What is Preventive Fraction?

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Podcast: What is Preventive Fraction
Length: 10 minutes 50 seconds
Written and read by the author

To begin with, this statistic commonly associates itself with vaccine challenge studies, that is, we have two groups of animals, one vaccinated, one not, and they are both exposed or challenged with the infectious disease in question, and we observe who gets infected. Let’s say you have ten animals in each category, and 9 out of ten vaccinated animals are protected. 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 80% of normal 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.

The formula for Preventive Fraction is 1 minus the Relative Risk. We use Relative Risk in many types of scientific studies and is 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, vaccination, the Relative Risk number will be a small number whereas, in most other scientific studies, exposure is a harmful effect and produces a characteristicly large number.

In our example of 9 out of 10 vaccinated animals protected, let’s look at two different scenarios: in one we have all non-vaccinated, control animals getting sick and in the other, we have only 80% of control animals sick. For the first scenario, we don’t really need to calculate this but let’s do it anyways. 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. The Preventive Fraction is 90%, pretty great huh?

In the second scenario, our denominator is 20% or 2 diseased animals out of ten. This leads to a preventive fraction of only 50%. This means that only half of the animals that did not become infected were the sole result of the vaccine’s protection. Is this vaccine worth the time, money and energy to get? Possibly not, yet we had the same number of vaccinated animals remain uninfected following challenge. See why we can’t just ask, “What percentage were protected?”

              Preventive Fraction works to represent the protection afforded by the vaccine and attempts to factor in the disease’s natural infectivity. While the goal is real-world data, we must view this statistic with caution. Since this is done in an artificial laboratory setting, we need to critically evaluate what the researcher decided was a proper challenge model. For some situations, challenge mirrors the real world rather well. With tick-borne disease, the method of transmission and inoculative dose should be similar to what would occur in nature. Even with this situation, there are problems. For example, in the wild Amblyomma ticks are known for their aggressive behavior but become profoundly lazy in a laboratory setting. This generates laboratory frustration.

Other diseases create significant logistical and practical obstacles. How do you replicate a standard rabies attack dose? Would you put a rabid animal in with the challenge model? Beyond ethical concerns, consider handling issues, variation with bite numbers and bite location, and necessary survival throughout the ordeal. What about distemper, leptospirosis, and parvo challenge models.

For this reason, you cannot stand two separate vaccine challenge studies side by side and say, Vaccine A has a preventive fraction of 92% and is thus better than Vaccine B whose preventive fraction in a separate study was 89%. Different studies, different laboratories, different test animals and different challenge models to begin with.

When evaluating and choosing a vaccine, 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.

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