Transcript Infectious Disease Epidemiology

Infectious Disease Epidemiology
EPIET Introductory Course, 2011
Lazareto, 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
BSE
TSS
EHEC
Lyme
HIV
nvCJD
HEV
HCV
Hanta
Chikungunya
H5N1
West Nile
SARS
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
Person-toperson
transmission
Portal of
exit
Susceptible
host
Agent
Portal
of entry
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
Death
Clinical infection
Immunity
Subclinical infection
Carriage
Carriage
Non-immunity
Dynamics of disease and infectiousness
Latent period
Infectious period
Incubation period
Infection
Clinical disease
Onset of
symptoms
Non-infectious period
Recovery
Resolution
of symptoms
Time
Relationships between time periods
Transmission
Second patient
Latent period
Infectious period
Incubation period
First patient
Transmission
Latent period
Incubation period
Infection
Clinical disease
Infectious period
Clinical disease
Serial interval
or generation time
Time
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?
Reservoir
Person-toperson
transmission
Portal of
exit
Susceptible
host
Agent
Portal
of entry
Factors influencing disease transmission
Climate change
Megacities
Pollution
Vector proliferation
Environment
Vector resistance
Vectors
Animals
Food production
Agent
Intensive farming
Antibiotics
Infectivity
Pathogenicity
Virulence
Immunogenicity
Population growth
Migration
Behaviour
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
Measles
11-18
Average age at
infection*
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
Source: Anderson & May, 2006
R0
* In the absence of immunization
800
number of cases
700
600
500
400
300
200
100
0
1
2
3
4
generation
5
6
7
800
number of cases
700
600
500
400
300
200
100
0
1
2
3
4
generation
5
6
7
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
1200
number
1000
800
600
Susceptible
Incident cases
Immune
R=1
400
R<1
R>1
R=R0
200
0
0
0.05
0.1
time
0.15
0.2
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
Critical proportion, Pc
Pc = 1-1/Ro
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
rubella
0
2
4
6
8
10
12
14
16
Basic reproduction number, Ro
measles
18
20
Key points
• No question about continuing and increasing
threat from infections
• Infectious disease epidemiology is different
• Transmission dynamics important for
prevention and control
If you enjoyed this…
• 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; 192195
• Heymann D, Control of Communicable Diseases Manual, 19th
ed. 2008
• McNeill, WH. Plagues and Peoples, 3rd ed. 1998