BCG Vaccine

Stands for Bacillus Calmette-Guérin, named after the 2 Frenchmen who developed the attenuated strain of M. bovis in 1921.  It’s used widely in the 3^rd World, was routine in Western Europe until just a few years ago, & was never adopted in the U.S.  We refer to the vaccine simply as “BCG”.

BCG is like sourdough bread, one batch is used as a “starter” to prepare another.  As such, there’s no standardization in terms of potency.  And every country seems to have a different schedule as to age of immunization & numbers of boosters.

BCG can cause a response to PPD testing.  The amount of induration depends on how long after the last BCG dose, how many boosters had been placed, & simple individual variation.  But in general, BCG easily causes <10 mm of induration (interpreted as “negative” in terms of TB).  It might occasionally cause 10-15 mm, and rarely >15.

As such, the CDC says to ignore a history of BCG Vaccine when interpreting a PPD response.  In terms of mere probabilities, if a person from a high-prevalence country (e.g. the 3^rd World) tests >10 mm by PPD, the chances that the reaction represents true Latent TB Infection, vs. BCG effect, weigh enormously on the side of TB.

A meta-analysis of 26 studies (out of >1200) found that among children, BCG confers approximately 50% protection for TB overall, 64% for TB meningitis, & 71% for death.  In poor countries, it’s life-saving.  Since BCG may confound PPD testing, and the incidence of childhood TB in the US is so low, it’s rarely indicated (perhaps for children in a household with drug-resistant TB).

The Interferon-Gamma Release Assay (IGRA), like QuantiFERON^® and T-Spot.TB^®, are very specific for Mycobacterium tuberculosis, and don’t react to BCG.