. This information can also be used to target control measures appropriately to those at greatest risk of disease and at times of greatest need. 4.1: Transmissibility The ability of infectious agents to be transmitted between individuals results in an exponential increase of cases within a relatively short time period in a susceptible population.

If one person transmits to 3 others, and each of these transmits to three others, and so on (see diagram opposite), the number of cases can rapidly escalate.

4.2: Transmissibility As you saw in session EC03, by quantifying the transmissibility of an infection, epidemiologists are able to identify the potential scale of the problem.

The measurement of transmissibility, using the attack rate measure in different age, social or behavioural groups, can be used to identify the individuals at greatest risk of infection.

This can also identify the relative effectiveness of different modes of transmission, and may help to determine appropriate public health interventions. The secondary attack rate measure is generally used for infections that are transmitted to household or other close contacts (e.g. tuberculosis) where the contacts constitute an easily identifiable group of individuals that can be investigated.

By contrast, the basic reproduction number is used to measure transmissibility at a population level. This is difficult to measure directly, but can be inferred from seroprevalence surveys or the average age at first infection for a disease that confers life-long immunity. 4.3: Transmissibility The transmissibility of infectious agents implicates them as an important cause of disease increasingly so as the world's population grows, travels and congregates in urban centres.

In a susceptible population, this characteristic can lead to outbreaks of infection in which the disease burden is focused rather than being distributed throughout a population over time. The detection, investigation and control of outbreaks are a major focus of public health activities, increasingly so in countries where more and more diseases are being brought under control. In session EC04 you learned about the tools used to define an outbreak, to identify the cause and routes of transmission, and to control the spread of infection.

This involves establishing the existence of the outbreak, collecting data and describing the epidemiological characteristics of the disease. Subsequently, hypotheses about the mode and source of transmission need to be tested, after which an appropriate intervention must be recommended.

In the simulated outbreak investigation (session EC05), the exercise tried to capture the urgency of problem. The transmissibility of infectious agents dictates that the most important aspect of an outbreak investigation is rapid action. 4.4: Transmissibility For outbreaks and endemic infections, blocking the transmission of infections is the main target of public health activities. By studying the characteristics of the

infectious agent (EC01 & EC02) and its transmissibility (EC03), we can better inform public health activities.

The dynamic spread of an infection through a population (completely or partially susceptible) can help to identify whether or not an infection will be a problem in a particular setting. In session EC06 you were introduced to the methods of mathematical modelling of infectious diseases. These allow quantification of the rate at which an infection will be transmitted through a population, and can be used to indicate whether the infection will die out, persist endemically, persist with regular epidemic peaks, etc.

Such methods can be used, with caution, to predict the future course of an epidemic or the likely outcome of introducing a particular intervention. Section 5: Immunity Infectious agents may induce an immune response from the host in an attempt to control the infection and reduce any harmful effects. The initial response is non-specific, followed by a specific response involving the production of antibodies.

Some infectious agents are able to evade the immune response and can co-exist with the host (as in the case of many macroparasites), or kill the host (as in the case of HIV). When an effective immune response has been produced, the presence of specific antibodies or the body's ability to produce these antibodies can be used as an immunological marker in epidemiological studies. This is known as seroepidemiology, and the immunological markers are used to indicate previous infection or in some cases protection from subsequent infection. 5.1: Immunity The body's ability to defend itself from attack has been exploited by scientists in the development of vaccines. Vaccines induce a specific immune response that protects (partially or completely) against subsequent infection and/or disease due to a specific infectious agent.

This is one of the most important approaches to the control of infectious diseases, as it protects individuals and, if used on a large-scale, can benefit whole populations. Interaction: Tabs: Vaccine efficacy: However, vaccines are rarely completely protective to all individuals. The concept of measuring the efficacy and effectiveness of a vaccine was discussed in session EC07.

You saw that there are different methods of assessing vaccine efficacy, related to study design. The relative benefits of these designs can vary according to the mechanism of protection of the vaccine.

Interaction: Tabs: Herd immunity:

The public health benefit of a vaccine will vary according to the amount of the population covered and according to the vaccine efficacy because of the concept of herd immunity.

The idea that an individual can be protected from infection because of the immunity of the surrounding individuals is the driving force in the use of immunisation to eliminate infections locally or to eradicate infections globally. Section 6: Summary This is the end of EC08. When you are happy with the material covered here please move on to session EC09. The main points of this session will appear below as you click on the relevant title. In this unit we have focused on the study and measurement of characteristics of infectious diseases. Such epidemiological studies are used to identify appropriate methods of control and to assess the benefit of various interventions.

You should now have the basic tools necessary to identify pertinent questions in infectious disease epidemiology and appropriate methodologies to answer such questions.

To integrate some of the issues that have been raised in this unit, read the following papers in your reader: Interaction: Tabs: Guyer/McBean: In the paper by Guyer and McBean (1981), the authors review the epidemiology and control of measles in Cameroon during and after the introduction of mass immunisation. Consider the epidemiological characteristics (age and time patterns) of the disease before and after the introduction of vaccination. Both surveillance data and serological studies were used to assess the epidemiology of the disease.

Note that the age profile of the disease shifted towards the older age groups after the introduction of vaccination. Interaction: Tabs: Mossong/Muller: Mossong and Muller (2000) have used a measles outbreak to estimate the local vaccine efficacy and the transmissibility of the infection, drawing together concepts from sessions EC03 C07. The paper includes some mathematics but do not worry if you don't understand the formulae.

Note that mathematical modelling is used to estimate the vaccination coverage necessary to minimise outbreaks in these partially vaccinated populations. However the estimates are locally specific, and might not have a wider application due to differences in vaccine coverage and vaccine efficacy.

Interaction: Tabs: Prevots et al: The paper by Prevots et al. (1997) describes an investigation of a polio outbreak in Albania.

Think about the epidemiological characteristics of poliovirus see http://www.cdc.gov/polio/ for more information on this. Look at the age-profile of the disease.

Review the methods used to investigate the outbreak. Pay close attention to the identification of different risk groups and the estimation of separate attack rates for these different groups.

Consider how, even for a highly efficacious vaccine, effectiveness can be reduced by logistic limitations. The epidemiology of infectious diseases requires a different approach to that for non-infectious diseases. Try to consider what you have learned about this when reading studies and reports about infectious diseases, and be aware of the complexities of this topic.

This is a very dynamic subject, with new discoveries being made all the time, and new methodologies being developed everyday; especially in the fields of immunology, molecular biology, genomics and bioinformatics. Apart from the development of new vaccines, these new tools may help to improve our study and understanding of the epidemiology of infectious diseases and methods to control them. This unit has aimed to introduce you to some of the main concepts of the epidemiology of infectious diseases.

We hope that you continue to read about this subject and learn more about the subtleties of the different diseases.

4.1: Transmissibility4.2: Transmissibility4.3: Transmissibility4.4: Transmissibility5.1: Immunity