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Does “95% Effective” Mean 5 Out of 100 Will Still Get COVID Despite Vaccination? Well … No

When tabulating instances of astonishingly bad luck, spare a thought for the person that participated in the Moderna vaccine trial, was given the placebo, contracted COVID, and died of complications relating to their diabetes.


Given the 95% efficacy of that vaccine, even for those with conditions like diabetes, had they been given the vaccine in the trial instead of the placebo, they may still be alive. In fact, it’s almost certain they would be, since no one in the vaccine cohort died, or even really experienced major “adverse events,” as they put it.


The Moderna vaccine is extraordinarily safe. Of the 600,000 doses administered in Canada by January 9, Health Canada had received only 90 reports of “adverse events” following the vaccination, 27 of which were considered “serious.” Quite often it is unclear whether the serious AEFI (adverse event following immunization) is causally linked to the vaccination, or due, say, to some bad salmon.


The United States FDA’s approval of the Moderna vaccine notes similarly that the vaccine is very safe. Common side effects are a sore arm, a bit of fever, wooziness, and a “general sense of ennui,” although they concede the latter may be related to finishing Bridgerton.


Most of us feel a lot worse for a long longer after polishing off a Margaret River Pinot Grigio.


But what do they mean they say it is 95% effective? That 5 out of every 100 will still get COVID even if vaccinated? Sort of. First up, we need to be clear that that 95% is related to “efficacy,” not effectiveness. Even Saskatchewan’s own vaccine cheat sheet gets those two mixed up. The efficacy figure of 95% is the Moderna study’s statistical determination based on the relation between the number of people in the trial that got the placebo and got COVID, and the number in the vaccine cohort that didn’t.


It might be easier to put it like this:


So far, there have been 33,000 vaccinations in Saskatchewan. If we took a random sample of 33,000 who hadn’t been vaccinated and compared them to the 33,000 which had, this is what we’d come up with–based on Moderna’s figures: 15 out of 33,000 in the vaccine cohort would contract COVID, compared to 270 in the placebo cohort.


The reason we don’t say that 5 out of every 100 would still get COVID even if vaccinated is because that would assume the rate of infection is 100%, which it clearly isn’t, and never will be.

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