Surveillance of Infectious Diseases (43 slides, 677KB)
Control of Viral Diseases
An Overview
Terms
Containment – to contain the disease as to prevent it from
becoming a worse problem. Containment is usually the
only option available for the majority of infectious
diseases.
Elimination – to eliminate the disease even though the
infectious agent may remain e.g. rabies and polio had been
eliminated in many countries, and probably SARS in 2003.
Eradication – to eradicate the infectious agent altogether
worldwide e.g. smallpox
Epidemiology (Gr.Studies upon people)
Study of health and disease as it occurs in the
community either in groups of person or the entire
population. It deals with
Nature of the disease
Distribution of the disease
Causation of the disease
Mode of transfer of the disease
Prevention and control of the disease
Surveillance of Infectious
Diseases
Strategies for Surveillance of
Infectious Diseases
Notifiable diseases – make it a statutory duty for
physicians to notify the disease.
Virus isolation or serologic evidence reported
through diagnostic laboratories
Specific Epidemiological Studies e.g. hantavirus,
hand foot and mouth disease surveillance
Notifiable viral diseases (Hong Kong)
Yellow fever
Poliomyelitis
Measles
Mumps
Rubella
Rabies (Human and Animal)
Viral Hepatitis
Dengue fever
Chicken Pox
H5N1 influenza
SARS
Requirements for surveillance based on
clinical case
Occurrence of clinical illness
Sufficient severity to seek medical care
Availability of medical care
Capability of physicians to diagnose illness
Laboratory support of diagnosis
Reporting of disease to Health Department
Collection and analysis of data by Health Department
Laboratory based surveillance
Scientific source of information
Coherent and consistent information on
trends of infection
Qualitative detail information
Control Measures Available
Control Measures Available
To control the spread of the disease in the
population by
Agent - removing the source of the agent by targeting
its reservoir
Controlling its transmission
Patient – immunization, prophylaxis, antiviral therapy.
Removing the Source
Every pathogen has a reservoir, which may be in humans, animals or
both. One may aim to remove the pathogen from the reservoir, or
remove the reservoir completely.
Human Reservoir
Isolating the patient
Curing the patient completely
Preventing infection in susceptible individuals by vaccination
Animal Reservoir
Isolating/observing the animal e.g. rabid dog
Eradicate the animals involved e.g. slaughter of rabid dog, vector control
Vaccinating the animals e.g. vaccination of dogs and foxes. It is very
difficult to vaccinate wild animals.
Controlling its transmission
Prophylactic chemotherapy or vaccination
individuals exposed to or susceptible to infection.
among
Contact tracing
Improvement in hygiene and living standards
Modification of lifestyle and behavior
Health education
Screening of potential sources of infection e.g. blood,
foods, water
Controlling vectors that may be involved in transmission
Man-Arthropod-Man Cycle
Animal-Arthropod-Man Cycle
Examples of Arthropod
Vectors
Aedes Aegyti
Culex Mosquito
Assorted Ticks
Phlebotmine Sandfly
Vaccination
Types of Vaccination
Strategies
There are two types of vaccination policies:
Universal Vaccination – every person is vaccinated in the hope of
eliminating/eradicating the disease from the community
Selective Vaccination – only individuals in particular risk groups
are vaccinated.
Both policies are in use for rubella.
The US started off with universal vaccination.
The UK and HK started off with selective vaccination of primary
school girls but decided to switch to universal vaccination because
the uptake rate was not good enough.
Characteristics of vaccines
The characteristics of the vaccine used is a major determinant on
the outcome of the vaccination strategy. Factors to consider
include
Response rate
Type of protection
Duration of protection
Local immunity
Side effects
Route of administration
Stability
Cost
Developing a vaccination
policy
The following questions should be asked when a vaccination policy
against a particular virus is being developed.
1.
2.
3.
4.
5.
6.
What proportion of the population should be immunized to
achieve eradication.
What is the best age to immunize?
How is this affected by birth rates and other factors
How does immunization affect the age distribution of susceptible
individuals, particularly those in age-classes most at risk of
serious disease?
How significant are genetic, social, or spatial heterogeneities in
susceptibility to infection?
How does this affect herd immunity?
Coverage Required for
eradication
Basic concept is that of the basic rate of the infection R0.
For most viral infections, R0 is the average number of secondary cases produced by a
primary case in a wholly susceptible population. Clearly, an infection cannot maintain
itself or spread if R0 is less than 1.
R0 can be estimated from as B/(A-D);B = life expectancy, A = average age at which
infection is acquired, D = the characteristic duration of maternal antibodies.
The larger the value of R0, the harder it is to eradicate the infection from the community
in question.
A rough estimate of the level of immunization coverage required can be estimated in the
following manner: eradication will be achieved if the proportion immunized exceeds a
critical value pinc = 1-1/R0.
Thus the larger the R0, the higher the coverage is required to eliminate the infection.
Thus the global eradication of measles, with its R0 of 10 to 20 or more, is almost sure to
be more difficult to eradicate than smallpox, with its estimated R0 of 2 to 4.
Critical Coverage
Av Age of
infection
Epidemic
Period
Ro
Critical
Coverage
Measles
4-5
2
15-17
92-95
Pertussis
4-5
3-4
15-17
92-95
Mumps
6-7
3
10-12
90-92
Rubella
9-10
3-5
7-8
85-87
Diptheria
11-14
4-6
5-6
80-85
Polio
12-15
3-5
5-6
80-85
Eradication of Small Pox
Eradication of Smallpox - 1
Smallpox was transmitted by respiratory route from lesions in the respiratory tract of
patients in the early stage of the disease
During the 12 day incubation period, the virus was distributed initially to the internal
organs and then to the skin.
Variola major caused severe infections with 20-50% mortality, variola minor with
An Overview
Terms
Containment – to contain the disease as to prevent it from
becoming a worse problem. Containment is usually the
only option available for the majority of infectious
diseases.
Elimination – to eliminate the disease even though the
infectious agent may remain e.g. rabies and polio had been
eliminated in many countries, and probably SARS in 2003.
Eradication – to eradicate the infectious agent altogether
worldwide e.g. smallpox
Epidemiology (Gr.Studies upon people)
Study of health and disease as it occurs in the
community either in groups of person or the entire
population. It deals with
Nature of the disease
Distribution of the disease
Causation of the disease
Mode of transfer of the disease
Prevention and control of the disease
Surveillance of Infectious
Diseases
Strategies for Surveillance of
Infectious Diseases
Notifiable diseases – make it a statutory duty for
physicians to notify the disease.
Virus isolation or serologic evidence reported
through diagnostic laboratories
Specific Epidemiological Studies e.g. hantavirus,
hand foot and mouth disease surveillance
Notifiable viral diseases (Hong Kong)
Yellow fever
Poliomyelitis
Measles
Mumps
Rubella
Rabies (Human and Animal)
Viral Hepatitis
Dengue fever
Chicken Pox
H5N1 influenza
SARS
Requirements for surveillance based on
clinical case
Occurrence of clinical illness
Sufficient severity to seek medical care
Availability of medical care
Capability of physicians to diagnose illness
Laboratory support of diagnosis
Reporting of disease to Health Department
Collection and analysis of data by Health Department
Laboratory based surveillance
Scientific source of information
Coherent and consistent information on
trends of infection
Qualitative detail information
Control Measures Available
Control Measures Available
To control the spread of the disease in the
population by
Agent - removing the source of the agent by targeting
its reservoir
Controlling its transmission
Patient – immunization, prophylaxis, antiviral therapy.
Removing the Source
Every pathogen has a reservoir, which may be in humans, animals or
both. One may aim to remove the pathogen from the reservoir, or
remove the reservoir completely.
Human Reservoir
Isolating the patient
Curing the patient completely
Preventing infection in susceptible individuals by vaccination
Animal Reservoir
Isolating/observing the animal e.g. rabid dog
Eradicate the animals involved e.g. slaughter of rabid dog, vector control
Vaccinating the animals e.g. vaccination of dogs and foxes. It is very
difficult to vaccinate wild animals.
Controlling its transmission
Prophylactic chemotherapy or vaccination
individuals exposed to or susceptible to infection.
among
Contact tracing
Improvement in hygiene and living standards
Modification of lifestyle and behavior
Health education
Screening of potential sources of infection e.g. blood,
foods, water
Controlling vectors that may be involved in transmission
Man-Arthropod-Man Cycle
Animal-Arthropod-Man Cycle
Examples of Arthropod
Vectors
Aedes Aegyti
Culex Mosquito
Assorted Ticks
Phlebotmine Sandfly
Vaccination
Types of Vaccination
Strategies
There are two types of vaccination policies:
Universal Vaccination – every person is vaccinated in the hope of
eliminating/eradicating the disease from the community
Selective Vaccination – only individuals in particular risk groups
are vaccinated.
Both policies are in use for rubella.
The US started off with universal vaccination.
The UK and HK started off with selective vaccination of primary
school girls but decided to switch to universal vaccination because
the uptake rate was not good enough.
Characteristics of vaccines
The characteristics of the vaccine used is a major determinant on
the outcome of the vaccination strategy. Factors to consider
include
Response rate
Type of protection
Duration of protection
Local immunity
Side effects
Route of administration
Stability
Cost
Developing a vaccination
policy
The following questions should be asked when a vaccination policy
against a particular virus is being developed.
1.
2.
3.
4.
5.
6.
What proportion of the population should be immunized to
achieve eradication.
What is the best age to immunize?
How is this affected by birth rates and other factors
How does immunization affect the age distribution of susceptible
individuals, particularly those in age-classes most at risk of
serious disease?
How significant are genetic, social, or spatial heterogeneities in
susceptibility to infection?
How does this affect herd immunity?
Coverage Required for
eradication
Basic concept is that of the basic rate of the infection R0.
For most viral infections, R0 is the average number of secondary cases produced by a
primary case in a wholly susceptible population. Clearly, an infection cannot maintain
itself or spread if R0 is less than 1.
R0 can be estimated from as B/(A-D);B = life expectancy, A = average age at which
infection is acquired, D = the characteristic duration of maternal antibodies.
The larger the value of R0, the harder it is to eradicate the infection from the community
in question.
A rough estimate of the level of immunization coverage required can be estimated in the
following manner: eradication will be achieved if the proportion immunized exceeds a
critical value pinc = 1-1/R0.
Thus the larger the R0, the higher the coverage is required to eliminate the infection.
Thus the global eradication of measles, with its R0 of 10 to 20 or more, is almost sure to
be more difficult to eradicate than smallpox, with its estimated R0 of 2 to 4.
Critical Coverage
Av Age of
infection
Epidemic
Period
Ro
Critical
Coverage
Measles
4-5
2
15-17
92-95
Pertussis
4-5
3-4
15-17
92-95
Mumps
6-7
3
10-12
90-92
Rubella
9-10
3-5
7-8
85-87
Diptheria
11-14
4-6
5-6
80-85
Polio
12-15
3-5
5-6
80-85
Eradication of Small Pox
Eradication of Smallpox - 1
Smallpox was transmitted by respiratory route from lesions in the respiratory tract of
patients in the early stage of the disease
During the 12 day incubation period, the virus was distributed initially to the internal
organs and then to the skin.
Variola major caused severe infections with 20-50% mortality, variola minor with