1. Communicating Complication Risks
from Influenza A (H1N1)
Tom´s J. Arag´n, MD, DrPH
a
o
Health Officer, City & County of San Francisco
Director, Population Health Division (PHD)
San Francisco Department of Public Health
Adjunct Faculty, Division of Epidemiology
UC Berkeley School of Public Health
Email: tomas.aragon@sfdph.org
February 7, 2014
Tom´s J. Arag´n, MD, DrPH (SFDPH)
a
o
Influenza Risk Communication
February 7, 2014
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2. Individual risk of complication vs number of complications
For simplicity, we focus on complications (e.g., death)
Types of individual risks
Risk of complication given infectiona
Risk of complication given influenza seasonb
a
b
driven by individual susceptibility
driven by individual susceptibility and prevalence of circulating subtype
It is important to distinguish the difference between the types of individual
risk and the number of cases in a season. You can have low individual
risks but large number of cases if there are many susceptibles. However,
the large number of complications may give the perception that the
individual risks are large.
In the next slide, we use the epidemic equation to help understand the
components that go into individual risks and number of complications.
Tom´s J. Arag´n, MD, DrPH (SFDPH)
a
o
Influenza Risk Communication
February 7, 2014
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3. Number of complications from influenza during a season
Epidemic equation
Number of complications = Pr(Complication) × NS
= c × P × p × Pr(D | I) × Pr(C | D) × NS
c = contact rate to potential infectious influenza cases
P = prevalence of influenza in the community*
p = transmission probability given contact with influenza case*
I = infection (transmission)
D = disease (symptomatic flu)**
C = complication (e.g., death)**
NS = number of susceptibles in population
* drivers for transmission of novel strains (i.e., spread)
** function of host predisposition (e.g., obesity) and novel strain virulence
Tom´s J. Arag´n, MD, DrPH (SFDPH)
a
o
Influenza Risk Communication
February 7, 2014
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4. Components of transmission probability (p)
Infectiousness of source
Susceptibility of host
Infection control
Tom´s J. Arag´n, MD, DrPH (SFDPH)
a
o
Influenza Risk Communication
February 7, 2014
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5. What explains increase in number of H1N1 deaths in
2013-2014 season?
Assuming no changes in the virulence of H1N1 (i.e., chance of
complication given infection), the only plausible explanation would be a
combination of the following:
Increase in circulating H1N1 (P , prevalence)
Large susceptible population (NS )
In an influenza pandemic, not everyone becomes infected with the pandemic
strain. In a non-pandemic season, the risk of infection is about 10%. This means
a large proportion of the population remained susceptible (NS ) to H1N1
assuming they did not get natural H1N1 infection (a reasonable chance), or did
not get vaccinated. Since 2009, H1N1 and H3N2 have been competing, and
which dominates will depend on available susceptibles. Finally, even with a stable
and low individual risk of complications (like H1N1), we can see many deaths if
the number of sususceptibles is large. Nonetheless, this gives the perception that
H1N1 may be worse (more virulent).
Tom´s J. Arag´n, MD, DrPH (SFDPH)
a
o
Influenza Risk Communication
February 7, 2014
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