Patients complete a short questionnaire regarding risk factors for HPV infection and undergo collection of blood samples for testing HPV16 and HPV18 levels on study.

Undergo collection of blood samples

Negative/positive status will be assessed using the standard predefined cutoffs established at the testing laboratory.

Will report the geometric mean titer levels and their 95% confidence intervals at years 1, 2, 3, 4, 7, 9, 11, 14, 16, 18, and 20 for the 1-dose, 2-dose and 3-dose groups and for both human papillomavirus (HPV)-16 and HPV-18 antibodies. Will estimate these quantities using a weighted linear mixed models with log-titer level as the dependent variable and year as the independent variable, adjusting for laboratory batch and including a random-effects for participant and, when needed, participant: study year. For the three-dose analysis, will also include a covariate for study group. Moreover, will model the 19-year trend (e.g. excluding peak titers at year 1) by including time as a single continuous variable (i.e. linear effect) and then model time flexibly using a natural spline to assess a potentially non-linear relationship.

For each dose regimen and for HPV 16, HPV 18, and a composite endpoint of either HPV type, will estimate the proportion of individuals who serorevert (i.e., become antibody negative by the assay cutoff) in years 1, 2, 3, 4, 7, 9, 11, 14, 16, 18, and 20. Will also report 95% confidence intervals for these estimates. Again, will follow the approach described in our previous work. Note, will consider both standard and updated cutoffs and will use batch-adjusted levels. Can obtain the relative risk for seroreversion and its 95% confidence interval using score tests. Will then use generalized linear mixed models with seroreversion as the outcome and all other parameters similar to above to assess trends over time.