I haven't looked at every covid thread so I apologize in advance if this has already been addressed.
Frequently I'm seeing covid vaccine efficacy rates touted in the 90 percentile range. At first blush, percentages like that sound wildly successful. But what does it really mean?
Let's take this study for example:
Peer-Reviewed Report on Moderna COVID-19 Vaccine Publishes
Data from Phase 3 Clinical Trial Confirm Vaccine is Effective
December 30, 2020
o Moderna's Covid vaccine, specifically messangerRNA-1273
o It was a trail that ran from July 27, 2020 thru November 21, 2020
o 30,420 participants in the trial
o 15,210 participants were injected with messangerRNA-1273
o 15,210 participants were injected with a placebo
o Thru November 25, 2020, 196 cases of symptomatic COVID-19 occurred
o That breaks down into 11 participants who were treated with the mRNA and 185 with placebo
o Moderna claims that the treatment had an 94.1% efficacy
How did they come up with 94.1% ?
Let's do the math:
In The Context Of This Study (ITCOTS), what was a placebo participant's chance of testing symptomatic for COVID-19?
(185÷15200) or 1.217%
ITCOTS, what was an mRNA participant's chance of testing symptomatic for COVID-19?
(11÷15200) or .072%
Plug those numbers into the following equation:
1 − ((11÷15200) ÷ (185÷15200)) results in .9405 Or as they state it 94.1%
As is common in these pharmaceutical trials, 94.1% is expressed as a RELATIVE difference. Stated another way, Moderna mRNA-1273 reduces the RELATIVE risk of symptomatic COVID-19 by 94.1%. Is that a good thing? It would be helpful to compare the RELATIVE difference to the ABSOLUTE difference.
Let's do the math:
1.217% - .072%, or 1.145%, is the ABSOLUTE percentage risk reduction
Stated another way, Moderna mRNA-1273 reduces the ABSOLUTE risk of symptomatic COVID-19 by 1.145%.
Which number is the barn--burner that gets your attention, 94.1% or 1.145% ?
In The Context Of This Study, stated in ABSOLUTE percentages:
o Placebo participant's had a 98.783% chance of not testing symptomatic COVID-19
o mRNA participant's had a 99.928% chance of not testing symptomatic COVID-19
I found this article to be helpful:
Reporting the findings: Absolute vs relative risk |