The risk of developing tuberculosis is highest in the first years following infection. This is shown here in the placebo groups of two prospective trials, the BCG trial in the United Kingdom and a preventive therapy trial among contacts of new cases in the United States.
The risk falls rapidly within the first years subsequent to infection when it is about 1% per year and then levels of to about 1 in a 1,000 years.
It is from such and similar observations that it had been estimated that the life-time risk of tuberculosis is about 10% for a young person: half of this risk falls into the first 5 years and the other half into the rest of the life, with a remainder lifespan of about 50 years (5 times 1% plus 50 times 1 per 1,000).
