Video 4.1: The basics

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En provenance du cours de The University of Hong Kong

Epidemics

25 notes

The University of Hong Kong

Epidemics

25 notes

“If history is our guide, we can assume that the battle between the intellect and will of the human species and the extraordinary adaptability of microbes will be never-ending.” (1) Despite all the remarkable technological breakthroughs that we have made over the past few decades, the threat from infectious diseases has significantly accelerated. In this course, we will learn why this is the case by looking at the fundamental scientific principles underlying epidemics and the public health actions behind their prevention and control in the 21st century. This course covers the following four topics: 1. Origins of novel pathogens; 2. Analysis of the spread of infectious diseases; 3. Medical and public health countermeasures to prevent and control epidemics; 4. Panel discussions involving leading public health experts with deep frontline experiences to share their views on risk communication, crisis management, ethics and public trust in the context of infectious disease control. In addition to the original introductory sessions on epidemics, we revamped the course by adding: - new panel discussions with world-leading experts; and - supplementary modules on next generation informatics for combating epidemics. -------------------------------------------------------- (1) Fauci AS, Touchette NA, Folkers GK. Emerging Infectious Diseases: a 10-Year Perspective from the National Institute of Allergy and Infectious Diseases. Emerg Infect Dis 2005 Apr; 11(4):519-25. What you'll learn - Demonstrate knowledge of the origins, spread and control of infectious disease epidemics - Demonstrate understanding of the importance of effective communication about epidemics - Demonstrate understanding of key contemporary issues relating to epidemics from a global perspective

À partir de la leçon

Theme Two: Spread (Infectious disease epidemiology)

Week 4 Introduction1:10

Video 4.1: The basics8:00

Video 4.2: Epidemic curve7:11

Video 4.3: Incubation period7:29

Video 4.4: Transmissibility of communicable diseases7:58

Video 4.5: Timescale of disease transmission5:51

Video 4.6: Severity of infectious disease7:17

Rencontrer les enseignants

Gabriel M. Leung (HKU)
Dean
Li Ka Shing Faculty of Medicine
Joseph T. Wu (HKU)
Associate Professor
Li Ka Shing Faculty of Medicine
Kwok-Yung Yuen (HKU)
Professor
Li Ka Shing Faculty of Medicine
Tommy Lam (HKU)
Assistant Professor
Li Ka Shing Faculty of Medicine
Maria Huachen Zhu (HKU)
Associate Professor
Li Ka Shing Faculty of Medicine
Guan Yi (HKU)
Chair Professor
Li Ka Shing Faculty of Medicine
Benjamin Cowling (HKU)
Professor
Li Ka Shing Faculty of Medicine
Thomas Abraham (HKU)
Associate Professor
Journalism and Media Studies Centre
Mark Jit (LSHTM)
Professor
Department of Infectious Disease Epidemiology
Malik Peiris (HKU)
Chair Professor
Li Ka Shing Faculty of Medicine
Marc Lipsitch (Harvard)
Professor
Department of Epidemiology

In this session, we will discuss some basic concepts

in infectious disease epidemiology.

Epidemiology is the basic science of public health.

In epidemiology, we study the patterns

and determinants of diseases in the population.

For example, where and when

does the disease of interest appear?

What is the burden of the disease in the population?

Do people who have the disease

have any common characteristics

that are not present among people

who do not have the disease?

As public health epidemiologists,

we aim to understand the epidemiology of diseases

for improving preventive

and control measures against them.

Infectious diseases are diseases caused by pathogens

including viruses, bacteria, and parasites.

The most important difference between

infectious and non-infectious diseases

is that most infectious diseases are communicable,

which means that these diseases can be transmitted

from person to person, either directly,

for example, via droplet or airborne transmission

for influenza, or indirectly,

for example, via mosquito bites for malaria.

The risk of getting a communicable disease

increases with the number of people

who have the disease in the population,

a feature that is absent for non-infectious diseases.

For example, the risk of a child getting influenza

is greatly increased if his classmate

sitting next to him has influenza.

However, the risk of a child getting diabetes

does not directly depend on

whether his classmate has diabetes.

Another important difference between

infectious and non-infectious diseases

is that for some infectious diseases

such as measles, a recovered person gains

long-lasting immunity against

re-infection by the same pathogen.

We say that an individual is susceptible

if he is neither infected nor immune,

and therefore can potentially be infected

if he encounters the pathogen.

Obviously, more immune people

means fewer susceptible people.

The spread of a communicable disease

is limited by the proportion

of population that are immune,

the so-called herd immunity effect

which we will learn later this week.

By the same token, the persistence

of a communicable disease in a population

relies on the replenishment of susceptibles.

For example, via newborns,

immigration of susceptibles, or waning of immunity.

In epidemiology, we say that a person has been

exposed to a pathogen if the person has

encountered the pathogen in a way

that the person could become infected.

For example, a child who has been sitting

next to another child infected with influenza

for an hour has been exposed to influenza.

Exposure does not necessarily lead to infection.

In our example, the child may not be infected

after being exposed for an hour.

If exposure leads to infection,

the infected person may develop symptoms

in which case the infection is clinical.

For many diseases, such as influenza and polio,

the infection may be subclinical or asymptomatic,

which means that the infected person

does not have any clinical symptoms

over the course of the infection.

Clinical infection may result in death

or recovery with or without immunity.

For example, almost all measles infections

are clinical with long-lasting immunity after recovery.

Similarly, subclinical infections result

in recovery with or without immunity.

Epidemiologic study of an infectious disease

starts with the case definition of the disease.

A consensus on case definition is essential

for comparing disease patterns

across time and geographical areas.

The case definition of a disease

could be based on laboratory results,

clinical symptoms as well as epidemiologic links.

For example, during the SARS epidemic in 2003,

the WHO definition for a suspected case of SARS

had a clinical and an epidemiologic component.

The clinical criterion was either

a person presenting after 1 November 2002

with history of high fever

and cough or breathing difficulty,

or a person with an unexplained

acute respiratory illness resulting in death

after 1 November 2002, but on whom

no autopsy has been performed.

The epidemiologic criterion was one or more

of the following exposures during the 10 days

prior to onset of symptoms.

First, close contact with a person

who is a suspected or probable case of SARS,

or two, history of travel to an area

with recent local transmission of SARS,

or three, residing in an area

with recent local transmission of SARS.

With a clear case definition, we can then begin

to describe disease burden by tracking

the occurrence of cases.

Incidence is defined as the number of cases

that occur during a defined time period,

divided by the population size.

The time period can be days, weeks, months or years.

For example, in 2012, there were 86 cases

of AIDS in Hong Kong which had a population size

of around 7.2 million.

So the incidence of AIDS in Hong Kong in 2012

was about 1.2 per 100,000.

Incidence is often calculated

from clinical cases seeking medical care,

in which case the true number of infections

would be underestimated if

(i), a substantial proportion of infections are subclinical

or (ii) a significant proportion of clinical cases

do not seek medical care and therefore

not observed by the healthcare system.

For example, a UK study recently reported

that among their subjects,

around 3/4 of influenza infections were subclinical

and that only 4% of all influenza infections

were medically attended.

Prevalence is another useful measure

for disease burden.

It is defined as the total number of cases

at a specific time, divided by the population size.

For chronic or protracted infections,

prevalence is the product of incidence

and average disease duration.

Most infectious disease epidemiologic studies

are observational science, which means

that we cannot perform reproducible,

controlled experiments as we could in lab settings

for physics and chemistry.

This is because it is impossible to set up

precisely the same set of conditions

to generate the same exact epidemics.

Furthermore, deliberate release of pathogens

in human populations for scientific purposes

is medically unethical.

Our best alternative is therefore to study

the cause and spread of diseases

by analyzing the data collected from

naturally occurring epidemics.

As such, robust disease surveillance

and meticulous medical records are essential

for our understanding of and combat against epidemics.

To summarize, in this session, we have discussed

some basic concepts in infectious disease epidemiology.

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