Force of infection by age and sex.

The event is inferred seroconversion based either on fold change, or change from below to above the absolute threshold. The person-years includes time between donations, and also from an inferred negative value to the first donation. In order to estimate the force of infection, periods after the first positive result for each person are not included, on the basis of not being at risk. People with missing age or sex are not included. Note that the overall rate cannot be obtained by simple division of the events and person-years between donations, because the exact times of the events were not observed.