The risk of tuberculous infection in Sweden.

Large sections of the population of Sweden were tuberculin tested during the 1940s prior to BCG-vaccination. This material has been used to estimate the risk of tuberculous infection and its trends with calendar year and age. An analysis of the results of approximately 200,000 tuberculin tests in military recruits and schoolchildren was made, using the method developed in the Tuberculosis Surveillance Research Unit. The risk of infection was estimated to be 2-8 per cent at age 10 in 1935, the rate decreasing by 9-0 per cent annually. There was also a substantial increase in the risk with each year of age, amounting to 8-6 per cent for each year of age. Separate analyses of the risk of infection and its trend with time for each of the individual 24 counties and the city of Stockholm, showed considerable variations in the level of the risk between the different areas. These are related to variations in tuberculosis in cattle in a further report. In seven areas information was available from earlier surveys in schoolchildren or recruits, using the same method of testing. In these areas separate estimates of the calendar trend of the risk of infection and the age trend have been made. The decrease in the risk with calendar year and the increase with age seem to apply in each of these counties, as in the whole of Sweden. The reliability of the findings was tested by comparing the estimated risk of infection for the whole of Sweden and the tuberculous meningitis mortality rate in children aged 0-4 years between 1925 and 1935. There was a similar trend in the risk and in the mortality rate. Between 1935 and 1940 the decline in the tuberculous meningitis mortality rate became steeper, and amounted to about 11-5 per cent annually between 1935 and 1945. Special measures for the protection of uninfected but exposed children became increasingly common during the 1920s and 1930s, and these were supplemented with BCG-vaccination at a later date. From July 1939 all milk sold for human consumption had to be pasteurised. It is suggested that all these factors may have contributed to the more rapid decrease in the tuberculous meningitis mortality in young children after about 1938.