Impact of the BCG vaccine on the prevalence of TB in badgers
The largest and most thorough trial of badger vaccination ever undertaken in the UK was conducted in Gloucestershire between 2006 and 2009. The report was peer reviewed and one of the authors was the APHA Chief Scientist Glyn Hewinson.In the vaccination trial about 200 badgers were vaccinated each year over 4 years over an area of about 55 sq km. Badgers were vaccinated on a group by group basis instead of over one wide area. This means that the mixing between vaccinated and unvaccinated badgers would have been greater in the trial than if the treatment had been implemented in a wide-area roll out. As such the challenge to the vaccine was greater in the trial than in a typical rollout.
What impact did the BCG vaccine have on the prevalence of TB in badgers after 4 years? This is an important question because it is hoped that vaccination will build herd immunity in treated badger populations over the course of time.
The scientists failed to detect a significant herd effect in terms of prevalence in the 200 badgers vaccinated at the end of the 4 years. In fact when these badgers were triple tested in 2009, the prevalence level in the 100 unvaccinated controls was 36.8% and in the 200 vaccinated treated badgers was 33.8%. Another words the prevalence level of TB in badgers had dropped by (36.8 - 33.8) / 36.8% = 8% over the course of 4 years2. When 95% confidence levels are considered this effect is statistically insignificant.
So where is this 79% reduction in risk in unvaccinated cubs coming from?
This is the amount by which TB incidence reduced in unvaccinated cubs. It is thought that TB in these cubs dropped because they were subjected to less infection largely on account of the vaccinated badgers they came into contact with.
Ultimately however what counts is the amount by which TB levels (i.e. prevalence) drops. Incidence is a measure of the number of events which gives rise to TB level. If the incidence drops but does not lead to a drop in TB level, there would have been no benefit.
Although the impact of the treatment on incidence is encouraging, the ultimate test is if TB levels drop. If the treatment is causing badgers to live longer and hence spread their reduced infection during the course of a longer lifetime, it is possible that the treatment had no impact on the overall extent to which they were exposing other badgers (and cattle) to their infection.
To see the report, please refer to,
BCG Vaccination Reduces Risk of Tuberculosis Infection in Vaccinated Badgers and Unvaccinated Badger Cubs