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Epidemiology lecture-1-intro.pptx
- 1. Epidemiology Epidemiology is the branch of medical science that investigates all the factors that determine the presence or absence of diseases and disorders NIH The study of the incidence, distribution, and control of
- 2. Q: What is Epidemiology to me? A: Ecology Humans are animals Ecologists study the population dynamics of plants, animals, bacteria, even viruses. What causes populations to change in number? Resources Sun, space, food, mates, etc Predators Competition Intra and interspecific Parasites This is where epidemiology comes in However, all of the above determines the presence and population sizes of parasites as well.
- 4. Back to humans In ecology we care about everything In medicine we only care about humans (and maybe our pets) The population dynamics of humans (and everything else) Are determined by: Life expectancy Fecundity (how many, when, and for how long) All of those are functions of the factors we mentioned before
- 5. How long do humans live? Depends Why?
- 6. FIGURE 5-6 Population Pyramid for the Population of the United States, 2020
- 8. World demographic pyrimad Current growth rate 1.05% Peak was in 1960 2.2% 1.8 billion women at reproductive age Increases each year Aged out replaced with young women
- 10. Epidemiology Host pathogen dynamics More complicated than simple growth formulas Nt+1 = Growth rate x Nt0 Nt = Growth ratet x Nt0 Types of individuals in a population Susceptible Infected Recovered/immune
- 11. Also 1) Differences in infection at different ages 2) Latent period of infection (infected but not transmissible) 3) Vertical transmission (mother to newborn) These models can get very complicated We will simplify for sanity
- 12. Threshold density What you really want to know You may have heard of something called herd immunity An infectious disease will only spread if the susceptible hosts exceed the threshold density. Math! No really, fun math.
- 13. Variables S = density of susceptible individuals I = density of infected individuals For spread to occur, a susceptible individual must contact an infected individual. The likelihood of that encounter is a function of the density of both. S*I If the population is 80% susceptible and 20% infected: .80*.20 = .16 so there would be a 16% chance any encounter between two individuals will be with an infected person.
- 14. But an encounter does not mean infection The next big variable is the transmission coefficient (beta) How easily the disease is transmitted from the infected to the susceptible Some diseases (measles for example) have an incredibly high other diseases do not. In many cases, specific to the interaction (sex, physical contact, bodily fluid, etc) Therefore, 署 is the disease transmission. How much the disease is spreading
- 15. As the disease spreads (at the rate determined by 署) the density of infected individuals increase, increasing that rate of rise. But what happens to infected individuals? This isnt Girl with all the Gifts Individuals die or recover Not much difference to the disease, neither (normally) can be infected New variable: =death or recovery rate.
- 16. Bringing it all together = 署 is the change in the density of infected individuals at each instant in time. A disease will establish and spread when the numbers of infected individuals increases over time. > 0 or 署 > 0
- 17. Threshold density We can rearrange 署 > 0 > And to figure out what number of susceptible individuals necessary are for the disease to spread: = This is the Threshold density
- 18. So lets do an example How many susceptible individuals do we need to have on campus for Covid to spread? Can we figure out our variables?
- 19. Beta=0.5 m=0.05 per week Die or recover
- 20. As long as there are 90 susceptible individuals, the disease will spread This would mean heard immunity at 90% immunized or recovered. beta 0.5 raw by population m 0.05 St 0.06 90 population 900 beta 0.5 raw by population m 0.01 St 0.02 18 population 900 beta 0.8 raw by population m 0.01 St 0.0125 11.25 population 900
- 22. R0 : How many people each person will infect Infectious period in days (how long you can get people sick) Infectious contacts per day 0 = We know R0 = 8 We know infectious period for Delta is 5 days 8 = 5 * 1.6 = 1.6 infectious contacts per day
- 23. R0 First conclusions: 0 = What does this tell us we could do to reduce R0 Reduce contact Reduce proportion of contacts resulting in infection How do we do these things?
- 24. Another way of thinking about 0 0 = = rate of contact between susceptible and infected (per day) T = transmissibility Can we do something with ? (days infectious)