Communicating Complication Risksfrom Influenza A (H1N1)

Tomas J. Aragon, MD, DrPH

Health Officer, City & County of San FranciscoDirector, Population Health Division (PHD)San Francisco Department of Public Health

Adjunct Faculty, Division of EpidemiologyUC Berkeley School of Public Health

Email: tomas.aragon@sfdph.org

February 7, 2014

Tomas J. Aragon, MD, DrPH (SFDPH) Influenza Risk Communication February 7, 2014 1 / 5

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

adriven by individual susceptibilitybdriven by individual susceptibility and prevalence of circulating subtype

It is important to distinguish the difference between the types of individualrisk and the number of cases in a season. You can have low individualrisks but large number of cases if there are many susceptibles. However,the large number of complications may give the perception that theindividual risks are large.

In the next slide, we use the epidemic equation to help understand thecomponents that go into individual risks and number of complications.

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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 casesP = 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

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Components of transmission probability (p)

Infectiousness of source

Susceptibility of host

Infection control

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What explains increase in number of H1N1 deaths in2013-2014 season?

Assuming no changes in the virulence of H1N1 (i.e., chance ofcomplication given infection), the only plausible explanation would be acombination 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).

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