Download Public Health in Tropics :Further understanding in infectious disease

Public Health in Tropics
:Further understanding in infectious
disease epidemiology
Taro Yamamoto
Department of International Health
Institute of Tropical Medicine
Definitions and concepts
• Today, I briefly touch upon a number of
definitions and concepts necessary for
understanding the literature on infectious
disease epidemiology
• Every self-respecting branch of science needs to
have its own language and concepts because it is
not deter the novice but rather to lay a foundation
for precise communication.
Incidence
• Incidence is defined as the number of
individuals who fall ill with a certain
disease within a defined time period,
divided by the total population.
• If the time is not clearly stated, it is usually
assumed to be one year.
• What do you mean that the incidence of hepatitis B
in Sweden is about two per 100,000?
Prevalence
• The prevalence of disease is the number
of individuals who have that disease at a
specific time, divided by the total
population.
• What do you mean that the prevalence of HIV
infection in some African countries is about 20 per
100 population?
• P=I ×D (I: average incidence, D: average duration
of disease)
– This alternatively can be expressed in words “prevalence
is the product of incidence and duration”
Mortality
• Mortality indicates what proportion of
entire population die from the disease
each year.
• Ex) In western Europe a disease such as rabies
has a high case fatality rate* but low mortality.
– Case fatality rate: the proportion of people who will die of
a certain disease out of those who contract it
Incubation and latent period
• Incubation period
extends from the
moment a person is
infected until they
develop clinical
symptoms of disease.
• On the other hand,
latent period is the
time duration from
infection until a
person has infectivity.
What is important thing to learn is that
there is time period in which a person has
Infectivity but no symptoms of disease at all
Basic reproductive rate
• The potential for infectious disease to
spread from person to person in a
population is called reproductive rate.
• If a given population is completely
susceptible, it is called basic reproductive
rate.
The average number of individuals directly infected by each case is
(1+2+0+1+3+2+1+1+2+1+2)/10=1.5
What is the epidemiological implication of R0?
• Definition again: R0 is the average number of individuals
directly infected by each case when he/she enters into
an entirely susceptible population.
• A certain disease is introduced into a theoretical, totally
susceptible population, three possible situations are
considered.
– R0 < 1: the disease will eventually disappear
– R0 = 1: the disease will become endemic
– R0 > 1: the disease will spread and be epidemic
Coffee break
• The reasoning mentioned in the previous slide has also
important implication for the vaccination coverage.
• Suppose that we have a disease for which R0 is 4. If,
instead of letting natural infection gradually increase the
proportion of immune individuals, vaccine can give a
immunity to those who have it, what proportion of
vaccine coverage is needed for preventing infection from
spreading, or being epidemic?
A piece of cake
• If R0 is 4, in the unvaccinated natural stage, a primary case of
disease will thus infect 4 other individuals on average.
• However, if 25% of the population have already been immunized
against disease, then one out of four individuals who should have
been infected can escape infection, only three among four will be
infected.
• What about that the half population have been immunized? What
about 75% population? If 75% of the population have been already
immunized, then three out of four who should have been infected
can escape infection, and thus only one out of those four will be
infected. It leads the situation that reproductive rate is one on
average and thus infection will neither spread or disappear.
A piece of cake 2
• If you put vaccine coverage as p, reproductive rate is
given by following equation.
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R= R0×(1-p)
If R is less than 1, infection eventually will disappear.
R= R0×(1-p)<1
1-p< 1/R0
P>1- 1/R0
In this case, R0 is 4. thus P>0.75
What determined R0?
• The basic formula that give R0 acctual value is as follows:
• R0 ＝β×κ×Ｄ
• where beta is the risk transmission per contact, kappa is
the number of such contacts that an average person in the
population would normally have per time unit, and D is the
duration of infectivity of a infected person on average,
measured by same time unit as kappa.
R0 of HIV in CSW and general population
• R0 ＝β×κ×Ｄ
• where beta is the risk transmission per contact, kappa is the number
of such contacts that an average person in the population would
normally have per time unit, and D is the duration of infectivity of a
infected person on average, measured by same time unit as kappa.
• The risk transmission per contact (β) is between 0.01 and 0.001
without any clinical manifestations in genitalia caused by STI, and
between 1 and 0.1 with them.
• The number of contact (κ) is, in this case, the number of sexual
partners newly acquired.
• The duration of infectivity (D) is life long after infection.
Coffee break
• If safe sex is fulfilled, or you use condom, what is
expected to happen in terms of reproductive rate of HIV?
• The possible answer is that the risk transmission rate (β)
is reduced to a greater extent.
• Or you might be able to say that number of contact
effective in transmission from virus view point declines.
• It depend on the viewpoint you take, though,
reproductive rate is expected to effectively decrease.
Coffee break
What do you expect to happen to reproductive rate and which
factor do you think to be affected,
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if you wear mask in flu season?
If you prescribe antiviral to AIDS patients?
If you take school close policy in action when measles is getting in
epidemic situation?
Generally, it is likely to say that public health measure such as
distance policy reduce contact frequency and medical procedure
such as drug administration reduce either/ both the probability of
transmission per contact or/ and duration of infectivity.
One more thing to be added
Through the lecture today, it should be clear that the
higher the reproductive rate of a certain disease is, the
greater the risk will be of encountering such disease
early in life. That’s is to say that high reproductive rate
inversely correlates with an average age that they
contract infection.
A simple approximate formula exists.
R0 ＝1 + L/A
Where L is the average life span of the individuals in the
population, and A is the average age at infection.
One more thing (con’t)
• If average age at infection of measles in your
country is 5 y.o., and average life span is 60,
what is estimated R0?