Some studies suggest that having a concussion during athletic activities increases the risk (as expressed through ‘rate ratios’) of subsequent concussion,1 potentially because of incomplete recovery after brain trauma. However, these analyses are flawed for causal interpretation because participants with one concussion likely have different inherent risks compared with those with no concussions.2 The issue is clinically important because patients may decide to return to their sport if their risk of concussion is simply due to the body type, sport and their personal style of play (ie, unchanged compared with prior to the injury) versus being at twice or thrice the risk of the first concussion because the concussion has caused permanent damage.

To address whether a first concussion causally increases the risk of subsequent concussion, we analysed data from Cirque du Soleil (CdS) artists who had two or more concussions.2 3 We provide methodological details in the online supplementary appendix. The analysis in this paper is a matched analysis comparing risk of first concussion to risk of second concussion in the same individuals, thus controlling for different inherent risks similar to case-cross-over designs.4 These matched strategies assume that the increased risk for a …