Estimation of the reproductive number and the serial interval in early phase of the 2009 influenza A/H1N1 pandemic in the USA

Background The United States was the second country to have a major outbreak of novel influenza A/H1N1 in what has become a new pandemic. Appropriate public health responses to this pandemic depend in part on early estimates of key epidemiological parameters of the virus in defined populations. Methods We use a likelihood-based method to estimate the basic reproductive number (R-0) and serial interval using individual level U.S. data from the Centers for Disease Control and Prevention (CDC). We adjust for missing dates of illness and changes in case ascertainment. Using prior estimates for the serial interval we also estimate the reproductive number only. Results Using the raw CDC data, we estimate the reproductive number to be between 2 center dot 2 and 2 center dot 3 and the mean of the serial interval (mu) between 2 center dot 5 and 2 center dot 6 days. After adjustment for increased case ascertainment our estimates change to 1 center dot 7 to 1 center dot 8 for R-0 and 2 center dot 2 to 2 center dot 3 days for mu. In a sensitivity analysis making use of previous estimates of the mean of the serial interval, both for this epidemic (mu = 1 center dot 91 days) and for seasonal influenza (mu = 3 center dot 6 days), we estimate the reproductive number at 1 center dot 5 to 3 center dot 1. Conclusions With adjustments for data imperfections we obtain useful estimates of key epidemiological parameters for the current influenza H1N1 outbreak in the United States. Estimates that adjust for suspected increases in reporting suggest that substantial reductions in the spread of this epidemic may be achievable with aggressive control measures, while sensitivity analyses suggest the possibility that even such measures would have limited effect in reducing total attack rates.