Seasonal Influenza in the United States, France, and Australia: Transmission and prospects for control.

Recurrent epidemics of influenza are observed seasonally around the world with considerable health and economic consequences. A key quantity for the control of infectious diseases is the reproduction number, which measures the transmissibility of a pathogen and determines the magnitude of public health interventions necessary to control epidemics. Here we applied a simple epidemic model to weekly indicators of influenza mortality to estimate the reproduction numbers of seasonal influenza epidemics spanning 3 decades in the United States, France, and Australia. We found similar distributions of reproduction number estimates in the three countries, with mean value 1.3 and important year-to-year variability (standard error 0.09). Estimates derived from two different mortality indicators (pneumonia and influenza excess deaths and influenza-specific deaths) were in close agreement for the United States (correlation =0.61, P<0.001) and France (correlation=0.79, P<0.001), but not Australia. Interestingly, high prevalence of A/H3N2 influenza viruses was associated with high transmission seasons (P=0.006), while B viruses were more prevalent in low transmission seasons (P=0.004).

The current vaccination strategy targeted at people at highest risk of severe disease outcome is suboptimal because current vaccines are poorly immunogenic in these population groups.  Our results suggest that interrupting transmission of seasonal influenza would require a relatively high vaccination coverage (>70%) in healthy individuals who respond well to vaccine, in addition to periodic re-vaccination due to evolving viral antigens and waning population immunity.

References:

• G. Chowell, M.A. Miller, C. Viboud. Seasonal influenza in the United States, France and Australia: Transmission and prospects for control. Epidemiology and Infection 2007 Jul 18;:1-13(pdf)
• M. Nuno, G. Chowell, X. Wang, C. Castillo-Chavez. On the role of cross-immunity and vaccination in the survival of less-fit flu strains.Theor. Pop. Biol. 71, 20-29 (2007) (pdf)

 

Fig 1 :Time series of weekly number of P&I and influenza-specific deaths per 100,000 in 3 countries(France, United States and Australia, blue curve). The red dashed line indicates the baseline mirroring the expected level of P&I mortality in the absence of influenza epidemic activity.

Fig 2 :Boxplots of the reproduction number (Rp) of influenza seasons (1972-1997) in the United States, France, and Australia. The boxes have lines at the 25, 50 and 75 percentiles. The whiskers show the extent of the rest of the data extending to a maximum of 1.5 times the interquartile range. Points outside the ends of the whiskers are indicated with a + symbol.