5 5IntroSurveillance Slides

8/10/2019 5 5IntroSurveillance Slides

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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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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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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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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.
http://www.cdc.gov/epiinfo/maps.htmhttp://www.cdc.gov/epiinfo/maps.htm


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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
http://www.cdc.gov/epiinfohttp://www.cdc.gov/epiinfo


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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.

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