BCG Vaccine Effectiveness

BCG is a vaccine with important applications because it immunises against a highly contagious, and all too often fatal, disease: tuberculosis. The tuberculosis infection is thought to affect at least a third of the world’s total population, 90% of whom will be carrying an asymptomatic, latent infection. Of the 10% who suffer an activated infection, at least half will find the disease fatal. With such a high mortality rate, tuberculosis is considered a major threat to global health.

This threat is further exacerbated by complications involved in the delivery of treatment for the disease. To effectively remove tuberculosis from a patient’s system, a long course of antibiotics needs to be undertaken, and if this course is not completed, then the underlying infection can often come back with a vengeance. Unfortunately the treatment clears up symptoms quite quickly, and at this point many patients stop taking their antibiotics, giving rise to a growing population of drug resistant strains of TB bacteria.

All in all the current state of tuberculosis as a global concern makes prevention a major priority to healthcare organisations across the world. An effective vaccination programme would dramatically reduce the spread of the infection, and BCG is currently the main contender in the fight to prevent TB. Unfortunately BCG has also shown a huge variability in its effectiveness, and in this article we look at this particular phenomenon and what it means for global TB vaccination initiatives.

This variation is a concern to us here in the UK because if TB is to ever be eliminated like other contagious diseases in the past (e.g. smallpox), an effective global scheme of immunisation is needed.

How does the effectiveness of BCG vary across the world?

In the UK the effectiveness of this vaccine has been proven to be at about 60-80% after a single dose. The immunity provided by this injection is long lasting here, and therefore BCG has proven to be an effective tool in reducing incidences of TB in the UK.

Unfortunately other parts of the world have reported hugely variable efficacy rates after BCG injection. Reports from parts of the US like Alabama and Georgia recorded an efficacy rate as low as 14%, however further investigations into communities in America showed that native Americans were immunised for as long as 60 years after BCG injection with no significant reduction in immunity to TB.

Moreover immunity seems to vary between different types of TB. For instance, in some countries where the protective effects of the vaccine against the standard pulmonary infection were shown to be minimal, the same vaccine proved to be highly effective against TB meningitis and military TB. The former of which affects the lining of the brain, and the latter of which is a spread of tiny pockets of bacteria across the entire body.

Further studies are needed to outline this effect in more detail, and such information would be extremely useful in determining why the effectiveness of the vaccine has shown such dramatic variation.

Why does the effectiveness of BCG vary across the world?

At present this phenomenon is not entirely understood, however there are a number of theories around based on the evidence we have to date.

One theory suggests that in places where TB is endemic (present in high background levels), the majority of the population are carrying a latent infection which renders the vaccine useless. This would account for the low efficacy of the vaccine in TB hotspots like India.

Others argue however that it is genetic variation in the strains of bacteria used to generate the BCG vaccine that is responsible for the variation in effectiveness. BCG is a live vaccine which is made up of a weakened strain of TB derived from cows. Genetic variation in the populations of the vaccine used for vaccination would affect the body’s immune response to these strains, potentially accounting for this phenomenon.

Along those same lines is the theory that it is genetic variation between different populations across the world that causes the reported variation. However this theory is somewhat undermined by recent studies showing that populations in the UK originating from parts of the world where TB is endemic still demonstrate high vaccine effectiveness unlike the efficacy rates reported from their native countries.

Interference from other bacteria or parasitic infections is thought to be a potential factor. This is an effect referred to as ‘masking’, where an existing infection is effectively distracting the immune system from the vaccine, meaning that the body is therefore not as effectively immunised against TB.

Finally exposure to UV radiation has been recently raised as a potential explanation. Areas nearer the equator have reported lower efficacy rates, and it is thought that the increased UV radiation native to these parts of the world may impact the effectiveness of the BCG vaccine. This theory has yet to be thoroughly investigated, however a recent investigation into the effects of UV on the success of BCG immunisation in guinea pigs has supported the possibility of exposure to UV radiation being a factor.

Is BCG still used across the world because of this variability?

As mentioned earlier on in this article, BCG doses that prove ineffective against pulmonary tuberculosis have shown evidence of being effective against other versions of the disease. While TB largely infiltrates the lungs, it can spread to other parts of the body. So even where the BCG vaccine has not proven massively efficacious, it still has some sues, and this is one of the reasons why its uptake is still widespread.

BCG immunisation is still routine in countries like India, Brazil, Pakistan, and many others.

Vaccination technology is constantly improving, and the hope is that progress will reveal more about why the BCG vaccine’s effectiveness varies so much.

---

« Who gets the BCG Vaccine in the UK? Safety of the BCG Injection »