Hey all - was actually in Vienna for the TB Vaccines for the World conference all last week, so missed this wonderful thread until just now 🙂 Glad the troll is gone, but I got a good laugh out of some of his posts, anyway!
I appreciate all of your thoughtful (and very well-informed, by the way!) posts about the BCG vaccine. I've been working at non-profit org. that does research on several levels (basic science, preclinical, clinical) into new TB vaccines for the last year, and it has been really eye-opening. Actually, I'm surprised at the high level of BCG vaccinees in this poll (20%! which I think is higher than the national average.... perhaps there is a higher percentage of foreign-born individuals applying to med school than there is in the population? Interesting...)
As many of you noted, one of the main problems with the BCG vaccine is that it causes a positive reaction to the PPD test, which is the common diagnostic tool for TB. PPD is short for purified protein derivative, and its UNBELIEVEABLY archaic, and basically just purified proteins from tuberculosis. Because BCG is an attenuated strain of M. bovis, the cow form of TB, it shares a lot of similar proteins with TB, hence the positive reactions. There are new diagnostic tests being developed that test reactions to specifically TB (and not BCG, or other environmental mycobacterial) antigens, but these have not proved totally reliable yet (though the WHO has begun supporting the Quantiferon Gold test, I think....) The OTHER big problem with BCG is that its efficacy is so variable. The only reason it is still given in many countries is that it has shown some (but still variable in clinical trials) efficacy against the more severe forms of tuberculosis in infants and small children (namely, meningeal and disseminated tuberculosis, not pulmonary) in areas where the epidemic is the worst. However, the protective efficacy against adult pulmonary tuberculosis, which is where much of the disease burden (and threat of disease spread!) lies, is very controversial. Some trials have shown negative efficacy, some have shown up to 80%. Basically, its just not good enough. And, in HIV positive or otherwise immunocompromised individuals, BCG (which is a live vaccine) can spread in the host and result in disease equivalent to TB (both pulmonary and disseminated). So, as the threat of HIV/AIDS is increasing, BCG is becoming an even more undesirable vaccine.
Furthermore (again, as many of you astutely noted), the treatment is long (9 months minimum usually for latent infection, with two drugs - isoniazid and rifampin - both very hepatotoxic, and 6-12 months for active disease with 4 antibiotics) and if not conducted properly can result in creation of drug-resistant strains. Thankfully, the DOTs program works well when applied completely, but its difficult to implement in many areas. And, we can't rely totally on treatment to stop the spread of TB, as diagnosis is difficult and cases often go unchecked for a long time (allowing spread) before treatment. Because TB/BCG are slow-growing organisms, and because there is no good established animal model of infection, and no established immune correlates or surrogates for protection, TB vaccine discovery is pretty slow and difficult. Like malaria and HIV/AIDS, TB is mainly protected against by cellular immunity, and we all know how difficult it has been to make vaccines against these other two pathogens (most of the vaccines currently licensed protect against diseases that require humoral immunity- antibodies).
Anyway I apologize for this incredibly long rant, but I find the field of TB vaccines to be really fascinating and am always excited to talk about vaccination issues. Hopefully in the next 10-15 years we will see some progress so that the 20% on this poll can go up to 100%, with a vaccine of 100% efficacy!!!
Love,
P