Randomization and masking
Meningococcal vaccination was chosen as the control because meningococcal antibodies offer potential clinical benefits and do not impact HPV outcomes. Participants were randomized to (1) nonavalent HPV vaccination (Gardasil-9), (2) bivalent HPV vaccination (Cervarix) or (3) meningococcal (control) vaccination. Following randomization, a single dose of each vaccine was administered.An unblinded statistical analyst generated the randomization sequence using SAS v.9.4. Randomization was stratified by site, using a fixed block size of 15 and a 1:1:1 allocation. Blinded study assignment was implemented via 