DIPHTHERIA IMMUNITY IN UK BLOOD-DONORS

Immunity to diphtheria was determined in serum samples from 1000 UK-born blood donors at the North London Blood Transfusion Centre during a three-month period in 1993; 125 women and 125 men were stratified in 10-year age groups, from 20 to 59. A tissue (vero cell)-culture toxin-neutralisation assay was used to measure serum diphtheria antitoxin concentrations. According to internationally accepted definitions (antitoxin <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 were susceptible to diphtheria, 31.5% had basic protection, and 30.9% were fully protected. Log-linear modeling of the influence of age and sex on population immunity showed a significant trend (p<0.001) of decreasing immunity with increasing age: 25.2% of donors aged 20-29 were susceptible 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 susceptible, but fewer women had full protection. There was no age-sex interaction on immunity (p=0.454). Our results suggest that booster immunisation of adults is necessary to increase herd immunity of the adult population.