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

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1 2 3 4 5 6 7n

um

be

r o

f c

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generation

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1 2 3 4 5 6 7n

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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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