DIPHTHERIA IMMUNITY IN UK BLOOD-DONORS

Immunity to diphtheria was determined in serum samples from 1000 UK-bo rn blood donors at the North London Blood Transfusion Centre during a three-month period in 1993; 125 women and 125 men were stratified in 1 0-year age groups, from 20 to 59. A tissue (vero cell)-culture toxin-n eutralisation assay was used to measure serum diphtheria antitoxin con centrations. According to internationally accepted definitions (antito xin <0.01 IU/mL=susceptibility, 0.01-0.09 IU/mL=basic protection, and greater than or equal to 0.1 IU/mL=full protection), 37.6% of donors w ere susceptible to diphtheria, 31.5% had basic protection, and 30.9% w ere fully protected. Log-linear modeling of the influence of age and s ex on population immunity showed a significant trend (p<0.001) of decr easing immunity with increasing age: 25.2% of donors aged 20-29 were s usceptible compared with 52.8% of those aged 50-59. There was a small sex effect (p=0.052); similar proportions of men and women were suscep tible, but fewer women had full protection. There was no age-sex inter action on immunity (p=0.454). Our results suggest that booster immunis ation of adults is necessary to increase herd immunity of the adult po pulation.