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Table 5 Definitions and analysis of cases and time at risk

From: Measuring vaccine effectiveness against persistent HPV infections: a comparison of different statistical approaches

Analysis
Method
Case definition Person-time definition
Conditional exact method for comparing two independent Poisson rates using a binomial distribution Two consecutive measurements positive for the same HPV type. The participant is counted as a case if one or more persistent infections occur. Data for two consecutive rounds counts as 1 person-year, each additional consecutive round adds another person-year. After a missing data point counting continues. Counting stops after event or at the end of follow-up.
Cox PH Two consecutive measurements positive for the same HPV type. The participant is counted as a case if one or more persistent infections occur. Data for two consecutive rounds counts as 1 person-year, each additional consecutive round adds another person-year. Person time is censored at event, loss to follow-up or end of follow-up; half-time censoring was applied.
GEE Poisson Two consecutive measurements positive for the same HPV type. Multiple events can occur within one participant. In our study to be counted as next infection after at least one negative round was observed. The number of infections is counted. Data for two consecutive rounds counts as 1 person-year, each additional consecutive round adds another person-year. After a missing data point counting continues. Counting stops at the end of follow-up.
PWP-TT Two consecutive measurements positive for the same HPV type. Multiple events can occur within one participant, in our study to be counted as next infection; at least one negative round should be observed. The number of infections is counted. Analyses are stratified for sequential events. Data for two consecutive rounds counts as 1 person-year. After a missing data point counting continues. Counting stops at the end of follow-up.