Immunity to diphtheria in young British adults.

Of 906 recruits aged between 16 and 20 years who were Schick-tested upon entry to the Royal Air Force, 775 were tested for circulating Corynebacterium diphtheriae antitioxin by means of an indirect haemagglutination (IHA) method. Of the total population, 95.7% were Schick-test negative, 3.5% were Schick-test positive and 0.8% gave pseudo-reactions. Of those tested by the IHA test, 37.7% were regarded as immune, 47.0% as 'immune-susceptible' and 15.3% as non-immune. Discrepancies between the two tests were discovered. Of those persons found to be Schick-test positive, 34.6% possessed circulating antitoxin; of those Schick-test negative, 13.5% were regarded as non-immune. A history of diphtheria immunisation in childhood was found to be a poor predictor of immunity. A protocol for selective diphtheria immunisation of adults is proposed.