Epidemiological data sets

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• Epidemiological data sets

Dr. Judy Omumbo Associate Research Scientist, IRI
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Public Health

• The science and art of preventing disease,
  prolonging life, and promoting health through the
  organized efforts of society including
• Monitoring disease and other health-related
  conditions (surveillance) and
• Identifying and quantifying important trends
  (epidemiology).

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Surveillance

• The monitoring of the occurrence of a disease or
  other health-related condition.
• Active surveillance
• Actively searching for people with a disease or
  condition (usually through sampling).
• Passive surveillance
• Dependent on reporting from external sources e.g.
  birth or mortality records

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What information?

• Types of information epidemiologists use for
  monitoring community health status
• Demographic data
• Vital statistics data
• Surveillance data
• Health status and behavioral data
• Socioeconomic data
• Utilization (e.g. health facilities) data
• Time (when?) variables Include
• Long term trends
• Annual occurrence
• Seasonal occurrence
• Weekly occurrence
• Daily occurrence
• Hourly occurrences (outbreak)

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Population / Denominator

• Group of people with a common characteristic like
  age, race, sex or place of residence
• Membership can be permanent or transient

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Defining a case
- Active case detection systems through
structured surveys and sampling - Passive case
detection through surveillance or reporting
systems
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Direct and indirect consequences of Plasmodium
falciparum infection
Sequelae
Infected
Mild clinical disease
Severe clinical disease
Uninfected
Anaemia
Death
Undernutrition
Low birth weight
Increased susceptibility to other infections
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Endemicity
In epidemiology, a disease is said to be endemic
in a population when it is maintained in the
population without the need for external inputs
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Endemicity stable vs. unstable
Endemic areas for P. falciparum malaria
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Spatial data - maps
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Points and polygons
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Grids
Map of the probability of an area having an
infection prevalence gt50 for schistosomiasis
(Brooker et al., 2001 TMIH)
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Disease measures
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Prevalence
Prevalence refers the number of existing cases of
a disease Prevalence No. of cases observed at
time t Total of individuals at time t
Prevalence is a proportion
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Incidence
Incidence is a measure of new cases that develop
in a population during a specified time period
Incidence is a rate
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Measuring incidence
Cumulative Incidence Number of new cases of
disease Number in at risk over a specified
time period Incidence Rate Number of
new cases of disease Person-time of
observation in at risk population
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Prevalence and Incidence
Prevalence depends on incidence rate and duration
of disease
Ways to increase prevalence
Ways to increase prevalence
Ways to increase prevalence
Increased incidence
Increased average duration of disease (decreased
outflow)
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Recovered (immune)
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Non-Infected (non-immune)
Herd immunity
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Infected (non-susceptible)
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Active disease transmission
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Prevalence 16
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Vulnerability, hazard and risk
Risk Hazard X Vulnerability
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Measures Used to Show Statistical Correlation
- Is this disease increasing in incidence? - Is
it more frequent in my area? - Does its incidence
correlate with some suspected cause? - Have
things changed since control measures were
introduced? We answer these questions by
comparing between two or sets of rates
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Relative Risk, Odds Ratio and Attributable Risk
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Relative Risk
The relative risk (RR) is the probability that a
member of an exposed group will develop a disease
relative to the probability that a member of an
unexposed group will develop that same
disease. Relative Risk 1 means no
difference Relative Risk gt 1 shows existence
of an association between exposure and disease
The Relative Risk is usually determined in cohort
studies
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Odds Ratio
The ratio of the odds of a condition or disease
among exposed individuals to the odds of the same
condition or disease in a non-exposed individuals
Ratio 1 indicates no association Ratio gt 1
indicates an association between exposure and
disease
The Odds Ratio is usually determined in case
control studies
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Attributable Risk

Attributable Risk is the proportion of exposed
cases that would not have developed the disease
if they had not been exposed to the risk
factor. It is calculated by subtracting the rate
of disease in a population that does not have a
risk factor from the rate of disease in a
population that has the risk factor
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Added value of climate data analysis

• Stratification of risk areas (determining long
  term averages)
• Program implementation when and where to
  spray (seasonal data)
• Impact evaluation controlling for the effects
  of climate variability (time series data)

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Establishing the usual conditions
- Routine surveillance - Incidence
data - Monitoring confounding factors -
Seasonality
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Calculating anomalies
- An anomaly is the deviation of a variable from
the long-term average. - Anomalies are
created by subtracting long-term average values
from observed data - Values can be positive
(greater than the long-term average or negative
(less than the long-term average)
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Analyzing disease/ climate associations
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