Thursday, March 6, 2014

When Vaccines Don't Seem to Make a Difference
A research project recently reported that vaccinations in young calves did not change rates of sickness, death and growth among vaccinated vs. non-vaccinated calves. On the surface one might conclude that vaccinating young calves is a poor risk management practice.
Let's dig a little into the research design to see if that is the appropriate conclusion. For the research report click HERE .
First, 89 percent of the 2,874 calves in the study had successful passive transfer of immunity using 5.2 g/dL threshold. Thus, protection from colostrum against respiratory disease was quite high. 
Second, the research design had four treatments: (1) control - injected with neutral liquid at 2 and 5 weeks, (2) treatment 1 = vaccinated at 2 weeks only, (3) treatment 2 = vaccinated at 5 weeks only, and (4) vaccinated at both 2 and 5 weeks. This design means that 72 percent of the calves on each farm were vaccinated.
Herd immunity (or community immunity) describes a form of immunity that occurs when the vaccination of a significant portion of a population (or herd) provides a measure of protection for individuals who have not developed immunity.[1] Herd immunity theory proposes that, in contagious diseases that are transmitted from individual to individual, chains of infection are likely to be disrupted when large numbers of a population are immune or less susceptible to the disease. The greater the proportion of individuals who are resistant, the smaller the probability that a susceptible individual will come into contact with an infectious individual.[2][Wikipedia definition. 

Ah, so with nearly three-quarters of the population vaccinated, the chances of an un-vaccinated calf coming in contact with a sick calf was, in my opinion, drastically reduced. 

Third, 44 percent of the pneumonia cases in this population occurred before 5 weeks of age - that would be before the vaccinations were completed. Thus, vaccines still in the bottle had no chance of reducing treatment rates (average age at which first treatment occurred was 30 days). 

Fourth, overall 22 percent of the calves were treated at least once for respiratory disease. Unfortunately the report does not describe the feeding program for the calves. One might guess that regardless of vaccination status, some of these cases might have been prevented by a more generous feeding program especially among the younger calves (i.e., less than one month). 

Further, with lots more energy and protein available from a more liberal feeding program the response to vaccination might have been stronger.

Bottomline

In my opinion this study had significant flaws in its design and subsequent data analysis such that its conclusion (vaccination did not change rates of sickness, rates of growth and death) ended up being so specific to the study population its finding have limited use to the rest of us. To their credit the authors did point out that their conclusions did have limited applicability to dairy farms in general. 

Please do not take the conclusions reached by authors uncritically.  
 

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