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≥ 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 modelling 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.