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these reasons, continuing scientific and public health attention to the spread of this novel virus is essential.

As officials prepare and plan for the growth of this pandemic, estimates of epidemiological parameters are needed to mount an effective response. Decisions about the degree of mitigation that is warranted – and public compliance with efforts to reduce transmission – depend in part on estimates of individual and population risk, as measured in part by the frequency of severe and fatal illness. Knowledge of the serial interval and basic reproductive number are crucial for understanding the dynamics of any infectious disease, and these should be reevaluated as the pandemic progresses in space and time (Fraser et al. 2004). The basic reproductive number R0 is defined as the average number of secondary cases per typical case in an otherwise susceptible population, and is a special case of the more general reproductive number, which may be measured even after some of the population is immune. R0 quantifies the transmissibility of an infection: the higher the R0, the more difficult it is to control. The distribution of the serial interval, the time between infections in consecutive generations, determines, along with R0, the rate at which an epidemic grows. Estimates of these quantities characterize the rates of epidemic growth and inform recommendations for control measures; ongoing estimates of the reproductive number as control measures are introduced can be used to estimate the impact of control measures. Previous modeling work has stated that a reproductive number exceeding two for influenza would make it unlikely that even stringent control measures could halt the growth of an influenza pandemic (Hallroan et al., 2008).

Prior work has placed estimates for the serial interval of seasonal influenza at 3.6 days (Cowling et al., 2009) with a SD of 1.6 days. Other work has estimated that the serial interval is between 2.8 and 3.3 days (White and Pagano, 2008a). Analysis of linked cases of novel A/H1N1 in Spain yields an estimate of a mean of 3.5 days with a range from 1 to 6 days (Surveillance Group for New Influenza A (H1N1) Virus Investigation and Control in Spain, 2009). Fraser et al. (2009) estimate the mean of the serial interval to be 1.91 days for the completed outbreak of respiratory infection in La Gloria, Mexico, which may have resulted from the novel H1N1 strain. There have been many attempts made to estimate the reproductive number. Fraser et al. (2009) estimate the reproductive number to be in the range of 1.4–1.6 for La Gloria but acknowledge the preliminary nature of their estimate. For the fall wave of the 1918 pandemic, others have estimated the basic reproductive number to be approximately 1.8 for UK cities (Ferguson et al., 2005), 2.0 for U.S. cities (Mills et al., 2004), 1.34–3.21 (depending on the setting) (White and Pagano, 2008a), and 1.2–1.5 (Andreasen et al., 2008). Additionally Andreasen et al. (2008) estimate, in contrast, that the reproductive number in the 1918 summer wave was between 2.0 and 5.4.

In what follows we employ a likelihood-based method previously introduced (White and Pagano, 2008a,b) to simultaneously estimate the basic reproductive number and the serial interval. We make use of data from the Centers for Disease



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