Spread of Disease and Interventions Flashcards
What is an intervention?
An intervention is a strategic action intended to improve individual and population health status
- interventions targeted at any level, from the individual to the community, the nation, and the world, can be effective at improving personal and public health
Prevention Logic
Maintaining good health status through preventitive interventions is less costly than paying for rehabilitation after health crisis “prevention is better than a cure”
Prevention Science
The study of which preventive health interventions are effective (includes which are more economic) in various populations, how successful the interventions are, and how well they can be scaled up for widespread implementation.
The three levels of prevention:
Primary, secondary, tertiary
The ‘natural history of disease’
refers to the progression of the disease process in an individual over time in the absence of intervention
Every disease has its own natural history
Disease Causation: What is a critical premise of epidemiology?
That disease and other health events do not occur randomly in a population, but are more likely to occur in some members of the population than others because of the risk factors that may not be distributed randomly in the population
Epidemiologic Triad
consists of an external agent, a susceptible host, and an environment that brings the host and agent together
In this model, disease results from the interaction between the agent and the susceptible host in an environment that supports transmission of the agent from a source to that host
Causal Pies
The individual factors are called component causes
The complete pie, which might be considered s causal pathway, is called a sufficient cause (a disease may have more than one sufficient cause)
A component that appears in every pie or pathway is a necessary cause
Can help us understand variation in outcomes
Causal Web
looks more like a flowchart
-> useful for more chronic or non-infectious disease causation
–> offers a metaphor for the inteconnectedness of characteristics and different levels of direct and indirect causes (micro, macro, individual)
Infectious Diseases are…
disorders caused by organisms – such as bacteria, fungi, or parasites
The Chain of Infection: What are the 6 links?
the infectious agent, reservoir, portal of exit, mode of transmission, portal of entry, and susceptible host
The way to stop germs from spreading is by interrupting this chain at any link
The 3 types of reservoir for disease
Human, Animal, Environmental
Human reservoir
Many common infectious diseases have human reservoirs. Diseases that are transmitted from person to person without intermediaries include the sexually transmitted diseases, measles, mumps, streptococcal infection, and many respiratory pathogens
Animal Reservoirs
(Zoonosis)
Humans are also susceptible to diseases that have animal reservoirs. Zoonosis refers to an infectious disease that is transmissible under natural conditions
Environmental Reservoirs
Plants, soil, and water in the environment are also reservoirs for some infectious agents
Portal of Exit
Portal of exit is the path by which a pathogen leaves its host. Usually corresponds to the site where the pathogen is localized.
Modes of Transmission
Direct: direct contact, droplet infection, contact with soil, inoculation into skin or mucosa, trans-placental (vertical)
Indirect Transmission: vehical-borne, vector-borne (mechanical, biological), fomite-born, unclean hands and fingers
What does susceptibility of a host depend on?
genetic or constitutional factors, specific immunity, and nonspecific factors that affect an individuals ability to resist infection or to limit pathogenicity (the ability of an organism to cause disease)
What does the concept of herd immunity suggest?
It suggests that if a high enough proportion of individuals in a population are resistant to an agent, then those few who are susceptible will be protected by the resistant majority, since the pathogen will be unlikely to “find” those few susceptible individuals (too great a challenge for pathogen)
The degree of herd immunity necessary to prevent or interrupt an outbreak varies by disease
What are the conditions under which herd immunity functions best?
- single reservoir
- direct transmission
- total immunity
- no shedding agents by immune hosts (was of interest during height of covid)
- uniform distribution of immunes
- no overcrowding
One problem with herd immunity is that in highly immunized populations, the relatively few susceptible persons are often clustered in subgroups defined by socioeconomic or cultural factors
The Basic Reproduction Number (Ro)
It is used to measure the transmission potential of a disease
It is the average number of secondary infections produced by a typical case of infection in a population where everyone is susceptible
Scientists use Ro to estimate how many other people one sick person is likely to infect
Key concept in studying susceptibility and disease transmission
What factors affect the basic reproduction number?
- the rate of contacts in the host population (C) (how many people we interact with on a daily basis)
- the probability of infection being transmitted during contact (P)
- the duration of infectiousness (D) (pretty big range for covid)
Ro = CPD
In general for an epidemic to occur in a susceptible population, Ro must be >1 (means you are infecting more than just yourself on average) (if less than 1, will die out)
If Ro = 1, talking abt communities where it has become epidemic
What is the limitation to Ro?
In many circumstances not all contacts will be susceptible to infection
Do airborne or physical contact diseases tend to have a higher Ro value than those spread through direct contact?
Airborne illnesses
Can contract from someone without ever touching them
In contrast, diseases transmitted through bodily fluids are not as easy to contract or transmit- will need to come into contact with infected blood, saliva, or other bodily fluids to contract them
Effective Reproductive Number (R)
takes it a step further than unrealistic assumption that everyone will be totally susceptible
is the average number of secondary cases per infectious case in a population made up of both susceptible and non-susceptible hosts
Can be estimated by the product of the basic reproduct number and the fraction of the host population that is susceptible (x) so R = Ro times X
If R>1 then epidemic
if R <1 then disease should die out
If R = 1 then disease will continue to persist endemically
Herd Immunity Threshold (HIT)
usually used to design vaccination programs
proportion of a pop that need to be immune in order for an infectious disease to become stable in that community. If this is reached, for ex through immunization, then each case leads to a single new case (R=1) and the infection will become stable within the population
HIT = Ro-1 / Ro or 1 - (1/Ro)
Endemicity provides stability + predictability so can be a good thing
The levels of disease
Sporadic: a disease that occurs infrequently and irregularly
Endemic: the constant presence and/or usual prevalence of a disease or infectious agent in a population wtihin a geogrpahic area
Hyperendemic: persistent, high levels of disease occurrence
Factors that affect spread of disease
infectivity, pathogenicity, virulence ( a pathogens or microorganisms ability to cause damage to a host)
Infectiousness
Infection Rate (IR) =
The capacity of an infectious agent to cause infection in a susceptible host exposed to the agent
Both clinical and subclinical (no symptoms, but could test for something)
Infection Rate (IR) = Number of infected / Number susceptible and exposed (x100)
Pathogenicity
Infection to disease… the capacity of an infectious agent to cause disease in an infected host (the ability to cause/actually develop the disease) (doesnt necessarily mean severity, just means that we have moved from the subclinical to the clinical)
measured by clinical to subclinical ratio
CL to SubCL = # clinical cases : # subclinical cases
Virulence
the ability of an infectious agent to cause severe disease or death in a host
CFR or HR (Case fatality rate or hospitalization rate)
CFR = Number fatal cases / total number of cases (x100)
HR = number hospitalized cases / total number of cases (x100)
Infectivity and virulence often appear to be correlated positively so as the pathogen is able to infect more hosts, also seems to increase in level of harm (so knowing infectivity can be a good predictor of virulence)
