Immunity to diphtheria in Helsinki in 1975.

To determine the status of the population's immunity to diphtheria, the Schick test was performed on 489 and the diphtheria antitoxin level determined in 404 hospital patients, all residents of Helsinki. A low ("unprotective", less than 0.01 IU/ml) antitoxin level was found in not more than 5% among persons under 20. The highest percentage, 48%, was found in the 50--59 years age group. A low antitoxin level was more common among women than men in those over 40 years of age. Taking into consideration the age structure of the population of Helsinki in 1975, 23% of the population had an antitoxin level less than 0.01 IU/ml. The percentage of Schick-positives among different age groups was almost the same as the percentage of low antitoxin levels. Of 100 serum samples from military recruits coming from different regions of the country, 15% had an antitoxin level below 0.01 IU/ml.