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I work at a COVID-19 vaccine clinic. Here’s what people ask me when they’re getting their shot — and what I tell them

In simple terms, 95 per cent efficacy means that vaccinated people have a 95 per cent lower chance of developing COVID-19.

September 2, 2021  by Ehsan Misaghi, Clinician-Scientist Trainee, Faculty of Medicine & Dentistry and Faculty of Science, University of Alberta

Once the mRNA vaccine is injected, it carries instructions to cells for how to make the SARS-CoV-2 virus’s spike protein, so the immune system can learn to recognize it and build an immune response to it. (AP Photo/Rogelio V. Solis)

As a medical student working with Alberta Health Services to vaccinate people against COVID-19, I have been asked my fair share of questions about the COVID-19 vaccines — from the need for booster doses to rare side effects.

A few days ago, I told an individual who was about to receive her second dose of the Moderna vaccine, “We are expecting about 95 per cent immunity two weeks from today.” She paused and asked, “What does that even mean?”

That scenario has repeated itself a few more times since then. I usually respond with, “It means you have 95 per cent less chance of developing COVID-19 two weeks after you have been vaccinated with the second dose of an mRNA vaccine.”

But what’s the long story behind that?

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mRNA vaccines

There are multiple vaccines against COVID-19. I’ll focus on the Moderna and Pfizer-BioNTech mRNA vaccines used in Canada. They both received emergency use authorization from the U.S. Food and Drug Administration (FDA) and Health Canada in December 2020. The Pfizer-BioNTech vaccine also received full FDA approval in August 2021. Now that it is fully approved, the Pfizer-BioNTech vaccine is also known by a brand name — Comirnaty — but it’s the exact same vaccine that’s been in use since December 2020.

Both the Moderna and Pfizer-BioNTech vaccines require two doses given at least three to four weeks apart. The mRNA (or messenger RNA) in the vaccines contains the instructions for how to make the now-well-known spike protein on the surface of SARS-CoV-2, the virus that causes COVID-19.

Once it is injected into a muscle in the upper arm, this mRNA gives the muscle’s cells the instructions to make the spike protein. The immune system practises combating this protein and learns how to react when it recognizes something that has that spike protein on it.

Down the line, if we are exposed to the actual SARS-CoV-2 virus, our body knows how to defend against the virus because it has built immunity by making antibodies against the spike protein on the surface of the virus. These antibodies are our bodies’ protective proteins against SARS-CoV-2.

Efficacy vs. effectiveness

The Pfizer-BioNTech and the Moderna vaccines are extremely efficacious and effective against COVID-19. But what do efficacious and effective mean in the context of a vaccine?

Vaccine efficacy is defined as the reduction in the rate of developing disease in vaccinated people compared to unvaccinated people. First, we would calculate the difference in cases between the two groups and then divide it by the rate of unvaccinated cases. For example, if eight out of 21,830 vaccinated people and 162 out of 21,830 unvaccinated people develop the disease, the efficacy of that vaccine would be calculated as:

(162 / 21830 – 8 / 21830) / (162 / 21830) = 95 per cent

These numbers are the actual numbers out of the Pfizer-BioNTech trial, which reported 95 per cent efficacy in its clinical trials. Moderna reported a similar number (94.5 per cent efficacy) in its trials.

Vaccine efficacy is usually measured under specific controlled environments and in the setting of double-blind randomized controlled trials (RCTs). A double-blind RCT is a study in which the participants are randomly assigned to either a placebo (no vaccine) or intervention (vaccine) group and neither the researchers nor the participants are aware which group they are assigned to. This setting reduces bias and increases the accuracy of the studies.

Now that we know how efficacy is measured, let’s see what 95 per cent efficacy really means. In simple terms, 95 per cent efficacy means that vaccinated people have a 95 per cent lower chance of developing COVID-19. So, if out of 10,000 unvaccinated people, 100 people get the disease, out of 10,000 vaccinated people, only five people might get the disease.

Real-world effectiveness

Vaccine effectiveness, however, is different from efficacy. Effectiveness is how well a vaccine works in reducing the rate of disease in vaccinated people compared to unvaccinated people under real-world conditions.

It’s worth noting that most studies have defined developing disease as testing positive for COVID-19 and having at least one symptom. The efficacy numbers can change based on the circumstances under which the vaccines are tested. For example, the location of testing, the method of testing, the presence of specific strains or variants of a disease-causing virus and the diversity of the participants can affect the efficacy numbers. That’s why demographic information is collected in clinical trials, including Moderna’s and Pfizer-BioNTech’s vaccine trials.

This means we can’t directly compare the efficacy of one vaccine to another if they have not been tested under the exact same conditions.

How well are the mRNA vaccines working?


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