return to

Dr. Chris Oura's "Clarification on vaccines efficacy and lab results interpretation" sent to ProMed. It appeared on September 21 2008 at following the article from the Farmers Weekly (which appeared onn Saturday) in which the low uptake of vaccine and worries about its efficacy were discussed.

[2] Clarification on vaccines efficacy and lab results interpretation
Date: Sat 20 Sep 2008
From: Chris Oura <<a href=""></a>>

Recent reports that not all animals vaccinated with inactivated BTV-8
vaccines seroconvert post-vaccination have been causing alarm. People
have concluded from this that the vaccine may be failing and may not
be protective.
We have found at the bluetongue CRL here at Pirbright that not all
sheep seroconvert after one shot of inactivated vaccine (when
antibodies are detected using the commercial routinely-used
competitive ELISAs). However, we have every reason to believe that
these animals are protected, as challenge experiments carried out by
the vaccine companies have proved that animals that have not
seroconverted are protected.
Antibodies are only a component of the immune system, and previous
scientific research indicates that there are likely to be
cell-mediated immune responses that are contributing to protection
against BTV. Also, low levels of antibodies may be present in
vaccinated animals that are below the threshold of detection of the
competitive ELISAs. This low level of antibodies may be sufficient to
produce an effective memory response on challenge.
It seems clear that, for animals vaccinated with inactivated BTV-8
vaccines on a single occasion, the absence of antibodies detected by
the cELISA tests does not correlate with a lack of protection. In
other words, seronegative vaccinated animals are still likely to be
protected from BTV-8. Therefore, reports of low levels of
seroconversion post-vaccination should not put people off vaccinating
their stock. However, this does cause a problem in that, at the
present time, we are unable to reliably confirm by testing that
animals have been successfully vaccinated.
Also, I would like to try to clear up another issue that is causing
some confusion about PCR testing. PCR detects viral RNA and not
infectious virus. Viral RNA detected by PCR persists in infected
animals often for around 100-200 days post-infection. However,
infectious virus persists for a maximum of 60 days post-infection.
Therefore, animals will be PCR-positive for a considerable time after
they have cleared infectious virus from their blood.
In the recent reports of PCR-positive animals imported into the UK,
these animals have been found to be PCR-positive in post-import
testing and have, in the majority of cases, had a PCR result
consistent with the animal being viraemic (early infection). In many
of the samples from these imported animals, we have gone on to
successfully isolate virus from them.
Therefore, it is important to be cautious about interpreting PCR
results, as animals will, in some cases, be PCR-positive; however,
they may not be at risk to the local midge population. Laboratories
however are able to go some way to interpret the PCR results and,
depending on the CT levels*, can predict if the animals are likely to
be in the early (viraemic) or late (non-viraemic) stages of
infection. The only test available to confirm that the animal is
viraemic is virus isolation, and the drawback of this is it takes at
least a week to perform.
[* The ct value -- or threshold cycle -- is the PCR cycle at which a
statistically significant increase of the fluorescence signal is 1st
detected. Ct values form the basis for quantitative comparison of
individual PCR reactions. The smaller the ct value is, the bigger the
quantity of target cDNA in a given PCR reaction].
Dr Chris Oura
Head of the CRL for Bluetongue,
IAH, PIrbright, UK.

The moderator, AS, commented:
    The above observation from an internationally recognized scientific authority, for which ProMED-mail is very grateful, is a welcome contribution in clarifying the vaccine efficacy issue raised in previous postings, such as Romania's questions in its recent presentation to SCFCAH (article 3, sub-article 5 in 20080918.2930).

    Hopefully, this detailed, pacifying information will encourage animal breeders in regions at risk, both in the UK as well as in continental Europe, to complete the timely vaccination of their susceptible animals against BTV-8. The peak disease season may continue until the end of October 2008, trailing beyond.

    Regarding the vaccine safety issue raised in item [1], according to the (45-year-long) experience of this moderator, all animal mass vaccination campaigns against any disease in any species in industrialized as well as in developing countries are accompanied by owners' complaints. Most of such complaints reflect honest but subjective observations of concerned individuals, in the majority of cases eventually found to be groundless, provided the vaccines in question are properly certified, and in particular when they are inactivated ones. This does not mean that accidents never happen on the manufacturing level but, mainly, during application; complaints should always be thoroughly investigated and their results published. - Mod.AS]