INFECTIOUS DISEASE MODELLING AND HEALTH ECONOMICS Flashcards
WHAT IS INFECTIOUS DISEASE MODELLING?
A TOOL THAT CAN BE USED TO STUDY THE MECHANISMS BY WHICH DISEASES SPREAD, PREDICT FUTURE OUTBRAKS AND/OR TO EVALUATE STRATEGIES TO CONTROL/PREVENT DISEASE
WHAT IS AN (INFECTIOUS DISEASE) MODEL?
- MODEL IS A SET OF MATHEMATICAL AND STATISTICAL TECHNIQUES DESIGNED FOR THE ANALYTICAL EVALUATION OF DECISION MAKING UNDER UNCERTAINTY
- UNCERTAINTY IS REPRESENTED BY EVENT PROBABILITIES AND ASSOCIATED DISTRIBUTIONS
- MODELS USED WHEN THERE IS LIMITED OR INCOMPLETE CLINICL TRIAL DATA OR WHEN WE WANT TO EXPAND ON THE KNOWLEDGE FROM TRIALS
- MODELS SOMETIMES USED TO COMPARE ALTERNATIVE COURSES OF ACTION INSTEAD OF RELYING ON CLINICAL TRIALS (WHICH ARE EXPENSIVE AND TIME CONSUMING)
- MODELS COMBINE MEDICAL AND ECONOMICAL EVIDENCE, INFORMING BEST DECISION/OUTPUT WITH LIMITED RESOURCES
EXMPLES OF MODELS?
- DECISION TREE
- MARKOV MODEL
- OTHERS (PATIENT LEVEL SIMULATION, DISCRETE EVENT SIMULATION, DYNAMIC MODELS)
DYNAMIC MODELS ARE USUALLY USED FOR INFECTIOUS OR NON INFECTIOUS DISEASE?
INFECTIOUS D
DESCRIBE THE MODEL: DECISION TREE
- PRESENTS SERIES OF POSSIBLE CONSEQUENCES AND COST ASSOCIATED WITH EACH ONE
- MOVE FROM LEFT TO RIGHT
- USED FOR E.G. A SCREENING PROGRAM OR VACCINATION PROGRAM
- USUALLY FOR INTERVENTIONS WITH SHORT DURATION (UNDER A YEAR)
DECISION TREE MODEL IS USED FOR INTERVENTIONS WHICH LAST LESS THAN?
1 YEAR
MARKOV MODEL VS DECISION TREE MODEL; WHICH ONE IS USED FOR INTERVENTIONS WITH LONGER DURATIONS?
MARKOV M
WHAT ARE ‘EXTERNALITIES’ IN DISEASE MODELLING?
- ANY COST OR BENEFIT OF ONE PARTY THAT MIGHT IMPACT ON OTHERS
- IGNORING EXTERNALITIES IN MODELLING MAY LEAD TO INCORRECT RESULTS AND THUS WRONG DECISION
- CAN BE NEGATIVE (LIKE SMOKING) OR POSITIVE (PERSON VACCINATED, PERSON SCREENED ASYMPTOMATICALLY, TREATMENT FOR SYMPTOMATIC PEOPLE..)
WHY HERD IMMUNITY FOR COVID MIGHT BE IMPOSSIBLE?
- VACCINE ROLLOUT UNEVEN
- NEW VARIANTS CHANGE ERD IMMUNITY
- IMMUNITY MAY NOT LAST FOREVER
- VACCINES CHANGE HUMAN BEHAVIOUR
- VACCINES NOT COMPLETEY EFFECTIVE
HERD IMMUNITY?
THE DIRECT PROTECTION EXPERIENCED BY UNVACCINATED INDIVIDUALS RESULTING FROM PRESENCE OF IMMUNE INDIVIDUALS IN A POPULATION
STEPS: HOW ARE MODELS SET UP?
- MODELS ARE SIMPLIFICATIONS OF REALITY
- SET UP USING A SOFTWARE OF ONE’S CHOICE
- MODEL BUILDING IS AN ITERATIVE PROCESS: STEPS CAN BE REVISITED
STEPS: - IDENTIFY THE RESEARCH QUESTION
- IDENTIFY RELEVANT FACTS ABOUT THE INFECTION
- CHOOSE THE MODEL STRUCTURE
- IDENTIFY MODEL INPUT PARAMETERS
- SET-UP MODEL
- MODEL VALIDATION
- PREDICTION AND OPTIMIZATION
WHICH RELEVANT FACTS ABOUT THE INFECTION NEED TO BE IDENTIFIED FOR INFECTIOUS DISEASE MODELLING?
- WHAT IS THE LATENT/PREINFECTIOUS PERIOD (TIME FROM BEING INFECTED TO BECOMING INFECTIOUS)
- HOW LONG ARE PEOPLE INFECTED
- WHAT IS THE BASIC REPRODUCTION NUMBER, R0
- ARE ALL AGE GROUPS AFFECTED EQUALLY
WHAT HAS TO BE COONSIDERED WHEN CHOOSING THE MODEL STRUCTURE FOR INFECTIOUS DISEASE MODELLING?
- NATURAL HISTORY OF THE INFECTION (MODEL STRUCTURE SHOULD REFLECT THE NATURAL HISTORY OF A DISEASE, SHOWING ALL PERIODS!!!!!!!!!!!)
- ACCURACY AND TIME PERIOD OVER WHICH MODEL PREDICITIONS ARE REQUIRED (MAY NEED TO INCORPORATE KEY ASPECTS OF DEMOGRAPHY LIKE BIRTH DATE, DEATHS AND MIGRATION; IF LOOKING AT LONG TERM IMPACTS OF INFECTION) + (MAY NEED TO CONSIDER DIFFERENT STRAINS OR CHANGES TO THE POPULATION’S IMMUNITY TO THE DIFF STRAINS)
- RESEARCH QUESTION (THE MODEL STRUCTURE IS A FUNCTION OF THE RESEARCH QUESTION, SHOULD BE CLEARLY DEFINED)
WHAT DOES THE ‘SEIR’ MODEL OF INFECTIOUS DISEASE MODELLING STAND FOR?
SUSCEPTIBLE—>PREINFECTIOUS (EXPOSED)—>INFECTIOUS—> RECOVERED
WHAT ARE DETERMINISTIC MODELS?
MODELS THAT DESCRIBE WHAT HAPPENS ON AVERAGE IN A POPULATION (SAME RESULTS EACH TIME THEY ARE RAN)
WHAT ARE STOCHASTIC MODELS?
THEY ALLOW THE NUMBER OF INDIVIDUALS WHO MOVE THROUGH COMPARTMENTS TO VARY THROUGH CHANCE
- USEFUL FOR ASSUMPTIONS AND WHEN THERE IS A LOT OF UNCERTAINTY IN THE MODEL
- INCLUDE RANGS OF VALUES, AND DIFFER EACH TIME THEY RUN
TYPES OF MODELS?
- COMPARTMENTAL (PEOPLE CLASSIFIED INTO WELL KNOWN CLEARLY DEFINED CATEGORIES)
- INDIVIDUAL BASED OR MICROSIMULATION
- TRANSMISSION DYNAMIC OR DYNAMIC TRANSMISSION (ALLOWS CONTACT BETWEEN PEOPLE IN THE MODELS, MOSTLY FOR INFECTIOUS DISEASE)
- STATIC (NO MIXING BETWEEN THE INDIVIDUALS)
- NETWORK (OFTEN USED FOR STIs, VERY COMPLEX)
EAMPLES OF INFECTIOUS DISEASE MODEL INPUT PARAMETERS?
- RISK THAT SUSCEPTIBLE PEOPLE BECOME INFECTED BETWEEN A SPECIFIED TIME POINT
- PROPORTION OF INFECTED PEOPLE WHO BECOME INFECTIOUS
- PROPORTION OF INFECTIOUS INDIVIDUALS WHO RECOVER (I.E. BECOME IMMUNE, BUT THENMIGHT HAVE TO CONSIDER HOW LONG THE IMMUNITY LASTS)
- RELIABLE DATA
- UNCERTAINTY IN THE DATA
WHAT IS FORCE OF INFECTION (λ)?
THE RATE AT WHICH SUSCEPTIBLE INDIIDUALS BECOME INFECTED PER UNIT OF TIME/ THE PROBABILITY PER UNIT TIME THAT A SUSCEPTIBLE PERSON ACQUIRES AN INFECTION
BY MULTIPLYING THE FORCE OF INFECTION BY THE NUMBER OF SUSCEPTIBLE PEOPLE, WE GET?
RATE OF INFECTION
BASIC REPRODUCTION NUMBER (R0; R NAUGHT) INTERPRETATION?
- THE HIGHER THE NUMBER, THE MORE INFECTIOUS A CONDITION IS
R0=1, THE INCIDENCE OF THE DISEASEIN THE POPULATION REMAINS THE SAME, AND THERE IS NO CHANGE
R0<1, THE INCIDENCE OF THE DISEASE WILL DECREASE, AND THE DISEASE WILL EVENTUALLY BE ELIMINATED
R>1, THE INCIDENCE OF THE DISEASE WILL EXPONENTIALLY INCREASE
BASIC REPRODUCTION NUMBER IS…
AVERAGE NUMBER OF SECONDARY INFECTIOUS PERONS RESULTING FROM ONE INFECTIOUS PERSON FOLLOWING INTRODUCTION INTO A TOTALLY SUSCEPTIBLE POPULATION
NET REPRODUCTION NUMBER IS… (Rn)
AVERAGE NUMBER OF SECONDARY INFECTIOUS PERSONS RESULTING FROM ONE INFECTIOUS PERSON IN A GIVEN POPULATION (WHERE SUSCEPTIBILITY VARIES, NOT THE WHOLE POPULATION IS SUSCEPTIBLE/ NOT TOTALLY SUSCEPTIBLE)
EXAMPLES OF PROGRAMS/SOFTWARES THAT CAN BE USED FOR SETTING UP MODELS?
EXCEL, C++, R, PYTHON, BERKELEY MADONNA…
