BCG and vole bacillus vaccination in adolescence and mortality from leukaemia.

In total, 28 deaths from leukaemia are known to have occurred among the 54,239 participants in the Medical Research Council tuberculosis vaccines trial (initially aged about 14 years), from its beginning in 1950 to the end of 1979. There is evidence that this total is likely to be very nearly complete. During the entire period (average 27.1 years) the leukaemia mortality per million person-years was 25 in the BCG vaccinated group, 20 in the vole-bacillus vaccinated group, and 21 in the randomly-allocated unvaccinated (control) group. Although there was neither a benefit nor a disadvantage from vaccination over the whole period, detailed figures suggested that there may have been a beneficial effect against leukaemia during the first 15 years after entry (that is between age 15 and 30 years) and a deleterious effect after longer intervals (or above the age of 30 years). Alternatively this swing may simply represent an unusual chance fluctuation. There has been no sign in recent years of any substantial decrease in leukaemia mortality in England and Wales at ages 15-29 years, although the proportion of children given BCG vaccine at about age 13 years has risen from less than 1 per cent (for the cohort aged 15-19 years in 1951-1955) to more than 70 per cent 25 years later. It has therefore been provisionally concluded that chance is the explanation for the apparent swing from prevention to enhancement, and that BCG vaccination at age 13-14 years in Great Britain does not affect subsequent mortality from leukaemia.