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Impact of BCG vaccine on badgers

The impact of vaccinating badgers with the BCG vaccine is reported in Reference 1 and 5. This field study was carried out by annually vaccinating about 200 badgers across a 55 km2 area in Gloucestershire for 4 years between 2006 and 2009.

The following graphs show data taken from Table S5 referred to in Reference 1 and shown in Reference 2. The graphs below illustrate the impact of vaccination on real, unscreened populations of badgers. These unscreened populations include initially infected as well as uninfected badgers. Only uninfected badgers are sensitive to the vaccine and are protected by it. The impact of the vaccine on these unscreened populations can be seen by comparing the proportion of TB in badger controls (unvaccinated population) with the proportion of TB in the vaccinated badgers for each year. The BCG was administered on recapture at a rate of one dose per calendar year, resulting in some individuals receiving multiple vaccinations over the course of the study.

It should be noted that in this field trial, badgers were vaccinated on a group by group basis and not across a wide area so the mixing between vaccinated and unvaccinated groups was greater. This presented a greater challenge to those groups which were vaccinated and reduced the force of infection to those groups which were left as controls.4

Although the small sample sizes used and the large associated confidence intervals mean that observations may not be representative, Figs 108a and 108e, and to a lesser extent Fig 108b, give the impression that the impact of the vaccination programme in the last year was less than in the preceding year.

Proportion (%) of badgers found infected when IGRA tested during 2006 to 2009 badger vaccination trial in Gloucestershire

Proportion (%) of badgers found infected when Stat-Pak tested during 2006 to 2009 badger vaccination trial in Gloucestershire

Proportion (%) of badgers found infected when culture tested during 2006 to 2009 badger vaccination trial in Gloucestershire

Proportion (%) of badgers found infected when dual tested during 2006 to 2009 badger vaccination trial in Gloucestershire

Proportion (%) of badgers found infected when triple tested during 2006 to 2009 badger vaccination trial in Gloucestershire

Proportion (%) of badgers found infected during 2006 to 2009 badger vaccination trial in Gloucestershire

Fig 108e, which shows results using the most thorough test combination (the triple test), shows that a very high proportion of badgers were found to be infected. In fact, in 2006, infection was detected in 53% of the 112 badger controls tested. This proportion is much bigger than the proportion of cattle in which infection is detected. In fact, in the worst affected counties of South West England this proportion is less than 2% (see here) so is less than one twenty fifth of that found in this subset of badgers. However prevalence in badgers was only measured in a small 55km2 area of Gloucestershire and this area is thought to be one of the worst affected areas in the country.

In badgers, the sensitivity of the triple test (Fig 108e) was reported to lie between 61% and 86%. If this is taken into account, this together with the 449 positive tests out of the 1201 performed, implies that during the 4 years from 2006 to 2009 the actual proportion of badgers infected in this 55km2 area exceeded 50%. This is even when the vaccine was administered to two thirds of the badgers each year during the course of these 4 years.

Fig 108e also suggests that in such high-disease areas, disease levels remained high after completing the 4-year vaccination programme. In fact, infection levels in vaccinates were found to be 33.8% compared to 36.8% in controls. This translates to the vaccine reducing badger infection by 8%. (36.8 - 33.8) / 36.8% = 8%. Although these percentages would be subject to sampling error in view of sample size and the close proximity of vaccinated and control groups may have reduced the impact of the vaccine,4 the small benefit seen in 2009 does cast some doubt on the benefit of vaccinating badgers in areas of high disease. This gives no reassurance that a noticeable impact will be seen in cattle after 5 years in the Intensive Action Area of Wales. Indeed Professor Chris Pollock, who was acting Chief Scientist in Wales, resigned in April 2012 citing the main reason for his resignation as follows.

... more significantly from my standpoint was that in the scientific report [which] the Minister commissioned, there was a specific comment about the problems of using vaccination in an area where you obviously had a high incidence of disease.

Using vaccination in the targeted Intensive Action Area, where the level of infection in badgers is extremely high, in my view goes against the recommendations of his own scientific review.

I was certainly not happy about that and as such did not believe I could continue to be a member of the programme board if I did not fully support the programme being pursued3.

In the Intensive Action Area, Wales are currently looking at the combined impact of enhanced cattle control measures which started on 1st May 2010 and vaccinating badgers which started on 1st May 2012. These measures are being taken in the absence of culling badgers.

References
  1. BCG Vaccination Reduces Risk of Tuberculosis Infection in Vaccinated Badgers and Unvaccinated Badger Cubs. PLOS ONE. Stephen P. Carter et al. Published 12th Dec 2012.
  2. TB prevalence of badgers in vaccination study in Gloucestershire 2006-2009. PLOS ONE. Stephen P. Carter et al. Published 12th Dec 2012.
  3. Top scientist resigns over Welsh badger cull U-turn. farmersguardian.com. 27 April 2012.
  4. BCG Vaccination Reduces Risk of Tuberculosis Infection in Vaccinated Badgers and Unvaccinated Badger Cubs. Comments submitted by SP Carter (principal author) in response to reader comments. www.plosone.org [Static copy. Last comment captured: 27 Sep 2013.]
  5. Impact of the BCG vaccine on the prevalence of TB in badgers.

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