Given our findings, the overall transmissibility of pH1N1 in South African during 2009 was more similar to that of seasonal influenza strains than the 1918 pandemic, and comparable to lower end estimates of the latter pandemics. However, by showing variation in transmissibility with time, we demonstrate that shortly after introduction of pH1N1 into the country, transmission of the virus reached anR R t of 2.9, resulting in exponential growth of the local epidemic and widespread illness. Nonetheless, we show that after a period of less than 2 months of heightened transmission,R R t dropped below 1, corresponding to a decline in the incidence of new cases; likely a result of a combination of herd immunity, There are several limitations in this analysis which merit discussion. First, we assume that all cases are known and reported. It has been shown previously that, if cases are not reported, this may bias estimates generated using this method [39] . If the proportion of cases reported remains consistent over the study, then the estimates of transmissibility will not be biased; however, if the reporting fraction varies through time, then biased estimates of the reproductive number and serial interval may result. Likewise, variation in case ascertainment with time may bias our estimates of the temporal variation of R t . Generally higher reporting rates may be anticipated in the early phase, with reporting fatigue later becoming a factor. Secondly, data for this study are derived from laboratory-based surveillance data from several regions across South Africa; a large and diverse country. Our findings do not incorporate heterogeneities (such as spatial and demographic differences) that likely exist in transmission patterns, or assess the degree to which these impact aggregate measures of initial R t . Methodologies that incorporate heterogeneities inherent in public health data warrant further study.