Risk assessment of events involving infectious diseases

Preben Aavitsland

Department of Infectious Disease Epidemiology

Norwegian Institute of Public Health

at

EpiTrain V, Vilnius, October 24 2007

Contents

• Background

• Risk assessment– what, why and when

• Risk assessment tool– EpiRisk

• Example

What is risk assessment of events?

• Pragmatic: The determination of the potential impact of an event by assessing the likelihood that the event will worsen and the impact if it does

• Risk = likelihood * impact

• Risk analysis = risk assessment + risk management + risk communication

Why do risk assessment of events?

• Basis for risk communication

• Basis for risk management– Appropriate use of resources

– Level of countermeasures (cost vs potential impact averted)

• Basis for decision to notify

• Guides information gathering

When do risk assessment of events?

• Whenever there is a communicable disease ”event” (outbreak, threat, etc)

• As early as possible– Gather information

• Update the assessment– Whenever new important information becomes

available

– Anyway after some time

How to do risk assessment?

• We already do it automatically and unsystematically

• Use a systematic approach– EpiNorth Risk Assessment of Events Tool (”EpiRisk”)

– Pilot version for testing by EpiTrain-participants

– Qualitative assessment

• Quantitative assessments usually look falsely precise

• Document the findings

1. Event description

• Disease / syndrome– Outbreak? Threat?

Disease?

• Agent / cause– What is the agent or

cause?

• Time– When did it happen?

• Place– Where dit it happen?

• Person– Who were involved

(gender, age, profession, other characteristics…)

2. Rapid disease overview

• Chain of transmission

• Time periods– Incubation period

– Period of infectiousness

– Natural history

• Diagnosis and treatment

• General epidemiological situation

The chain of transmission

Agent

Susceptible host

Source

Portal of entry

Portal of exit

Route of transmission

Six links of the chain

Agent: virus, bacteria, fungi, protozoa, helminths, ectoparasitesSource (reservoir): Humans and aminals (symptomatic,

incubating, convalescent, asymptomatic carrier) insects, environment (water, soil)

Portal of exit: Respiratory tract (nose, mouth), gastrointestinal tract (anus), genitourinary tract (urethra and cervix), skin, mucosa, placenta.

Route of transmission: Direct (contact or droplet), indirect (vehicle, vector), airborne

Portal of entry: Respiratory tract (nose, mouth), gastrointestinal tract (anus), genitourinary tract (urethra and cervix), skin, mucosa.

Susceptible host: anyone who is not immune

3. Impact of event

• Assess the current and future (or as yet undetected) impact in terms of– Severity

– Cost

– Public interest

• Give a summary measure

Severity of event

Now• Number of cases

(incidence rate)• Severity of disease;

case fatality• Age of patients• (DALYs lost)

0 – 3 points

Future• Assess the as yet

undetected part of the event and the development of the event or the (with the same indicators) in the abscence of preventive measures

0 – 3 points

Cost of event

Now• Cost incurred for

treatment• Cost incurred for sick

leave• Cost incurred for

sequelae

0 – 3 points

Future• Assess the as yet

undetected part of the event and the development of the event or the (with the same indicators) in the abscence of preventive measures

0 – 3 points

Public interest in event

Now• Media interest• Public interest,

especially parents, children

• Politicians’ interest• Historical context

0 – 3 points

Future• Assess the as yet

undetected part of the event and the development of the event or the (with the same indicators) in the abscence of preventive measures

0 – 3 points

Sum of impact

Now

Severity 0 – 3 points

Cost 0 – 3 points

Interest 0 – 3 points

Sum 0 – 9 points

Future

Severity 0 – 3 points

Cost 0 – 3 points

Interest 0 – 3 points

Sum 0 – 9 points

4. Potential for spread

• Assess the potential using– Experience from previous events

– Expert judgement

– Analysis of the chain of transmission for this event

– Other information

• Avoid assessing impact now– You have already assessed impact

The chain of transmission

Agent

Susceptible host

Source

Portal of entry

Portal of exit

Route of transmission

Analyse the chain

• Every disease has its chain of transmission

• The disease can spread only if all six links are present

• Analyse the chain for this event (disease + circumstances)– Who are the sources? Infectious periods?– How is it spread?– Are there susceptible hosts?

• Is further spread likely?

Likelihood of spread

Conclude: What is the likelihood that the event will spread in the abscence of preventive measures?

• Very low likelihood (It will not happen)

• Low likelihood (It will probably not happen)

• High likelihood (It may happen)

• Very high likelihood (It will happen)

5. Preventability

• Assess using– Experience from previous events

– Expert judgement

– Analysis of the chain of transmission for this event

– Other information

• Primary prevention– Stop event from happening

– Reduces likelihood

• Secondary prevention– Stop event from having consequences

– Reduces impact

The chain of transmission

Agent

Susceptible host

Source

Portal of entry

Portal of exit

Route of transmission

Possible interventions

Agent: antimicrobial treatment, disinfectionSource (reservoir): behavioural change, isolationPortal of exit: mask, condom, bandageRoute of transmission: hand hygiene,

environmental hygiene, vector control, food and water control, sanitation

Portal of entry: mask, gloves, gown, glasses, condom

Susceptible host: vaccination, passive immunisation, chemoprophylaxis, nutrition, behavioural change

Analyse the chain

• Which link(s) can be cut to stop transmission?

• Analyse the chain for this event (disease + circumstances), taking each link in turn

• Conclude: Is it possible to reduce the risk?– Primary: Reduce likelihood of spread

– Secondary: Reduce impact if it spread

6. Conclude: Current and future risk

EpiNorth Risk Assessment of Events Tool (EpiRisk)

Impact

Very low

(0-2 p)

Low

(3-4 p)

High

(5-6 p)

Very high

(7-9 p)

Situation now

Likelihood of future spread

Very low

Low

High

Very high High riskMedium ris

kLow ris

k

Example

• The infectious disease doctor at the local hospital calls you and reports that she over the last couple of weeks have seen three students with acute hepatitis B. They had the same address and they seemed to know each other. She had noted injection scars in one of them.

1. Event description

• Event: Outbreak of hepatitis B

• Cause: Unknown, maybe shared injection equipment or sex

• Time: November 2007

• Place: Latgale University Students’ Dormitory

• Person: Five students in the dormitory, both genders

2. Rapid disease overview

HBV

Almost all (no vaccination)

Carriers (asymptomatic)

Genitourinary tract, through

skin

Genitourinary tract, blood

Direct contact(sex, injection)

2. Rapid disease overview

• Time periods– Incubation period: 45 – 180 days

– Period of communicability: Weeks before symptom onset and for life (for carriers)

– Natural history: Mostly asymptomatic, less than 10% of adults become carriers, then increased lifetime risk of liver disease

• Diagnosis and treatment:– Blood test for antigen and antibodies

• General epidemiological situation:– Few domestic cases, many immigrant carriers

3. Impact of event

Now

Severity 1 point

Cost 1 point

Interest 2 points

Sum 4 points

Future

Severity 2 points

Cost 1 point

Interest 3 points

Sum 6 points

4. Potential for spread

HBV

Almost all (no vaccination)

Carriers and newly infected (asymptomatic)

Genitourinary tract, through

skin

Genitourinary tract, blood

Direct contact(sex, injection)

High likelihood

5. Preventability

HBV

Almost all (no vaccination)

Carriers (asymptomatic)

Genitourinary tract, through

skin

Genitourinary tract, blood

Direct contact(sex, injection)

Vaccinate all students in dormitory

Provide condoms and needles/syringes

Find the infected and carriers and stimulate to behavioural change

6. Conclude: Current and future risk

EpiNorth Risk Assessment of Events Tool (EpiRisk)

Impact

Very low

(0-2 p)

Low

(3-4 p)

High

(5-6 p)

Very high

(7-9 p)

Situation now X

Likelihood of future spread

Very low X

Low

High X

Very high

Conclusion

• Risk assessment is important in our work

• We should do it more systematically and documented

• The EpiRisk tool has six steps and may be useful– Event description

– Rapid disease overview

– Impact of event

– Potential for spread

– Preventability

– Conclusion

• Try it!

Exercise

• The groups do the story that corresponds to their number

• Prepare a presentation in template

• If time, do the other stories also on paper (follow the sequence 1-2-3-4-5-1-2…)

Event number 1

• The school health nurse calls you. She has just received information that the new cook in the school kitchen is a former intravenous drug user with hepatitis C. She has heard rumours that a student who is now away from school, got hepatitis C last month. She wants you to remove the new cook.

Event number 2

• The local medical doctor in a small farming community calls you. He tells you that the local newspaper has reported rumours that two young African asylum seekers in the refugee’s shelter in the area are HIV positive and have been observed dancing in the pub with local girls. The mayor wants the Africans removed from the community. He fears an HIV epidemic.

Event number 3

• At the national surveillance centre you receive reports of around 100 shigellosis cases from several cities in one region of the country during just one week. Usually there are only 100 cases a year. You believe that there may be a foodborne outbreak related to a dairy product. Journalists have started calling you to get comments.

Event number 4

• Five hundred teenaged folk singers are gathered in a village outside Vilnius for the annual song festival. They live in tents. On Friday evening, a young girl is admitted to hospital with meningitis symptoms. The next morning the hospital calls you to inform that it is meningococcal disease, group C. They also tells you that another, possible case from the festival has been admitted the same morning.

Event number 5

• The local doctor tells you that he has detected an outbreak of measles involving five children age 1-6 years in a group of traveller families (Rom) that has just arrived in their vans and settled in his city (10 000 inhabitants). They are some 75 people. He fears further spread, and informs you that the vaccination coverage in the local population is around 80%.