What does the ‘rate of effectiveness’ of a vaccine really mean?

What does the ‘rate of effectiveness’ of a vaccine really mean?

The rate of effectiveness of a vaccine is advertised to the public as the relative rate of effectiveness rather than the absolute rate. The difference between the two is substantial.

Here is an example to explain the difference by comparing 100 experimental (vaccinated) people with 100 control (unvaccinated) people.

Absolute value–

In the experimental group (vaccinated), 1 in a 100 gets COVID-19. (1%, 1/100 or 0.01)

In the control group (unvaccinated), 2 in a 100 get COVID-19.  (2%, 2/100 or 0.02)

The absolute value is 0.02 – 0.01 = 0.01 or 1%

This is to say that 1% more people are helped by the vaccine.

Relative value —

In the experimental group, 1 in a 100 gets COVID-19 (0.01 or 1%)

In the control group, 2 in a 100 get COVID-19  (0.02 or 2%)

The relative value is the absolute value (0.01) divided by the number infected in the control group (0.02)     0.01/0.02 x 100 = 50%

This states that the vaccine is 50% effective in reducing infections.

Let’s use a real example. The Ontario Ministry of Health published an information sheet to inform people of benefits and risks prior to vaccination. Here is a quote from page 2-

Ministry of Health COVID-19 Vaccine Information Sheet Version 9.0 – July 19 2021   

The Pfizer-BioNTech vaccine has been demonstrated to be highly effective at protecting against COVID-19 for individuals 12 and over. The Pfizer-BioNTech clinical trial studied 2,260 youth aged 12 to 15 years old in the United States. In the trial, there were 18 cases of COVID-19 in the group that did not get the vaccine (the “placebo” group) compared to zero cases in the vaccinated group. Based on these results, the vaccine was calculated to be 100% effective in the trial.”

As you can see, the relative rate of effectiveness was calculated by using the formula explained above.

(18-0) divided by 18 multiplied by 100% equals 100%.

How can one argue with these results??

The Absolute Rate of Effectiveness, however, considers the whole test group, not just the ones infected. If we assume that the group of 2,260 was divided equally between vaccinated and unvaccinated, there would be 1,130 in each group. In this example, like the one above, vaccinated people are the experimental group (0 infections) and the unvaccinated are the control group. (18 infections). Absolute Rate of Effectiveness using the formula described above would be –

(18/1130 in the experimental group) minus (0 in the control group), multiplied by 100% = 1.6%

In other words, 1.6% of the vaccinated children in this study were helped by the vaccine. For 98.4% of them, it made no difference whether they were vaccinated or not.

The question is—now that you are better informed, would you rush to have your 12-15 year old child vaccinated when, if they are in the 1.6% that become infected, data shows that they will not experience significant symptoms but will achieve a robust natural immunity probably for life?  

It is also worth considering how well powered was the clinical trial to show safety concerns. For example, in the Pfizer-BioNTech clinical trial mentioned earlier, any safety issues that did not occur in 1/1000 people probably would not have been detected.

For a complete explanation of relative and absolute effectiveness ratings for Pfizer and Moderna vaccines, visit https://pubmed.ncbi.nlm.nih.gov/33652582/



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