Indeed, there have been additional "2009 H1N1" neuraminidase sequences since the submission of this manuscript for review. Between April 29 th and May 8 th , 45 new sequences became available. Overall, 3 mutations occur in multiple (S95G, V106I and N248D) while 2 mutations are restricted to single patient virus isolates (V83M, V241I), so far. We have included a section about these mutations identified in new patient sequences and mapped them to the structure. As discussed in the main text, the intra-strain variation is typically found on the surface and at positions expected to be variable as judged by the conservation analysis among all NA subtypes. Interestingly, one of the mutations among patients (N248D) is critically affecting one of the antibody binding sites.