12,630 laboratory-confirmed pH1N1 cases were captured by the South African central registry during 2009. The overall demographic, spatial and temporal distribution of these cases has been previously described [23] . Data on date of symptom onset was available for 758 (6%) cases and date of specimen collection for 12,500 (99%) cases. The first case reported illness onset of June 12, 2009 The lag-time between symptom onset and specimen collection was significantly associated with the provincial location of specimen collection, as well as the collection of a specimen on a weekend day (Table 1) . We used these two covariates in the multiple-imputation to predict the date of symptom onset where missing for all cases (Figure 1 ). Other available variables, including date of specimen collection (period during the epidemic), patient age and sex were not significantly associated with the lag-time between symptom onset and specimen collection and, therefore, not included in the final model. Analyses to simultaneously estimate initial R t and serial interval, and estimate initial R t given a known serial interval, were performed over the exponential growth phase of the epidemic from June 21 to August 3, 2009.