Estimation of the Serial Interval of Influenza

Background: Estimates of the clinical-onset serial interval of human influenza infection (time between onset of symptoms in an index case and a secondary case) are used to inform public health policy and to construct mathematical models of influenza transmission. We estimate the serial interval of laboratory-confirmed influenza transmission in households. Methods: Index cases were recruited after reporting to a primary healthcare center with symptoms. Members of their households were followed-up with repeated home visits. Results: Assuming a Weibull model and accounting for selection bias inherent in our field study design, we used symptom-onset times from 14 pairs of infector/infectee to estimate a mean serial interval of 3.6 days (95% confidence interval = 2.9-4.3 days), with standard deviation 1.6 days. Conclusion: The household serial interval of influenza may be longer than previously estimated. Studies of the complete serial interval, based on transmission in all community contexts, are a priority.