A century-old tuberculosis vaccine protects well in Britain and barely at all near the equator
BCG, first used in 1921, is still the only licensed vaccine against tuberculosis, and around 100 million children a year were receiving it by 2004. Yet its protection is a puzzle: British trials found 60 to 80%, other trials found none, and nobody has proved why. It also doubles as a bladder cancer treatment.
The vaccine is named after Albert Calmette and Camille Guerin, who developed it from Mycobacterium bovis, a relative of the TB germ found in cattle. It is weakened but alive, given as a single injection into the skin of the upper arm, and usually leaves a raised scar that often serves as proof of vaccination. Where tuberculosis or leprosy is common, newborns get one dose as early as possible; elsewhere only high-risk children are typically immunised, and it is rarely given in the United States.
Effectiveness varies enormously. A 1994 review put the reduction in TB risk at about 50%, and a 2014 meta-analysis found infections cut by 19 to 27% and progression to active disease by 71%. Protection appears to fade the nearer a population lives to the equator. How long it lasts is also unclear: a Medical Research Council study saw it fall to 59% after 15 years and vanish by 20, while Native Americans immunised in the 1930s still showed benefit 60 years later. It seems strongest against severe forms such as TB meningitis, which keeps it in use even where it does little for lung disease.
Several explanations compete. Genetic differences between people looked plausible until a 1988 Birmingham trial found 64% protection in children of families from the Indian subcontinent, where efficacy had been measured at zero. Another idea is masking: exposure to harmless environmental mycobacteria may already prime immunity, so BCG adds little, as parallel studies in British and Malawian schoolchildren suggested. Parasitic worm infections might also blunt the immune response, and strain differences in the vaccine itself could matter.
Side effects are usually a red, sore swelling and sometimes a small ulcer. Because the vaccine is live, it can cause dangerous spreading infection in people with badly weakened immunity, and in 2007 the WHO stopped recommending it for infants with HIV. Delivered into the bladder through a catheter, BCG has been standard care for early bladder cancer since 1977 and prevents recurrence in up to 67% of superficial cases. It also gives partial protection against leprosy.
Source: BCG vaccine
