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 ( )/10=1.5

What is the epidemiological implication of R 0 ? Definition again: R 0 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. –R 0 < 1: the disease will eventually disappear –R 0 = 1: the disease will become endemic –R 0 > 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 R 0 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 R 0 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. R= R 0 ×(1-p) If R is less than 1, infection eventually will disappear. R= R 0 ×(1-p)<1 1-p< 1/R 0 P>1- 1/R 0 In this case, R 0 is 4. thus P>0.75

What determined R 0 ? The basic formula that give R 0 acctual value is as follows: R 0 ＝ 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.

R 0 of HIV in CSW and general population R 0 ＝ 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 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, 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. R 0 ＝ 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 R 0 ?