Download immune - 中華民國防疫學會

傳染病傳染力之評估
中華民國防疫學會
王任賢 理事長
Infectious Disease Epidemiology:
Major Differences
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A case can also be an exposure
Subclinical infections influence epidemiology
Contact patterns play major role
Immunity
There is sometimes a need for urgency
What is infectious disease
epidemiology?
Epidemiology
 Deals with one
population
 Risk  case
 Identifies causes
Infectious disease
epidemiology
 Two or more
population
 A case is a risk factor
 The cause often
known
What is infectious disease
epidemiology?

Two or more populations
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Humans
Infectious agents
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Vectors
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Helminths, bacteria, fungi, protozoa, viruses, prions
Mosquito (protozoa-malaria), snails (helminthsschistosomiasis)
Blackfly (microfilaria-onchocerciasis) – bacteria?
Animals
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Dogs and sheep/goats – Echinococcus
Mice and ticks – Borrelia
What is infectious disease
epidemiology?
A case is a risk factor …
Infection in one person can be transmitted to others
What is infectious disease
epidemiology?
The cause often known

An infectious agent is a necessary cause
What is infectious disease epidemiology then used for?
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Identification of causes of new, emerging infections, e.g. HIV,
vCJD, SARS
Surveillence of infectious disease
Identification of source of outbreaks
Studies of routes of transmission and natural history of infections
Identification of new interventions
(www)
Concepts Specific to Infectious
Disease Epidemiology
Attack rate, immunity, vector, transmission,
carrier, subclinical disease, serial interval,
index case, source, exposure, reservoir,
incubation period, colonization, generations,
susceptible, non-specific immunity, clone,
resistance, repeat episodes …
Infectious Disease
Definitions
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Infectious diseases
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Measles
vCJD
Caused by an infectious agent
Communicable diseases
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Tetanus
Transmission – directly or indirectly – from an infected person
Transmissible diseases
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Transmission – through unnatural routes – from an infected person
Note
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Infections are often subclinical – infections vs infectious
diseases!
Antonyms not well-defined
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Non-communicable diseases – virus involved in pathogenesis of
diabetes?
Chronic diseases – HIV?
Routes of transmission
Direct
Indirect
Skin-skin
 Herpes type 1
 Mucous-mucous
 STI
 Across placenta
 toxoplasmosis
 Through breast milk
 HIV
 Sneeze-cough
 Influenza
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Food-borne
 Salmonella
 Water-borne
 Hepatitis A
 Vector-borne
 Malaria
 Air-borne
 Chickenpox
 Ting-borne
 Scarlatina
Exposure
A
relevant contact – depends on the agent
Skin,
sexual intercourse, water contact, etc
Some Pathogens that Cross the
Placenta
Modes of Disease Transmission
Exposure to Infectious Agents
No infection
Death
Clinical
Carrier
Sub-clinical
Immunity
Outcome
Carrier
No immunity
Timeline for Infection
Dynamics of
infectiousness
Latent
period
Infectious
period
Non-infectious
Susceptible
Time
Dynamics of
disease
Incubation
period
Symptomatic
period
Non-diseased
Susceptible
Time
Transmission
Cases
Index – the first case identified
 Primary – the case that brings the infection into a population
 Secondary – infected by a primary case
 Tertiary – infected by a secondary case
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T
S
Susceptible
Immune
Sub-clinical
Clinical
P
S
S
T
Person-to-Person Transmission
Data from Dr. Simpson’s studies in England (1952)
Measles
Chickenpox
Rubella
Children exposed
Children ill
251
201
238
172
218
82
attack rate
0.80
0.72
0.38
Attack rate =
ill
exposed
Epidemiologic Triad
Disease is the result of
forces within a dynamic
system consisting of:
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agent of infection
host
environment
Factors Influencing Disease
Transmission
Agent
Environment
• Weather
• Infectivity
• Housing
• Pathogenicity
• Geography
• Virulence
• Occupational setting
• Immunogenicity
• Air quality
• Antigenic stability
• Food
• Survival
• Age
• Sex
Host
• Genotype
• Behaviour
• Nutritional status
• Health status
Epidemiologic Triad-Related
Concepts
Infectivity (ability to infect)感染力
(number infected / number susceptible) x 100
Pathogenicity (ability to cause disease)致病力
(number with clinical disease / number infected) x 100
Virulence (ability to cause death)毒性
(number of deaths / number with disease) x 100
All are dependent on host factors
Predisposition to Infections
(Host Factors)
Gender
Genetics
Climate and Weather
Nutrition, Stress, Sleep
Smoking
Stomach Acidity
Hygiene
Chain of Infection
Horton & Parker: Informed Infection Control Practice
(www)
Mathematical Modelling in
Infectious Disease
Epidemiology
Reproductive Number, R0
A measure of the potential for transmission
The basic reproductive number, R0, the mean number of individuals directly
infected by an infectious case through the total infectious period, when
introduced to a susceptible population
probability of transmission per contact
R0 = p • c • d
duration of infectiousness
contacts per unit time
Infection will …..
disappear, if
become endemic, if
become epidemic, if
R<1
R=1
R>1
(www)
Reproductive Number, R0
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Useful summary statistic
Definition: the average number of
secondary cases a typical infectious
individual will cause in a completely
susceptible population
Measure of the intrinsic potential for an
infectious agent to spread
(www)
Reproductive Number, R0
If
R0 < 1 then infection cannot invade a population
implications: infection control mechanisms
unnecessary (therefore not cost-effective)
If
R0 > 1 then (on average) the pathogen will
invade that population
implications: control measure necessary to
prevent (delay) an epidemic
Reproductive Number, R0
Use in STI Control
R0 = p • c • d
p
c
condoms, acyclovir, zidovudine
D
case ascertainment (screening,
partner notification), treatment,
compliance, health seeking
behaviour, accessibility of services
health education, negotiating skills
(www)
What determines R0 ?
p, transmission probability per exposure – depends on the infection
 HIV, p(hand shake)=0, p(transfusion)=1, p(sex)=0.001
 interventions often aim at reducing p
 use gloves, screene blood, condoms
c, number of contacts per time unit – relevant contact depends on infection
 same room, within sneezing distance, skin contact,
 interventions often aim at reducing c
 Isolation, sexual abstinence
d, duration of infectious period
 may be reduced by medical interventions (TB, but not salmonella)
Immunity – herd immunity
If R0 is the mean number of secondary cases in a susceptible population, then
R is the mean number of secondary cases in a population where a proportion, p,
are immune
R = R0 – (p • R0)
What proportion needs to be immune to prevent epidemics?
If R0 is 2, then R < 1 if the proportion of immune, p, is > 0.50
If R0 is 4, then R < 1 if the proportion of immune, p, is > 0.75
If the mean number of secondary cases should be < 1, then
R0 – (p • R0) < 1
p > (R0 – 1)/ R0 = 1 – 1/ R0
 If R0 =15, how large will p need to be to avoid an epidemic?
p > 1-1/15 = 0.94
The higher R0, the higher proportion of immune required for herd immunity
Endemic - Epidemic - Pandemic
R>1
R=1
R<1
Time
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Endemic
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Epidemic
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Transmission occur, but the number of cases remains
constant
The number of cases increases
Pandemic
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When epidemics occur at several continents – global
epidemic
Number of Cases of a Disease
Endemic vs Epidemic
Endemic
Time
Epidemic
Levels of Disease Occurrence
Sporadic level: occasional cases occurring at
irregular intervals
Endemic level: persistent occurrence with a
low to moderate level
Hyperendemic level: persistently high level of
occurrence
Epidemic or outbreak: occurrence clearly in
excess of the expected level for a given time
period
Pandemic: epidemic spread over several
countries or continents, affecting a large
number of people
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