introduction to surveillance
DESCRIPTION
Introduction to Surveillance. Questions. What do you think of when you hear the word “surveillance”? What do you suppose is the definition of surveillance when applied to infection prevention and control? . Learning Objectives. - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/1.jpg)
1
INTRODUCTION TO SURVEILLANCE
![Page 2: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/2.jpg)
Questions
• What do you think of when you hear the word “surveillance”?
• What do you suppose is the definition of surveillance when applied to infection prevention and control?
![Page 3: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/3.jpg)
Learning Objectives
• Define surveillance and its role in an infection prevention and control program
• Describe types of surveillance and advantages and disadvantages of various surveillance strategies
• Give examples of basic measures of disease frequency and describe applications
![Page 4: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/4.jpg)
Public Health Surveillance Definition
The ongoing, systematic collection, analysis, and interpretation of health-related data essential to the planning, implementation, and evaluation of public health practice, closely integrated with the timely dissemination of these data to those responsible for prevention and control
![Page 5: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/5.jpg)
Surveillance in the Healthcare Setting
Surveillance of healthcare—acquired infections (HAI) is the foundation for organizing, implementing, and maintaining an effective infection prevention and control (IPC) program in the health care facility
“If you don’t measure it, you cannot improve it”
~ Lord Kelvin
![Page 6: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/6.jpg)
Considerations for Surveillance
Frequency Severity Cost Preventability Communicability
![Page 7: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/7.jpg)
Methodologic Issues
GoalsCausal pathwayInformation needs Data sources, methodsCase definitionEvaluation: Balance of attributesInterpretation of data
![Page 8: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/8.jpg)
Interpretation of DataIssues to Consider
Source of dataReliability of diagnosisPotential biases in
detection/diagnosisDefinition of a caseCompleteness of dataReporting biasConsistency in data collectionCompleteness Context
![Page 9: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/9.jpg)
Surveillance in the Healthcare Setting:Objectives
Establish endemic or baseline rate of infections
Compare HAI rates within/between health care facilities
Engage clinical team to adopt best practices Introduce evidence-based and cost-effective
interventions to reduce HAIIdentify and control outbreaksEvaluate success of the ICP interventionsIdentify priority areas to allocate resources
Ultimate aim is to reduce HAI
![Page 10: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/10.jpg)
Components of a Strong Surveillance Program
SystematicOngoingData CollectionAnalysisInterpretationDisseminationAction
![Page 11: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/11.jpg)
Surveillance in the Healthcare Setting
Data must be Collected Validated Analyzed Interpreted Disseminated in a timely manner
Collecting and recording data is useless if no further action is taken Surveillance is synonymous with the premise of
“information for action”
![Page 12: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/12.jpg)
Surveillance in the Healthcare Setting:Methods
When applicable, data set should include:Information on the infected patient or
residentInformation on medical treatment or
procedures at the time of infectionAny underlying medical risk factors of the
patient
Information on both numerator and denominator data should be collected for the calculation of rates of infection
![Page 13: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/13.jpg)
Surveillance in the Healthcare Setting:Methods
Flexible to address challenges Technological changes within the health care facility Short lengths of stay Healthcare worker shortage and turnover Increased frequency of invasive procedures or devices Post-discharge surveillance, as appropriate
![Page 14: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/14.jpg)
Characteristics of a Strong Surveillance Program
Targets Infection prevention Performance improvement Patient safety Public health activities
Engages in mandatory and public reporting
![Page 15: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/15.jpg)
Characteristics of a Strong Surveillance Program
Able to identify risk factors for infection Adverse events Outbreaks Emerging infectious diseases Antibiotic-resistant organisms Bioterrorist events
Implements control or risk-reduction measures
Monitors the effectiveness of intervention
![Page 16: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/16.jpg)
Various Methods of Surveillance Used in Infection Control
Methods Source of Data Comments
Continuing surveillance of all patients (Prospective, active surveillance)
Medical, nursing, laboratory records
Time-consuming and not cost-effective. Infection rates are low in some specialties.
Ward liaison Twice-weekly visits to wards
Discuss all patients with staff and review records
Less comprehensive than continuing surveillance, with similar disadvantages.
Laboratory-based Laboratory records only Depends on samples taken and information on request form.
Laboratory-based ward surveillance/selected continuing surveillance
Reporting of laboratory records and outbreaks by ward staff and continuing surveillance in special units or infections
Early detection of outbreaks and incidence in studies in selected areas of infection.
Adapted from: Glenister HM, Taylor LJ, Bartlett CLR, et al. An evaluation of surveillance methods for detecting infections in hospital inpatients. Journal of Hospital Infection 1993; 23:229-42.
![Page 17: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/17.jpg)
17
Various Methods of Surveillance Used in Infection Control
Strategy Pros Cons
Incidence Provides data on infections due to all organisms, on all infection sites, and on all unitsIdentifies clusters
Establishes baseline infection rates
Allows outbreaks to be recognized early
Identifies risk factors
Expensive and labor intensive
Large amounts of data collected with little time for a analysis
No defined prevention objectives
Difficult to develop interventionsNot all infections are preventable
Prevalence Inexpensive
Efficient use of time; can be done periodically
Over-/underestimates infection rates; does not capture data on relevant differences
Limited value in small facilities
![Page 18: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/18.jpg)
Various Methods of Surveillance Used in Infection Control
Strategy Pros Cons
Site-specific Flexible and can be combined with other strategies
Identifies risk factors
No defined prevention strategies or objectives
Denominator may be inadequate
Unit specific Focuses on patients at greater risk
Simplified and reduces personnel
May miss clusters
Denominator may be inadequate
Objective or priority-based
Can be adaptable to facilities with special populations or resources
Focuses on specific issues at the facility
Identifies risk factors
Baseline infection rates are not available
May miss clusters or outbreaks
Adapted from: Perl TM, Chaiwarith R. Surveillance: An overview. Practical Healthcare Epidemiology, 3rd Ed., pp. 111-142, Chicago, IL: University of Chicago Press, 2010.
![Page 19: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/19.jpg)
19
Types of Surveillance: Outcome Surveillance
Objective: COUNT number of HAI Informs the magnitude of the problem
Disadvantages: No information on what factors contribute
to the problem No internationally agreed definitions on
surveillance Most commonly used: CDC/NHSN (USA) and ECDC
(Europe) Assumes availability of good diagnostic laboratory
support
![Page 20: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/20.jpg)
20
Types of Surveillance: Process Surveillance
Objective: MONITOR adherence to evidence-based or best practices
Essential to prioritize which processes/steps to monitor
Disadvantages: Reliability of data Good compliance does not equate with
effectiveness
![Page 21: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/21.jpg)
21
Example of Outcome vs. Process Surveillance
Hand Hygiene
Daily Review
Skin Disinfection
Maximum Barrier
Precautions
Optimal catheter
site selection
OUTCOME SURVEILLANCE
Counting number of CR-
BSIs
PROCESS SURVEILLANCE
Monitoring compliance with CVC care bundle elements
Adapted from Damani, N. Manual of Infection Prevention and Control, Third Edition. New York: Oxford University Press, 2012.
![Page 22: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/22.jpg)
22
Recommended Minimum Elements in a Data Set for Surveillance
Patient /resident information– Name or unique identifier, DOB, sex, MRN, ward or unit
in facility, name of consultant, date of admission, onset date, date of discharge or death, site of infection/colonization, organism isolated with antibiotic sensitivities
Medical treatment/procedures– At time of infection– Underlying medical risk factors, clinical outcome,
assessment of whether the incident was preventable
![Page 23: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/23.jpg)
23
Linelist: Example
![Page 24: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/24.jpg)
24
Considerations
All types of surveillance are expensive and time-consuming
Essential that definitions and objectives of surveillance must be agreed with the clinical team
Identify resources
Personnel involved in surveillance must be trained
![Page 25: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/25.jpg)
25
Summary
Assess population and identify those at greatest risk for outcome or process of interest
Select outcome or process for surveillance– Examples of outcomes: HIA, infection or colonization
with a specific organism, sharps injuries– Examples of processes: Central line insertion practices,
influenza vaccination rates, personnel compliance with protocols
Determine observation period
![Page 26: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/26.jpg)
26
Summary (continued)
Choose surveillance methodology
Monitor for outcome or process using standardized definitions for all data collected
Collect appropriate denominator data, if rates are to be calculated
Analyze data
Report in a timely manner
![Page 27: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/27.jpg)
ReferencesDamani, N. Manual of Infection Prevention and Control, Third Edition. New York: Oxford University Press, 2012.
DHHS/CDC . Outline for Healthcare-Associated Infections Surveillance, 2006.
Tokars JI, Richards C, Andrus M, et al. The Changing Face of Surveillance for Health Care—Associated Infections. Clinical Infectious Diseases 2004; 39: 1347-52.
Horan TC, Andrus M, Dudeck MA. CDC/NHSN surveillance definition of health care—associated infection and criteria for specific types of infection in the acute care setting. American Journal of Infection Control 2008; 36:309-32.
Haley RW, Culver DH, White JW, et al. The efficacy of infection surveillance and control programs in preventing nosocomial infection in US hospitals. (SENIC study). American Journal of Epidemiology 1985; 121(2):182-205.
Lee TB, Montgomery OG, Marx J, et al. Recommended practices for surveillance: Association for Professionals in Infection Control and Epidemiology (APIC), Inc. American Journal of Infection Control 2007; 35(7):427-40.
HICPAC guidance on public reporting of healthcare-associated infections: Recommendations of the Health care Infection Control Practices Advisory Committee. Infection Control Hospital Epidemiology 2005; 26(6):580-7.
![Page 28: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/28.jpg)
“Good surveillance does not necessarilyensure the making of right decisions,but it reduces the chances of wrong ones.”
-Alex Langmuir, NEJM 1963: 268:182-191
![Page 29: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/29.jpg)
MEASURES OF DISEASE FREQUENCY
![Page 30: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/30.jpg)
“One’s knowledge of science begins when he can measure what he is speaking about and express it in numbers”
Lord Kelvin 1824-1907
![Page 31: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/31.jpg)
Measures in General
Count Ratio Proportions Rate
![Page 32: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/32.jpg)
EPIDEMIOLOGIC MEASURES
Measures of frequency Incidence Prevalence Interrelationship between incidence and
prevalence
![Page 33: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/33.jpg)
COUNT
Simple measure of quantity
Example: The number of catheter-related bloodstream infections (CR-BSIs) in Facility X in 2012.
![Page 34: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/34.jpg)
RATIO
An expression of the relationship between a numerator and a denominator where the two are separate and distinct quantities.
Example: Injurious falls occur in twice as many women aged 65-69 years as in men of the same age group.
Ratio of women to men is 2/1 or 2:1
![Page 35: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/35.jpg)
PROPORTION
A type of ratio in which the numerator is
included in the denominator.
![Page 36: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/36.jpg)
EXAMPLE OF PROPORTION
650 HIV+ patients were seen at Facility X.
130 of these patients had Pneumocystis carinii pneumonia (PCP).
Proportion of HIV+ patients seen at Facility X with PCP is 130/650.
130/650 *100 = 20%
![Page 37: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/37.jpg)
RATE
An expression of the frequency with which an event occurs in a defined population.
A measure of time is an intrinsic part of
the denominator.
![Page 38: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/38.jpg)
EXAMPLE OF RATE
435/1,000 elderly individuals residing in assisted living facilities had colds in January.
(The 435 elderly residents with colds are part of the 1,000 residents in assisted living facilities.)
![Page 39: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/39.jpg)
TYPES OF RATES
Morbidity rates measure the frequency of
illness within a specific population. Incidence Prevalence Attack rate
Mortality rates measure the frequency of
death within a specific population. Crude death rate Cause-specific death rate Case-fatality rate
![Page 40: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/40.jpg)
MEASURES OF DISEASE FREQUENCY
Measures that characterize the occurrence of disease, disability or death in populations.
Incidence Prevalence
![Page 41: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/41.jpg)
MEASURES OF DISEASE FREQUENCY
Incidence (I): Measures new cases of a disease or health event that develop over a period of time.
Prevalence (P): Measures existing cases of a disease at a particular point in time or over a period of time.
![Page 42: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/42.jpg)
INCIDENCE
The number of new cases of disease
that occur in a specified period of time.
There are two kinds of incidence measures:· Cumulative incidence (CI)· Incidence density (ID) or incidence rate (IR)
![Page 43: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/43.jpg)
CUMULATIVE INCIDENCE
The proportion of unaffected individuals who contract disease during a specified time.
CI = # of new cases in a given time Total population at risk
(Estimate of individual risk)
![Page 44: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/44.jpg)
PROBLEMS WITH USING CI
To accurately calculate CI we need to follow the entire population for the specified time interval.
This is rarely possible for two main reasons People move in and out People may die from diseases other than disease
of interest
![Page 45: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/45.jpg)
INCIDENCE RATE
The instantaneous potential for change in disease status per unit of time.
IR = # new case in a given timeTotal person-time of observation
Ranges from 0 to
![Page 46: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/46.jpg)
WHAT DENOMINATOR DATA TO COLLECT?
For device-associated HAI incidence rates: Daily total number of patients AND Total number of ventilator-days, central
line-days, and urinary catheter-days in patient care area(s) under surveillance
Sum daily counts at the end of the surveillance period for use as denominators
Denominator data may be collected by someone other than the ICP as long as that person is trained
![Page 47: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/47.jpg)
INCIDENCE RATE EXAMPLE
Three people out of ten persons observed develop disease during a 30-day period of follow-up.
The cumulative rate = 3 cases in 30 days 10 people
or 1 per 100 per day
(3/10 = 0.3 * 100 = 30/30 days = 1 = incidence for one day per 100 people)
![Page 48: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/48.jpg)
48
ATTACK RATE
Another type of incidence rate Expressed as cases per 100 population (or
a percentage) Used to describe the new and recurrent
cases of disease that have been observed in a particular group during a limited time period in special circumstances, such as during an outbreak
Attack rate:Number of new and recurrent cases in a specified time period Population at risk for same time period
X 100
![Page 49: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/49.jpg)
PREVALENCE
Measures existing cases of a health condition
Two types of Prevalence Point prevalence Period prevalence
![Page 50: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/50.jpg)
POINT PREVALENCE
Point Prevalence = C / N
Where C = Number of observed cases at time tAnd N = Population size at time t
Point prevalence measures the frequency of disease at a given point in time.
![Page 51: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/51.jpg)
POINT PREVALENCEEXAMPLE
Suppose there are 150 individuals in a population and, on a certain day, 15 are ill with the flu. What is the estimated prevalence for this population?
P = 15/150 = 10%
![Page 52: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/52.jpg)
PERIOD PREVALENCE
Period Prevalence = [C + I] / N
C = the number of prevalent cases at the beginning of the time period.
I = the number of incident cases that develop during the period.
N = size of the population for this same time period.
![Page 53: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/53.jpg)
EXAMPLE
What is the prevalence of disease X on January 1, 1992?Point Prevalence = C/N = 0/10 = 0%
![Page 54: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/54.jpg)
EXAMPLE
What is the period prevalence of disease X between 1990 and 1995?Period Prevalence = [0 + 5] / 12 = 42%
![Page 55: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/55.jpg)
PREVALENCE
Useful for: Assessing the health status of a population. Planning health services.
Not Useful for: Identifying risk factors
![Page 56: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/56.jpg)
ANOTHER EXAMPLE
Suppose we followed a population of 150 persons for one year, and 25 had a disease of interest at the start of follow-up and another 15 new cases developed during the year.
What is the point prevalence at the start of the period?
What is the period prevalence for the year?
What is the point prevalence at the end of period?
What is the cumulative incidence for the one year period?
![Page 57: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/57.jpg)
ANOTHER EXAMPLE
Suppose we followed a population of 150 persons for one year, and 25 had a disease of interest at the start of follow-up and another 15 new cases developed during the year.
What is the point prevalence at the start of the period?25/150 = 0.17 = 17%
What is the period prevalence for the year?(25 + 15) / 150 = 0.27 or 27%
What is the point prevalence at the end of period?Not known
What is the cumulative incidence for the one year period?15/125 = 0.12 = 12%
![Page 58: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/58.jpg)
FACTORS THAT INCREASE PREVALENCE
Cases move into population Healthy people leave population Longer living with disease Longer duration of disease Increased number of susceptible/at-risk individuals
![Page 59: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/59.jpg)
FACTORS THAT DECREASE PREVALENCE
Cases move out of population Healthy people move into population People being cured Shorter duration of disease Decreased number of susceptible/at-risk
individuals
![Page 60: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/60.jpg)
INTERRELATIONSHIP BETWEEN INCIDENCE AND PREVALENCE
Prevalence depends on both incidence and disease duration.
If the incidence is low but the disease duration is
long, the proportion of the population with
the disease at a particular time is high
compared to the incidence.
![Page 61: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/61.jpg)
EXAMPLE OF INTERRELATIONSHIP
In the beginning of the AIDS epidemic, the incidence rate of AIDS increased quickly. However, the disease duration was short because everyone died in a few years. Therefore, the prevalence was low.
Incidence and duration = prevalence
![Page 62: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/62.jpg)
EXAMPLE OF INTERRELATIONSHIP
Today, the incidence rate is not increasing as quickly but the duration of survival is considerably longer. Therefore, the prevalence is now much higher.
Incidence and duration = prevalence
![Page 63: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/63.jpg)
PREVALENCE AND INCIDENCE
When the disease is stable:
Prevalence = Incidence * Disease Duration
![Page 64: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/64.jpg)
Incidence vs. Prevalence
Incidence (I): Measures new cases of a disease that develop over a period of time.
Prevalence (P): Measures existing cases of a disease at a particular point in time or over a period of time.
![Page 65: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/65.jpg)
Prevalence vs. Incidence
Prevalence can be viewed as describing a pool of disease in a population.
Incidence describes the input flow of new cases into the pool.
Fatality and recovery reflects the output flow from the pool.
![Page 66: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/66.jpg)
SUMMARY
Risk (cumulative incidence) is the probability that an event will occur within a given time-interval
Rate (incidence rate) is a measure of how rapidly the events occur in a population
In contrast to measures of incidence (risk and rate), prevalence deals with existing (as opposed to newly occurring) health-related states
![Page 67: Introduction to Surveillance](https://reader035.vdocuments.us/reader035/viewer/2022062520/56815d7f550346895dcb8d0e/html5/thumbnails/67.jpg)
67
Linelist: Example