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# Communicating Complication Risks from Influenza A (H1N1)

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### Communicating Complication Risks from Influenza A (H1N1)

1. 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 1/5
2. 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 2/5
3. 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 3/5
4. 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 4/5
5. 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 5/5