Pulse - September 2021 - 25

MYVETZONE@GMAIL.COM
MEDICAL LEEWAY
What Preventive Fraction Does and
Does Not Provide
Christopher A. Lee, DVM, MPH, DACVPM
H
ave you ever read a vaccine study and wondered,
" What's the bottom line here? How well does
the vaccine work? Just tell me what percentage
of pets are protected. " Then the conclusion section provides
the following statement, " The preventive fraction
is 93%. " In the language of scientific study, preventive
fraction represents clear concision. For those not regularly
reading or running these studies, the term appears
familiar, intuitive, but perhaps unclear as well. How synonymous
is a preventive fraction with percent protected?
What exactly is the definition of the preventive fraction,
and what does it tell you? Equally important, what
doesn't it tell you?
Researchers utilize preventive fractions in both correlative and
causal studies. In the context of vaccine challenge studies, researchers
take test animals and separate them into control (unvaccinated)
and exposed (vaccinated) animals. The exposure cohort is vaccinated,
and then, after appropriate medical timing, both groups
are challenged with the pathogen. After the incubation period has
passed, the researchers test and evaluate which animals are either
free of infection or clinical disease.
Let's say nine out of ten vaccinated animals are protected. So
then the vaccine works 90% of the time, right? Hold on. Not
all infectious diseases are equally contagious. What if, upon routine
exposure of a pathogen, only eight control animals become
infected. Then, is 9 out of 10 still 90%? No, but what is it? Here lies
the beauty of Preventive Fraction; it accounts for this discrepency.
The formula for Preventive Fraction is 1 minus the Relative
Risk. Relative risk finds its way into many scientific studies.
It represents a simple ratio of the percent diseased when exposed
divided by the percent diseased when not exposed. As exposure in
this situation is a protective effect, due to vaccination, the Relative
Risk number will be small.
In our example of 9 out of 10, infection-free, vaccinated animals,
let's look at two different scenarios: we have all non-vaccinated,
control animals getting sick, and in the other, we have only
80% of control animals sick. We don't really need to calculate this
for the first scenario, but let's do it anyway. The relative risk is 1
out of ten, divided by 10 out of 10 or one. Thus, 1 minus the relative
risk is just 1 minus 0.1, which comes out to 0.9. So the Preventive
Fraction is 90%.
In the second scenario, our denominator is 80% or eight diseased
animals out of ten. This leads to a preventive fraction of
87.5%. Intuitively, this number makes sense. The vaccine works,
one infected versus eight, but we know that the vaccine cannot
take full credit for ninety percent protection either. Preventive
PULSE
SEPTEMBER 2021
25
fraction works to represent the protection afforded by the vaccine
and attempts to factor in the pathogen's infectivity.
Of course, within animal vaccine challenge studies, this infectivity
occurs within an artificial laboratory setting. Some pathogens
create significant logistical obstacles. For example, how do
you replicate a standard rabies attack dose? Would you put a rabid
animal in with each animal as the challenge model? Beyond ethical
concerns, consider handling issues, variation with bite numbers,
different bite locations, inoculative dose, and strain. Consider
distemper, leptospirosis, and parvo challenge models similarly.
Thus, laboratory vaccine challenge studies and their reported
preventive fractions represent causative data with high internal
validity. In plain speak, these studies help us understand cause and
effect with minimum statistical bias interference.
For most studies, internal and external validity share an inverse
relationship. Thus, a laboratory challenge study with high internal
validity (cause and effect) naturally has gaps in external validity, or
how the findings can be generally applied to real-world situations
or different populations.
A preventive fraction in vaccine challenge studies provides a
wealth of knowledge. In light of the infectivity challenge, preventive
fraction provides insight into the protective ability afforded by
the vaccine. Unless vaccines are challenged within the same study,
we must take extreme care with comparing preventive fractions.
When we bring these vaccines into the real world, every vaccinated
individual represents an individual case. Thus, the study has
detailed why we utilize the vaccine for our at-risk patient populations
but does not provide percentages for an individual patient.
For example, if a vaccinated patient was exposed to a pathogen,
then the patient's percentages are absolute - either infected or not
after the incubation period.
When evaluating and choosing a vaccine, the preventive fraction
becomes one of the many factors we must consider. It
remains a vital
statistic when evaluating any vaccine challenge
study, and thus, when evaluating the vaccine itself. By understanding
the strengths and weaknesses of this statistic, we can better
evaluate the data and hopefully make better choices for ourselves
and our patients. P
References, supplemental reading, and podcasts
available at www.MyVetZone.com. To add in
iTunes, search " vetzone " on Spotify or iTunes.
http://www.MyVetZone.com

Pulse - September 2021

Table of Contents for the Digital Edition of Pulse - September 2021

Pulse - September 2021
Chapter Meetings & Calendar
President’s Perspective
Pulsepoints
SCVMA Profile
How Veterinary Managers’ Jobs Differ
Practical Pathology
Medical Leeway
UC Davis Update
Tools for Success
Angel Fund
Dear Tabby
The RVT
Industry Insights
Quick Reference
AVMA Diplomates
Digital Photography for Veterinarians
Resources
Disease Table
From the SCVMA Office
Pulse - September 2021 - Pulse - September 2021
Pulse - September 2021 - Cover2
Pulse - September 2021 - 1
Pulse - September 2021 - 2
Pulse - September 2021 - Chapter Meetings & Calendar
Pulse - September 2021 - President’s Perspective
Pulse - September 2021 - Pulsepoints
Pulse - September 2021 - 6
Pulse - September 2021 - 7
Pulse - September 2021 - 8
Pulse - September 2021 - 9
Pulse - September 2021 - 10
Pulse - September 2021 - SCVMA Profile
Pulse - September 2021 - How Veterinary Managers’ Jobs Differ
Pulse - September 2021 - 13
Pulse - September 2021 - 14
Pulse - September 2021 - 15
Pulse - September 2021 - 16
Pulse - September 2021 - 17
Pulse - September 2021 - 18
Pulse - September 2021 - 19
Pulse - September 2021 - 20
Pulse - September 2021 - 21
Pulse - September 2021 - 22
Pulse - September 2021 - 23
Pulse - September 2021 - Practical Pathology
Pulse - September 2021 - Medical Leeway
Pulse - September 2021 - UC Davis Update
Pulse - September 2021 - Tools for Success
Pulse - September 2021 - 28
Pulse - September 2021 - Angel Fund
Pulse - September 2021 - Dear Tabby
Pulse - September 2021 - The RVT
Pulse - September 2021 - Industry Insights
Pulse - September 2021 - Quick Reference
Pulse - September 2021 - AVMA Diplomates
Pulse - September 2021 - Digital Photography for Veterinarians
Pulse - September 2021 - 36
Pulse - September 2021 - 37
Pulse - September 2021 - 38
Pulse - September 2021 - Resources
Pulse - September 2021 - 40
Pulse - September 2021 - Disease Table
Pulse - September 2021 - 42
Pulse - September 2021 - 43
Pulse - September 2021 - 44
Pulse - September 2021 - 45
Pulse - September 2021 - 46
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Pulse - September 2021 - 50
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Pulse - September 2021 - From the SCVMA Office
Pulse - September 2021 - Cover3
Pulse - September 2021 - Cover4
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