5 5introsurveillance slides
TRANSCRIPT
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Introduction to Public HealthSurveillance
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Goals Define surveillance, explain surveillance
systems
Describe basic surveillance techniquesby person, place, time
Touch on importance of standardization
Provide overview of how to presentsurveillance data
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Surveillance For persons who need to carry out
surveillance activities but have little
prior experience or training
Also helpful for people who would liketo better understand the process and
reasoning behind surveillance methodsand interpretation
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What Is Surveillance? Centers for Disease Control and
Prevention (CDC): epidemiologic
surveillance is ongoing systematiccollection, analysis, and interpretationof health data essential to the planning,implementation, and evaluation of
public health practice, closely integratedwith the timely dissemination of thesedata to those who need to know.
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Why Is Surveillance
Important? Collecting data is merely one step
Critical goal is to control and/or preventdiseases
Any data collected must be organized andcarefully examined
Any results need to be communicated topublic health and medical communities
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Why Is Surveillance
Important?Vital to communicate results
During potential outbreak so public health
and medical communities can help withdisease prevention and control efforts
During non-outbreak times to provideinformation about baseline levels of
disease Baseline provides information to public health
officials monitoring health at community level,serves as reference in future outbreaks
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Surveillance Systems Classified as passive or active
Passive surveillance: local and state health
departments rely on health care providers orlaboratories to report cases of disease Primary advantage is efficiency: simple and
requires relatively few resources
Disadvantage is possibility of incomplete data dueto underreporting
Majority of public health surveillance systems arepassive
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Surveillance Systems Active surveillance: health department
contacts health care providers or laboratories
requesting information about conditions ordiseases to identify possible cases
Requires more resources than passive surveillance
Useful when important to identify all cases
Example: between 2002 and 2005, active surveillanceused to detect adverse events associated with smallpoxvaccine. (2)
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Why Is Surveillance
Important? Surveillance information has many uses:
Monitoring disease trends
Describing natural history of diseases Identifying epidemics or new syndromes
Monitoring changes in infectious agents
Identifying areas for research
Evaluating hypotheses
Planning public health policy
Evaluating public health policy/interventions
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Why Is Surveillance
Important? Examples of uses of surveillance data:
Evaluating impact of national vaccinationcampaigns
Identifying AIDS when unknown syndrome
Estimating impact of AIDS on US health caresystem in 1990s (using mathematical modelsbased on surveillance data)
Identifying outbreaks of rubella and congenitalrubella among Amish and Mennonite communitiesin 6 states in 1990 and 1991 (3)
Monitoring obesity, physical activity, otherindicators for chronic diseases
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How to Conduct Surveillance Surveillance data allow description and
comparison of patterns of disease by
person, place, and time Several ways to describe and compare
patterns, from straightforwardpresentations to statistically complexanalyses
Will concentrate on simple techniques
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How to Conduct Surveillance:
Person When available, demographic characteristics
such as gender, age, race/ethnicity,occupation, education level, socio-economicstatus, sexual orientation, immunizationstatus can reveal disease trends Example: looking at Streptococcus pneumoniae, a
common cause of community-acquired pneumonia
and bacterial meningitis, examining distribution ofcases by race provides important informationabout burden of disease in different populations
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How to Conduct Surveillance:
PersonNumbers and Rates Table 1 shows data
collected onStreptococcuspneumoniae from CDCEmerging InfectionsProgram Network, asurveillance programthat collects data frommultiple counties in 10US states (4)
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How to Conduct Surveillance:
PersonNumbers and Rates Data show majority of
cases reported amongwhites
Can draw only limitedconclusions because racenot recorded for 684 cases(15%)
Shows only numberof
reported cases, not rate Total number of
individuals by race neededto determine if there is adisproportionate burden ofdisease among races
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How to Conduct Surveillance:
PersonNumbers and Rates Table 2 shows same
data with 2006
populationestimates of totalnumber of personsin each racial
category used tocalculate diseaserates (4)
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How to Conduct Surveillance:PersonNumbers and Rates
While Table 1 showedthat whites had thehighest numberof cases,Table 2 indicates that therate of disease washighest among blacks
Using rates, stratifyingby race providesinformation aboutdisease burden indifferent populationsthat would not beapparent from total casenumbers
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More on Rates
RatesA rate is an expression of thefrequency with which an event occurs in adefined population
Using rates rather than raw numbers isessential to compare different classes ofpersons or populations at different times orplaces. (5)
Rate = number of events in a specified periodaverage population during the period
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How to Conduct Surveillance:Place
Best to characterize cases by place of exposurerather than by place at which cases reported
The two may differ and place of exposure is more
relevant to epidemiology of a disease Example: travelers on a cruise ship exposed to a disease just
prior to disembarking but become symptomatic and arediagnosed after return to various home locations
Example: person exposed to disease in small rural town but
referred to tertiary care center 100 miles away wheredisease is diagnosed and reported
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How to Conduct Surveillance:PlacePresenting Data
Data by geographic location can be presentedin a table
Also helpful to use maps to facilitaterecognition of spatial associations in data See FOCUS Volume 5, Issue 2: Mapping for
Surveillance and Outbreak Investigation fordiscussion of maps and visual presentation of
information
Inferential analysis can also be done usingmultilevel modeling, other statistical methods
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How to Conduct Surveillance:PlaceModeling Resources
Modeling of surveillance data by place isbeyond scope of this issue
Resources for further information: Centers for Disease Control and Prevention.
Resources for creating public health maps.http://www.cdc.gov/epiinfo/maps.htm. UpdatedAugust 14, 2008. Accessed August 22, 2008.
Clarke KC, McLafferty SL, Tempalski BJ. Onepidemiology and geographic informationsystems: A review and discussion of futuredirections. Emerg Infect Dis. 1996; 2(2):85-92.
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How to Conduct Surveillance:PlaceSpot Maps
Spot maps: maps on which a dot or symbol marks acase of disease
Made by indicating exposure locations of reported
cases of disease on hard copy map with pins orcolored pen
Or with geographic information systems (GIS) Computer programs designed for storing, manipulating,
analyzing, and displaying data in a geographic context
Very useful for mapping surveillance data by place
Epi Map (part of Epi Info) can be downloaded for free athttp://www.cdc.gov/epiinfoto assist with map making
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How to Conduct Surveillance:PlaceSpot Maps
Example: spot map used toshow geographic spread ofcases in 1995 outbreak oftoxoplasmosis thought to be
associated with a municipalwater system in BritishColumbia, Canada (5)
Spot maps show geographicdistribution of cases but not
population size at eachlocation, so should not beused to assess disease risk
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How to Conduct Surveillance:Time
Compare number of cases reported in timeperiod of interest (weeks, months, years) to
number of cases reported during similarhistorical period
Usually a delay (sometimes months to years)between disease onset and date when
disease is reported, so preferable to use dateof onset, if available, rather than date ofreport
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How to Conduct Surveillance:TimeLine Graphs
Especially helpful for examining data notlikely to have much short term variation
Example: there is limited variation in number ofAIDS cases reported each month
Provide valuable qualitative information;disease outbreaks often obvious from visual
inspection of data, may not require aquantitative analysis
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How to Conduct Surveillance:TimeLine Graphs
Example of line graphusing fabricated data:reported cases of
Salmonellatyphimuriumfor 2-yeartime intervals from1974 to 2002
Spike in 1994
indicating outbreak ofS. typhimurium obviouswithout quantitativeanalysis
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How to Conduct Surveillance:TimeIncidence Rates
May use line graph to plot incidence rates Incidence rate is number of new cases that occur
during a specified time interval in a population at risk
for developing the disease Number of new cases may be used as a proxy for overall
disease occurrence Value often multiplied by 1,000 or 100,000 to improve
interpretability
Reporting incidence rates rather than numbers
particularly important if population has changed insize or characteristics Example: addition of towns to a surveillance region has
increased population size, or influx of migrant workers hassignificantly changed the demographics
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Standardization
Rate made up of numerator and denominator
Surveillance data often numerator data (number ofcases reported in time period) Utility of these raw numbers is limited because do not take
into account size of population or distribution ofdemographic factors such as age or gender
Rates allow more meaningful comparisons over timewithin a population, among subpopulations, or
between populations Rates take into account size of the population and time
period involved (3)
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Standardization
Crude rates often calculated using surveillance data
Number of events of interest (such as reported casesof disease) for a specific period of time for the entire
population Only appropriate to compare crude rates if
populations are similar with respect to factors relatedto disease of interest, such as age, gender, race Example: would be inappropriate to compare rate of
prostate cancer in population with high proportion of elderlymen to rate in another population with mostly young men,since risk of prostate cancer increases with age
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Standardization
Standardization used to remove effects of differencesin confounding variables such as age whencomparing two or more populations Results in adjusted rates Is particularly useful when comparing rates in different
populations (e.g., comparing state data to national data)when comparison of crude rates may be misleading ifpopulations differ on key variables
Most common technique uses weighted average ratesspecific to potential confounding variables, based onspecified distribution of the variables (5)
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Data Presentation
Surveillance data must be presented in waythat is easy to understand and interpret
Many ways to display surveillance data: (3) Line graphs for displaying data by time
Maps for presenting data in geographic context
Graphical displays such as histograms, frequencypolygons, box plots, scatter diagrams, bar charts,pie charts, or stem-and-leaf displays
Spot or chloropleth maps
Single/multivariable tables
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Data Presentation
The choice of a particular graph or tabledepends on type of data, but presentationshould be simple and easy to follow
Should provide all information necessary tointerpret the figure without referring to text
Include concise title that describes subject or
disease, time, place (when relevant) Define any abbreviations or symbols
Note any data exclusions (3)
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Data Presentation
Additionaldisplay
guidelinesfor tablesand graphs
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Conclusion
Surveillance is valuable epidemiologictool that can serve many purposes
When surveillance data is collected,analyzed, interpreted, reportedappropriately, these data can provide
important information about diseasepatterns to inform public health practiceand policy
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References
1. Thacker SB, Berkelman RL. Public healthsurveillance in the United States. Epidemiol Rev.1988;10:164-190.
2. Thomas TN, Reef S, Neff L, Sniadack MM, MootreyGT. A review of the smallpox vaccine adverseevents active surveillance system. Clin Infect Dis.2008;46 Suppl 3:S212-S220.
3. Janes GR, Hutwanger L, Cates Jr W, Stroup DF,Williamson GD. Descriptive Epidemiology:Analyzing and Interpreting Surveillance Data. In:Teutsch SM, Churchill RE, eds. Principles andPractice of Public Health Surveillance. New York,NY: Oxford University Press, inc, 2000:112-167.
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References
4. Centers for Disease Control and Prevention. ActiveBacterial Core Surveillance Report (ABCs),Emerging Infections Program Network,Streptococcus pneumoniae, 2006.http://www.cdc.gov/ncidod/dbmd/abcs/survreports/spneu06.pdf2007. Published 2007. AccessedAugust 21, 2008.
5. Last JM, ed. A Dictionary of Epidemiology. 3rd ed.New York, NY: Oxford University Press, Inc, 1995.
6. Eng SB, Werker DH, King AS, et al. Computer-generated dot maps as an epidemiologic tool:Investigating an outbreak of toxoplasmosis. EmergInfect Dis.1999;5(6):815-819.