Tuesday, September 18, 2012

Vaccination myths

We are hearing the 'V' word from numerous sources at every photo opportunity at the moment. Vaccination is the holy grail for bTB it seems. But does it work? We covered the debacle surrounding badger vaccines in several postings as the Myth unfolded in 2010. As that 74 per cent efficacy lie bounced round the world, courtesy of the BBC and FERA, very belatedly Defra told its advisory group that 'the data should not be used to support this claim', and Jim Paice found the headline was 'misleading and unhelpful'.

Nevertheless, on the basis that it did no harm to badgers, BCG was licensed as a Limited Market Product by the Veterinary Medicine Directorate (VMD) and is now being administered to un-screened populations of badgers in several parts of England and Wales. And the reason?

 In 2011 Defra published this little gem:
a): Bovine tuberculosis Animal species: Badger vaccination: Description of the used vaccination, therapeutic or other scheme Badger BCG licensed in March 2010 has been used as part of the Badger Vaccine Deployment Project to build farmer confidence in vaccines as a key tool in an eradication programme.
To build farmer confidence? What an extraordinary reason for promoting a vaccine which doesn't work.

 So what about cattle vaccination?
 Many people, some of whom really should know better, slip this into their conversations at every opportunity. If we vaccinate the cattle (who are telling us the true level of badgerTB in our increasingly polluted environment) then all will be well? No it will not. It's far to late for that, even if it were effective or affordable. Despite Defra's contorted gymnastics to dumb down its own statistics for over spill into other mammals, it is happening on a wide and increasing level.

But back to BCG for cattle. Weybridge VLA have published a report where sentinel vaccinated yearling cattle were kept with reactor cattle in 10 pairings for twelve months. Post mortem results from this study published in Vet.Record found that the pen level transmission rate was 50 per cent but crucially:
"There was no difference in the number of infected sentinels in the non-vaccinated or vaccinated groups."
This should come as no surprise, as Defra put its collective thoughts into this paper in 2007 which gave efficacy of cattle BCG little encouragement:
Annex 3. 6:3. A BCG vaccine is likely to confer full protection against M. bovis infection to 50% of vaccinated animals. For both the epidemiological model and economic assessment it is assumed that the protection conferred will last a lifetime. Of the remaining 50% that remain susceptible to infection, it is estimated that over half will be partially protected and have a much reduced capability of transmitting M. bovis should they become infected. The benefits of vaccination are likely to last for at least 12 months.
The cost is assessed at £8.25 per dose. The skin test is still mandatory and thus the DIVA test which differentiates between vaccinates and infection, must be used. So how accurate is that?
2.5.2: The DIVA test would be used when a vaccinated animal gives a positive reaction to the skin test (i.e. will be used as an ancillary test to the skin test). In such cases the DIVA test will confirm whether the animal is indeed infected or whether the positive response to the skin test is due to vaccination with BCG. 62. However, the nature of the test makes it impossible to guarantee the disease status of an animal. As with existing antemortem diagnostic tests for TB, there will be a number of false positive and false negative test results since neither the specificity nor the sensitivity respectively is likely to be 100%. The diagnostic accuracy of the new test will have to be assessed in field trials of herds of known TB status, which has already been done for some prototype DIVA reagents. In order to get the test accepted in EU legislation it will need to be at least as good as the current skin test in terms of sensitivity. However, data is available to suggest that the prototype DIVA reagent will satisfy this criteria, although as noted above this will need to coincide with the recommended age of vaccination.
So giving variable sensitivity / specificity, the DIVA test will cost around £26.00.

 Trade implications are huge, in that a number of both International and EU directives would need to accommodate this country's love affair with wildlife infected with tuberculosis. Withdrawal periods for vaccines and bans on all exports out of vaccinated 'zones' are mentioned.

So in summary, Defra say they are 'pump priming' farmers to accept vaccination, however futile its efficacy - and they will have to pay for it. All of it. As explained in this posting which invites farmers to have a 'conversation' with them.

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