Figure 1.

A, Scenarios for febrile respiratory illness (FRI; defined as consultations at the primary healthcare clinic in which a temperature ≥37.5°C was detected, plus either cough or sore throat) episodes and intervals in time-to-event analyses. In scenario 1 (no subsequent FRI episode), a participant accumulates follow-up time from the first FRI episode (FRI-1) to the date of exit from basic military training (BMT). In scenario 2, follow-up time accrues between the first (FRI-1) and second (FRI-2) episodes (interval A, t₁–t₀), plus a second interval between FRI-2 and the date of exit from BMT (interval B, t₂–t₁). For ≥3 FRI episodes, additional intervals are used (eg, in scenario 3, interval C is t₃–t₂). B, FRI incidence rates by time from enlistment, with splitting of intervals into subintervals by BMT phase. Shown is an example based on a participant from the 17-week BMT intake. Interval A has a subinterval in weeks 11–12 of the BMT course and another in weeks 13–14 (with FRI incidences of 0.74 and 0.63 cases/1000 person-days, respectively). Likewise, interval B has 3 subintervals (weeks 13–14, 15–16, and 17–18). C, Levels within multilevel analysis. AdV, adenovirus; CoV, coronavirus; FluV, influenza virus; hMPV, human metapneumovirus; NA, not applicable; RSV, respiratory syncytial virus.