Infectious Disease Epidemiology EPIET Introductory Course, 2011 Lazareto, Menorca, Spain Mike...
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Transcript of Infectious Disease Epidemiology EPIET Introductory Course, 2011 Lazareto, Menorca, Spain Mike...
Infectious Disease Epidemiology
EPIET Introductory Course, 2011Lazareto, Menorca, Spain
Mike Catchpole, Bernadette Gergonne, James Stuart, Outi Lyytikäinen, Viviane Bremer
Objective
to review importance and specific concepts of infectious disease epidemiology
Why are infections important?
Which are the most important infections in the EU?
New kids on the block
TSS
EHEC H5N1
HIV
HEV
Lyme
BSE
HCV
Hanta
West Nile
nvCJD
SARS
Chikungunya
H1N1
Infectious vs. non-infectious disease epidemiology
Same • general rationale• terminology (mostly)• study methods• ways of collecting data
– blood samples, questionnaires, registries …
• analysis (statistics)
But some special features• A case may also be a risk factor
- Person with infection can also be source of infection
• People may be immune - Having had an infection or disease could result to resistance to an
infection (immunity)
• A case may be a source without being recognized - Asymptomatic/sub-clinical infections
• There is sometimes a need for urgency - Epidemics may spread fast and require control measures
• Preventive measures (usually) have a good scientific evidence
Case = exposure
• Primary case– Person who brings the disease/infection into a population
• Secondary cases– Persons who are infected by primary case
• Generation of infections (waves)– Secondary cases are infected at about the same time and
consequently tertiary cases• Index case
– First case discovered during an outbreak• Reproductive rate
– Potential of disease to spread in a population
Chain of Transmission
Reservoir and source of infection
• Reservoir of infection – Ecological niche where the infectious agent
survives and multiplies– Person, animal, arthropod, soil, or substance
• Source– Human– Animal– Environment
Transmission routes
Direct transmission Indirect transmission
Mucous to mucous membrane Waterborne
Across placenta Airborne
Transplants, blood Foodborne
Skin to skin Vectorborne
Sneezes, cough Objects/Fomites
Possible outcomes after exposure to an infectious agent
Exposure
No infection Clinical infection Subclinical infection
Death Immunity Carriage Non-immunity
Carriage
Dynamics of disease and infectiousness
Latent period Infectious period Non-infectious period
Incubation period Clinical disease Recovery
Infection
Time
Onset ofsymptoms
Resolutionof symptoms
Relationships between time periods
First patient
Second patient
Incubation period
Latent period Infectious period
Clinical disease
Infection
Incubation period
Latent period Infectious period
Clinical disease
Time
Transmission
Serial interval or generation time
Transmission
Disease occurrence in populations
• Sporadic – Occasional cases occurring at irregular intervals
• Endemic – Continuous occurrence at an expected frequency over a
certain period of time and in a certain geographical location
• Epidemic or outbreak– Occurrence in a community or region of cases of an illness
with a frequency clearly in excess of normal expectancy• Pandemic
– Epidemic involves several countries or continents, affecting a large population
What causes incidence to increase?
Climate change
Megacities
Pollution
Food production
Intensive farming
Antibiotics
Population growth
Migration
Behaviour
Vector proliferation
Vector resistance
Factors influencing disease transmission
Infectivity
Pathogenicity
Virulence
Immunogenicity
Antigenic stability
Reproductive rate• Potential of an infectious disease to spread
in a population• Dependent on 4 factors:
– Probability of transmission in a contact between an infected individual and a susceptible one
– Frequency of contacts in the population - contact patterns in a society
– Duration of infectiousness– Proportion of the population/contacts that are
already immune, not susceptible
Basic reproductive rate (R0)
Basic formula for the actual value: R0 = β * κ * D
• β - risk of transmission per contact (i.e. attack rate)– Condoms, face masks, hand washing β ↓
• κ - average number of contacts per time unit– Isolation, closing schools, public campaigns κ ↓
• D - duration of infectiousness measured by the same time units as κ
– Specific for an infectious disease– Early diagnosis and treatment, screening, contact
tracing D ↓
Basic reproductive rate (R0)
• Average number of individuals directly infected by an infectious case (secondary cases) during her or his entire infectious period, when she or he enters a totally susceptible population
• (1+2+0+1+3+2+1+2+1+2)/10 = 1.5– R0 < 1 - the disease will disappear
– R0 = 1 - the disease will become endemic
– R0 > 1 - there will be an epidemic
Approximate formula for R0
• For childhood diseases: R0 = 1 + L/A
– L - average life span in the population– A - average age at infection
R0 of childhood diseases
Infection/ infectious agent
R0 Average age at infection*
Measles 11-18 4-5
Pertussis 16-18 4-5
Mumps 11-14 6-7
Rubella 6-12 6-10
Diphtheria 4-5 11-14
Polio 6-7 12-17
* In the absence of immunizationSource: Anderson & May, 2006
0
100
200
300
400
500
600
700
800
1 2 3 4 5 6 7n
um
be
r o
f c
as
es
generation
0
100
200
300
400
500
600
700
800
1 2 3 4 5 6 7n
um
be
r of c
as
es
generation
Effective reproduction number R
• If the population is not fully susceptible, the average number of secondary cases is less than Ro. This is the effective reproduction number.
Effective reproduction number R• Epidemic in susceptible
population• Number of susceptibles
starts to decline• Eventually, insufficient
susceptibles to maintain transmission. When each infectious person infects <1 persons, epidemic dies out
Initial phase R = R0
Peak of epidemic R = 1
Changes to R(t) over an epidemic
0
200
400
600
800
1000
1200
0 0.05 0.1 0.15 0.2
time
nu
mb
er
Susceptible
Incident cases
Immune
R=R0
R>1
R=1
R<1
R, threshold for invasion
• If R < 1 – infection cannot invade a population – implications: infection control mechanisms
unnecessary (therefore not cost-effective)
• If R > 1 – on average the pathogen will invade that
population– implications: control measure necessary to
prevent (delay) an epidemic
What control measures might reduce the average number of cases an infectious
individual will generate?
Please talk to the person next to you.Can you think of one or two examples?
Herd immunity
• Level of immunity in a population which prevents epidemics even if some transmission may still occur
• Presence of immune individuals protects those who are not themselves immune
Herd immunity threshold
Minimum proportion (p) of population that needs to be immunized in order to obtain herd immunity
p > 1 - 1/R0
e.g.
if R0 = 3, immunity threshold = 67%
if R0 = 16, immunity threshold = 94%
Important concept for immunization programs and eradication of an infectious disease
Vaccination coverage required for elimination
0.00.10.20.30.40.50.60.70.80.91.0
0 2 4 6 8 10 12 14 16 18 20Basic reproduction number, Ro
Crit
ical
pro
port
ion,
Pc
Pc = 1-1/Ro
rubella measles
Key points
• No question about continuing and increasing threat from infections
• Infectious disease epidemiology is different
• Transmission dynamics important for prevention and control
• Anderson RM & May RM, Infectious Diseases of Humans: Dynamics and Control, 11th ed. 2006
• Giesecke J, Modern Infectious Disease Epidemiology, 2nd ed. 2002
• Barreto ML, Teixeira MG, Carmo EH. Infectious diseases epidemiology. J Epidemiol Community Health 2006; 60; 192-195
• Heymann D, Control of Communicable Diseases Manual, 19th ed. 2008
• McNeill, WH. Plagues and Peoples, 3rd ed. 1998
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