Background The effect of contact reduction measures on infectious disease transmission can only be assessed indirectly and with considerable delay . However, individual social contact data and population mobility data can offer near real-time proxy information . Aim To compare social contact data and population mobility data with respect to their ability to predict transmission dynamics during the first wave of the SARS-CoV-2 pandemic in Germany . Methods We quantified the change in social contact patterns derived from self-reported contact survey data collected by the German COVIMOD study from 04/2020-06/2020 (compared to the pre-pandemic period), and estimated the percentage mean reduction in the effective reproduction number R (t) over time . We compared these results to the ones based on R (t) estimates from open-source mobility data and to R (t) values provided by the German Public Health Institute . Results We observed the largest reduction in social contacts (90%, compared to pre-pandemic data) in late April corresponding to the strictest contacts reduction measures . Thereafter, the reduction in contacts dropped continuously to a minimum of 73% in late June . R (t) estimates based on social contacts underestimated measured R (t) values slightly in the time of strictest contact reduction measures but predicted R (t) well thereafter . R (t) estimates based on mobility data overestimated R (t) considerably throughout the study . Conclusions R (t) prediction accuracy based on contact survey data was superior to the one based on population mobility data, indicating that measuring changes in mobility alone is not sufficient for understanding changes in transmission dynamics triggered by public health measures.