BIOL 437 Week Five Part 1 Flashcards
reproductive number (R.)
- average number of secondary infections generated by the first infectious individual in a population of completely susceptible individuals
- continues until something slows the process down
R. affects
-rate of spread
R. increases with
- length of infection (L)
- longer a person is infected=longer they can transmit to other people
S
- number of susceptible hosts that can be infected
- higher=greater potential for first individual to transmit infection
L+S
-determines the potential number of contacts over the course of an infection
R. also depends on
-how transmissible a pathogen is
transmissibility depends on
-pathogen
-population of interest
>nutrionally stressed are more susceptible
*beta
outbreaks
- only likely to spread if R. is >1
* threshold for that pathogen
interventions to reduce R. threshold
- Decrease number of susceptible individuals
>vaccination - Decrease the length of time an individual is infectious
>ask sick people to stay home - Decrease transmissibiilty
>limit contacts or introduce sanitary measures
epidemics
-mostly associated wiht acute, highly transmissilbe, directly transmitted pathogens
SIR compartmental framework
- susceptible, infected, removed
- used to understand epidemic curves caused by these pathogens
- removal is either due to death or recovery with immunity
ex. smallpox, measles
epidemic curve: SIR
-assumes random interactions
-speed of transmission depends on the R. and the infectious period
>higher R., shorter infectious period = more explosive spread
effective reproductive number (Re)
- average number of new infections later in an epidmeic
- R. x individuals still susceptible
- if less than 1, epidemic is self-limiting
self limitation
-outweighs contagion when proportion of susceptible falls below 1/R
>when population reaches ‘herd immunity’
early in a new infectious disease
-Re will be close to R.
pace of epidemic determined by
- duration of the infectious period
- latent period
- eg. season flu is typically self-limiting after a month or two
population close to extinction
- due to acute, directly transmitted pathogens
- close to extinction but rarely to extinction due to the self-limiting nature
characteristics that increase chance of host extinction
- also reduce pathogen self-limitation
- alternative, resistant hosts that act as a reservoir
- vector transmission
determinants of susceptibility beyond immune status
- Genetics
- Microbiome
- Nutrition
positive social influence on immunity
-instrumental support
-emotional support
>higher=slower disease progression
negative social influences on immunity
- social stressors (ex. loneliness)
- smoking
- drinking
- activate stress symptoms that depress or disrupt immune functioning
force of infection
- way to measure degreee to which an infected individual can spread infection to susceptible individuals
- per capita rate at which susceptible individuals contract infection
factors that affect force of infection
- Transmissiblity
2. Prevelance of pathogen
high force of infection
-immunizing infection
>see infection in younger people
-pathogens with higher R.
low force of infection
-lower transmissibility
>lower contact rates
-individuals will be older on average by time of first exposure
-pathogens with lower R.
force of infection relationship
-between mean age of infection in a population and basic reproductive number
