Figure 2

Occurrence of NPC after screening. A total of 131 NPC cases (hatched bar) were detected at different times from 8 months to 15 years after enrollment, 56 from seropositive group (solid bar) and 75 from seronegative group (open bar). Occurrence was analysed in successive 2 years in the first 12 years and the final 3 years in years 13–15 of the study; incidence is expressed in cases per 105 person-year; prediction is calculated as percentage of cases detected from seropositive group; relative risk is calculated as ratio of incidence of seropositive or seronegative group to the corresponding value for the total population.