Determination of 
Participants had a nasopharyngeal swab taken at day 0 (immediately prior to LAIV receipt), days 7 and 21, using flocked swabs (FLOQSwabs) stored in RNAprotect (Qiagen). Samples were processed within 4 hours of collection and stored at −70°C until further processing. DNA was extracted from 200 μL of RNAprotect using the AGOWA Mag Mini DNA extraction kit (LGC Genomics) in combination with phenol-bead beating as previously described [Study profile: 320 participants were randomized 2:1 to the LAIV (day 0 vaccine) or control (day 21 vaccine) group. All participants (colonization study group) had 