TB jabs 'may cause tougher strains'

Vaccinating against TB could actually spread the lung infection by clearing the way for tougher strains to flourish, a new study claimed.

An immunisation programme might merely remove a weak form of tuberculosis and allow a virulent strain to break out, researchers from the University of Bristol found.

There are nearly nine million new cases of TB each year, and nearly two million deaths worldwide. Efforts to create a vaccine have produced more than 200 candidates, many of which are now undergoing animal testing and early clinical trials.

Large-scale genetic analysis of strains of TB mycobacteria has shown they are more varied than first thought and that vaccines may be effective against only some of the circulating strains.

Dr Caroline Colijn from Bristol's department of Engineering Maths, came up with several models to replicate how these various strains would react to certain drugs.

Dr Colijn said: "In most scenarios increasing vaccine coverage reduces the overall TB burden. However, the benefits are reduced if the preferential removal of one strain allows a previously suppressed strain to succeed.

"We found that there is a possibility that TB prevalence may increase due to a vaccination programme effective against a dominant strain, if that strain didn't provoke a good immune response.

"This creates a concern that vaccination policies could affect which strains are present in a population, potentially releasing strains with an increased potential for drug resistance, and could even result in an increase in disease levels."

This phenomenon has already been seen in the case of streptococcus pneumoniae - the bacterium that can cause pneumonia, and haemophilus influenzae, common bacteria that cause a wide variety of infections in children.

Dr Colijn added: "Our results indicate that the public health benefit of new TB vaccines and vaccination programmes will depend critically on the diversity of circulating strains, the in-host competition between the strains and on the strain specificity of the vaccines."

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