Evaluation of seasonal waveforms. The seasonality of disease occurrence was evaluated through calculation of autocorrelations for weekly case counts [2] . As yearly periodicity was observed with IPD, we estimated seasonal trends in disease occurrence of IPD and influenza A and B using Poisson and negative binomial regression models that incorporated sine and cosine oscillators, with 52-wk (annual) frequencies (i.e., incorporated fast Fourier transforms) [2, 11] . Using these parameters, the expression for the expected number of case counts for a given week, E(Y) is given by: E(Y) = e a + b1Nsin(2p(week/52))+b2Ncos(2p(week/52)) . Accordingly, the phase-shift of the composite waveform generated by combining sine and cosine components can be approximated as tan 21 (b 1 /b 2 ) and can be used to estimate the timing of peak disease incidence [11] . Similarly, the amplitude (i.e., the maximum disease incidence) can be approximated as