Chapter 3- Disease Surveillance and Measures of Morbidity

Question 1

Epidemiologic surveillance

Answer 1

The ongoing systematic collection, analysis, and interpretation of health data essential to the planning, implementation, and evaluation of public health data essential to the planning, implementation, and evaluation of public health practice integrated with the timely distribution of data to those who need to know. Surveillance can be carried out to monitor changes in disease frequency or to monitor changes in the levels of risks for specific diseases.

Question 2

What kinds of diseases is surveillance used to monitor for?

Answer 2

It has been used for both infectious and noninfectious diseases. It provides a lot of our information regarding morbidity and mortality of diseases. Surveillance is conducted to monitor changes in conditions like congenital malformations, environmental toxins, and for injuries and illnesses after natural disasters. It is also used to monitor for completeness of vaccination coverage and protection of a population and for prevalence of drug resistant organisms like drug resistant TB and malaria

Question 3

How does surveillance impact policy?

Answer 3

Surveillance provides policy makers with guidance for developing and implementing the best strategies for programs for disease prevention and control

Question 4

Surveillance case definition

Answer 4

A set of uniform criteria used to define a disease for public health. Case definitions are intended to aid public health officials in recording and reporting cases by standardizing definitions of disease and diagnostic criteria

Question 5

Passive surveillance

Answer 5

Surveillance in which available data on reportable diseases are used, or in which disease reporting is mandated or requested by the government or the local health authority. The responsibility for reporting falls on the health care provider or district health officer

Question 6

Passive reporting

Answer 6

Reporting done by the health care provider or district health officer. The completeness and quality of the data often depends on the individual and their staff, who may take on the role without additional resources or funding. Therefore, a disadvantage of passive reporting is underreporting or lack of completeness of the data. Reporting instruments should be simple and brief to minimize this problem. Another disadvantage is that local outbreaks may be missed because the relatively small number of cases becomes diluted within a larger population

Question 7

Examples of reportable diseases

Answer 7

Common STIs (syphilis, gonorrhea, and HIV), influenza

Question 8

Advantage of passive reporting systems

Answer 8

A passive reporting system is relatively inexpensive and relatively easy to develop initially. Many countries have systems of passive reporting for infectious diseases, so passive reporting allows for international comparisons that can identify areas that need assistance in confirming new cases and in providing appropriate interventions for control and treatment

Question 9

How is passive reporting done in communities?

Answer 9

To monitor flu outbreaks, Google searches and social media can be assessed

Question 10

Active surveillance

Answer 10

A system in which project staff are specifically recruited to carry out a surveillance program. They make field visits to healthcare facilities like clinics and hospitals to identify new cases of the disease or deaths from the disease that have occurred.

Question 11

Case findings

Answer 11

During active surveillance, when public health officials visit hospitals and clinics to identify new cases of the disease or the number of deaths from the disease

Question 12

What does active surveillance involve?

Answer 12

Involves interviewing physicians and patients and reviewing medical records. In developing countries and rural areas, villages and towns are surveyed to detect cases periodically or on a routine basis, or after an index case has been reported

Question 13

Passive surveillance vs active surveillance

Answer 13

Active surveillance is generally more accurate because it is conducted by individuals who are specifically trained and recruited to carry out the surveillance. With passive surveillance, physicians are often busy with their other responsibilities and view reporting diseases as peripheral to their other roles. With active reporting, outbreaks are often easier to identify. However, active reporting is more expensive and is usually more difficult to develop initially

Question 14

What problems are presented by surveillance in developing countries?

Answer 14

Areas in need of surveillance may be difficult to reach, and it may be difficult to maintain communication with these areas in order to make policy decisions and allocate necessary resources. Additionally, the definitions of disease used in developed countries may be inappropriate or unusable in developing countries due to a lack of the laboratory and other resources needed for full diagnostic evaluation of suspected cases. There may be underreporting of observed clinical cases

Question 15

Natural history of disease in an individual

Answer 15

The person starts off healthy (without the disease), but at some point biologic onset of disease occurs. There are usually no symptoms at this stage, but eventually, symptoms do occur. At this point, the person will seek medical care for their symptoms and will receive a diagnosis. Hospitalization may occur for diagnosis, treatment, or both. One of several outcomes can follow treatment

Question 16

Disease outcomes after treatment (4)

Answer 16

1. Cure
2. Control
3. Disability
4. Death

Question 17

What sources of data can be used to obtain information about the person’s illness?

Answer 17

If the illness requires hospitalization, medical and hospital records can provide data. If hospitalization is not required, a primary care provider’s medical records can be the best source of data. The patient or a family member can provide information about the illness before they sought care. The source of data from which cases are identified influences the rates that we calculate for expressing the frequency of disease. For example, if a patient only needed to seek care at a physician’s office, hospital records will not include that data

Question 18

Rates

Answer 18

Division of two numbers, time is always in the denominator. Tell us how fast the disease is occurring in a population

Question 19

Proportions

Answer 19

Division of 2 related numbers, numerator is a subset of the denominator. Tell us what fraction of the population is affected

Question 20

Incidence rate

Answer 20

The number of new cases of a disease that occur during a specified period of time in a population AT RISK for developing the disease. The incidence rate is a measure of risk

Question 21

What factors influence the incidence rate?

Answer 21

Influenced by the frequency of disease. Incidence is defined as a percentage, so rare diseases might require a higher multiplier than 1,000 (10,000 or 100,000). Increased frequency of screening could also contribute to increased incidence- more disease is detected

Question 22

Denominator of the incidence rate

Answer 22

The denominator represents the number of people who are at risk for developing the disease. For an incidence rate to be meaningful, any individual who is included in the denominator must have the potential to become part of the group that is counted in the numerator. For example, if calculating the incidence of uterine cancer, the denominator must only include women who have not had a hysterectomy- only people with a uterus can get uterine cancer. The denominator can also look at people at risk who are observed throughout a defined time period or it can look at person-time (units of time when each person is observed)

Question 23

Cumulative incidence proportion

Answer 23

In this situation, the individuals in the denominator have been followed for that entire time period. The choice of time is arbitrary- you could calculate incidence for 1 week, 1 month, 1 year, etc. The incidence is calculated using a period of time during which all individuals in the population are considered at risk for the outcome- this is a measure of risk

Question 24

Person-time

Answer 24

Consists of the sum of the units of time that each individual at risk was observed. It is expressed in terms of person-months or person-years of observation. Example- if 5 people are each observed for a 5 year period, the person-time is 25 person-years.

Question 25

Incidence density

Answer 25

This is calculated when every individual in the denominator cannot be followed for the full period (due to death, loss to follow up, and other reasons). In this case, the denominator is the person-time. This is because people at risk for the specific disease are observed for different periods of time

Question 26

Attack rate

Answer 26

In a situation of foodborne disease, the number of people exposed to a suspect food who become ill, divided by the total number of people who were exposed to that food. This situation is a point-source epidemic curve. The attack rate does not explicitly state the time interval, because many foodborne disease outbreaks occur within hours or days after consumption. The attack rate is actually a proportion because it tells us the proportion of all people who ate a certain food and became ill

Question 27

Prevalence

Answer 27

The proportion of the population that is affected by the disease at a certain time. The number of affected people present in the population at a specific time is divided by the number of people in the population at that time. Numerator represents old & new cases with no info on when disease occurred

Question 28

What is the difference between incidence and prevalence?

Answer 28

Prevalence can be viewed as a snapshot or a slice through the population at a point in time at which we determine who has the disease and who does not. However, when we do this, we are not determining when the disease developed- some people may have developed arthritis over the past few months, while some people may have had arthritis for 10 years. However, all of these people would be included in prevalence. In contrast, incidence includes only new cases or events, as well as a specific time period when the new events occurred.

Question 29

Point prevalence

Answer 29

Prevalence of the disease at a certain point in time. This is generally what is meant when people refer to prevalence. Example- asking someone if they currently have asthma

Question 30

Period prevalence

Answer 30

How many people have had the disease at any point during a certain time period? The time period can be arbitrarily chosen (one month, one year, etc.). Some people may have developed the disease during that period, while others may have already died or been cured from the disease. Every person in the numerator has had the disease at some point during the time period specified. Example- asking someone if they have had asthma during the last X years

Question 31

Cumulative incidence

Answer 31

Example- asking someone if they have ever had asthma

Question 32

Dynamic relationship between incidence and prevalence

Answer 32

To increase prevalence, incidence (new cases) must increase as well. To lower the prevalence, death or cure would have to occur- both of these would remove the number of people who have the disease. It is also important to remember that the development of life-sustaining (but not curative) treatment for a disease will increase the prevalence because patients with this disease are no longer dying. For example, the prevalence of diabetes increased when insulin first became available. This would also occur if new techniques are developed for early detection of the disease in more people

Question 33

How does prevalence assist with resource allocation?

Answer 33

Prevalence is a useful measure of the burden of disease in a community to inform resource allocation by decision-makers. Can be used as an indicator for how many providers and how many clinics are needed

Question 34

Etiology

Answer 34

The cause of disease

Question 35

Incidence and prevalence- etiology of disease

Answer 35

Data on incidence is needed in order to explore the relationship between an exposure and the risk of disease. However, prevalence data may be suggestive if not confirmatory in studying the etiology of certain disease. Prevalence can be used to look at time trends or geographic distribution, which can be linked to a certain disease

Question 36

Problems with numerators- incidence and prevalence (2)

Answer 36

1. Defining who has the disease. Some diseases are difficult to diagnose, and there can be multiple sets of diagnostic criteria. It can be difficult to decide which definition to use
2. Determining which people should be included in the numerator. Which study or data should be used to determine case numbers?

Question 37

Problems with denominators- incidence and prevalence

Answer 37

1. Selective undercounting of certain groups of the population could occur if the risk of an ethnic group for a certain disease is being discussed. Different definitions for defining ethnic groups may occur, so each study should state their working definition of an ethnic group
2. Everyone in the group represented by the denominator must have the potential to enter the group that is represented by the numerator. For some diseases, it can be difficult to determine who should be included

Question 38

Problems with hospital data (3)

Answer 38

1. Hospital admissions are selective- may depend on personal characteristics, severity of disease, associated medical conditions, and hospital policies
2. Hospital records are designed by patient care, not research. There may be problems with compatibility if using records from different hospitals
3. There may be problems with defining denominators- most hospitals in the US do not have defined catchment areas

Question 39

Catchment areas

Answer 39

Areas that require that all people in those areas who are hospitalized must be admitted to a particular hospital, and no one from outside that area can be admitted to that hospital

Question 40

Steady-state

Answer 40

When the rates are not changing and in-migration equals out-migration

Question 41

Who to consider during surveillance?

Answer 41

Host level factors- what populations or subpopulations are relevant to the health behavior or health outcome- the “who” is informed about your knowledge about the specific disease or health behavior. How does disease compare across subpopulations?

Question 42

How is surveillance used to determine when disease occurred?

Answer 42

Examine cases over time to consider agent and environmental factors- thinking about time in reference to the disease. Do you see patterns/trends across time? May be able to use this information to predict future trends

Question 43

How to determine where disease occurred?

Answer 43

Cases are mapped by place to determine whether there are patterns of disease across spatial units. For example, certain regions of the US have much lower life expectancies than others. What spatial unit is appropriate for analysis?

Question 44

Epidemic curves

Answer 44

Visual representation of the number of cases (y axis) and time (x axis)- often used for foodborne outbreaks to determine when exposures occurred. There are various dates to choose- people tend to wait various times to seek healthcare or to be tested to come to a definitive diagnosis. The data representing onset of the symptoms can be the most accurate/most standardized. There are 4 types of curves

Question 45

Outliers in epidemic curves

Answer 45

An outlier could represent the index or initial case

Question 46

Answer 46

Point source- specific event where everyone was exposed. The first case represents the minimum incubation period. The last case generally represents the maximum incubation period. These graphs can help to identify the exposure data as well

Question 47

Answer 47

Common, intermittent source. Where everyone is exposed to the same source but at different periods of time. Example- when a contaminated food is given to people at different times.

Question 48

Answer 48

Common continuous source. Everyone is exposed to the same source, but exposure occurs over a longer period of time

Question 49

Answer 49

Propagated outbreak- waves of illness. Occurs in situations of person to person spread, like the flu

Question 50

Uses of surveillance data (6)

Answer 50

1. Disease detection & changes in health problems/behaviors
2. Disease control/contact tracing
3. Estimate magnitude/scope of a health problem
4. Examine trends and generate data for descriptive epidemiology
5. Ecological assessment of program effectiveness
6. Develop hypotheses & stimulate research

Question 51

Sources of surveillance data (5)

Answer 51

1. Vital records, like birth certificates
2. Notifiable diseases or syndromes
3. Disease or immunization registries- providers are legally required to report vaccines given to children
4. Electronic health records
5. Population surveys

Question 52

Principle of negative constitution

Answer 52

The US constitution may govern public health and healthcare, but it does not mention health. The constitution may not say anything about healthcare, so we can conduct surveillance, but it is not required. Allows, but does not require, governments to protect public health or healthcare services

Question 53

Tenth amendment

Answer 53

The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people. This defines the relationship between state and federal government. States largely act in the interest of public health using police power

Question 54

Police power

Answer 54

Allows states to take actions to protect the “common good”. Court systems determine the limits of police power- police power is balanced with individual rights

Question 55

Where does the federal government derive its authority? (3)

Answer 55

1. Authority exerted through incentives to the states- shape state policies/laws by providing funding/matched funding
2. Interstate Commerce Clause from the U.S. Constitution
3. Supremacy clause from the U.S. Constitution

Question 56

Interstate commerce clause

Answer 56

Federal government can tax, spend, and regulate interstate commerce. The CDC is responsible for investigating food borne outbreaks across state lines

Question 57

Supremacy clause

Answer 57

Legitimate federal laws are the supreme laws of the land & they preempt/overrule state laws that conflict with national law. Example- labs report data to states, not the federal government. However, as a result of the covid CARES act, labs were required to report covid cases to the federal government. It was a temporary act during the covid health emergency. Additionally, states can request that the CDC come investigate a public health issue if they need more resources

Question 58

Balancing act of public health surveillance

Answer 58

Need to balance information needs with the feasibility of data collection. Surveillance data can be imperfect, but consistency is key

Question 59

Challenges for consistency of data collection (3)

Answer 59

1. Changing interest in identifying disease 2. Methods available to diagnose/identify disease
2. Adopting standard case definitions for disease

Question 60

Approaches to Public Health Surveillance (4)

Answer 60

1. Passive surveillance
2. Active surveillance
3. Sentinel surveillance
4. Syndromic surveillance

Question 61

Sentinel surveillance

Answer 61

Reporting of health events by health professionals who are selected to represent a geographic area or a specific reporting group- can be active or passive

Question 62

Syndromic surveillance

Answer 62

Focuses on one or more symptoms rather than a physician-diagnosed or laboratory-confirmed disease

Question 63

What is a population?

Answer 63

A group of people that share a common characteristic (s). Example- demographics, geography or proximity to spatial features, life event (like people giving birth), catchment population/service by a medical facility. Populations can be closed or fixed

Question 64

Closed (“fixed”) population

Answer 64

Membership defined by life event- new members cannot be added after
population is defined. Membership is permanent and only “exit” is through death. Example- people who had a baby in 2023

Question 65

Open (“dynamic”) population

Answer 65

Membership is based on condition- new members can be added or leave
if condition changes. Membership is transitory. Examples- residents of a city, people aged 20-30 years.

Question 66

Ratio

Answer 66

Division of 2 unrelated numbers

Question 67

3 factors to consider when quantifying disease

Answer 67

1. Disease: The number of people affected
2. Population: The size of the population from which the cases of disease arise
3. Time: The length of time that the population is followed