As a medical student working with Alberta Health Services to vaccinate people against COVID-19, I’ve been asked my fair share of questions about COVID-19 vaccines – from the need for booster doses to rare up to side effects.
A few days ago, I told a man who was about to get his second dose of Moderna Vaccine, “We’re expecting about 95 percent immunity two weeks from today.” She paused and said, “What does that mean?”
That scenario has repeated itself a few more times since then. I usually respond with, “That means you have a 95 percent lower chance of developing COVID-19 two weeks after being vaccinated with the second dose of mRNA vaccine.”
But what is the long story behind it?
There are many vaccines against COVID-19. I will focus on the Moderna and Pfizer-BioNtech mRNA vaccines used in Canada. They both received emergency use authorization from the US 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’s fully approved, there’s a Pfizer-BioNTech vaccine. Also known by a brand name – Comirnati – but this is exactly the vaccine that has been in use since December 2020.
Both Moderna and Pfizer-BioNTech vaccines require two doses spaced at least three to four weeks apart. The mRNA (or messenger RNA) in vaccines contains instructions on how to make the now-famous spike protein on the surface of SARS-CoV-2, the virus that causes COVID-19.
Once it is injected into the muscle in the upper arm, this mRNA instructs the muscle cells to make the spike protein. The immune system practices counteracting this protein and learns how to respond when it recognizes something that contains that spike protein.
Down the line, if we are exposed to the actual SARS-CoV-2 virus, our body knows how to defend itself against the virus because it has built up immunity by making antibodies against the spike proteins on the surface of the virus. These antibodies are our body’s protective proteins against SARS-CoV-2.
efficacy vs effectiveness
The vaccines from Pfizer-BioNtech and Moderna are highly effective and effective against COVID-19. But what does efficacious and effective mean in the context of a vaccine?
Vaccine effectiveness is defined as the reduction in the rate of developing disease in vaccinated people compared to unvaccinated people. First, we’ll 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 people were vaccinated and 162 out of 21,830 got the disease, the effectiveness of that vaccine would be calculated as:
(162 / 21830 – 8 / 21830) / (162 / 21830) = 95 percent
These numbers are actual numbers from the Pfizer-BioNTech trial, which reported 95 percent efficacy in its clinical trials. Moderna reported a similar number (94.5 percent efficacy) in its trials.
Vaccine efficacy is typically 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 participants are randomly assigned to either a placebo (no vaccine) or intervention (vaccine) group and neither the researchers nor the participants know which group they were assigned to. has been assigned. This setting reduces bias and increases the accuracy of the study.
Now that we know how efficacy is measured, let’s look at what 95 percent efficacy actually means. In simple words, 95 percent efficacy means that people who are vaccinated have a 95 percent lower chance of developing COVID-19. So, if 100 people out of 10,000 people get the disease, then only 5 out of 10,000 people can get the disease.
real world effectiveness
However, vaccine effectiveness differs 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 is worth noting that most studies have defined developing disease as testing positive for COVID-19 and having at least one symptom. Effectiveness numbers can change depending on the conditions under which vaccines are tested. For example, the location of the trial, the method of testing, the presence of specific strains or types of disease-causing virus, and the diversity of participants can affect efficacy numbers. That’s why demographic information is collected in clinical trials, including Moderna and Pfizer-BioNtech’s vaccine trials.
This means that we cannot directly compare the effectiveness of one vaccine to another if they have not been tested under the exact same conditions.
How well are mRNA vaccines working?
With more than five billion doses administered worldwide, we are at a point where we can even see the effectiveness of COVID-19 vaccines. Preliminary studies have shown that both mRNA vaccines are about 90 percent effective against COVID-19 in the real world. The Alberta government has reported 93 percent effectiveness from Moderna and 90 percent effectiveness from the Pfizer-BioNTech vaccine.
And why does it take two weeks to develop that level of immunity? There are several steps in the process of getting a vaccine that immunizes our body against a disease. Remember protective proteins called antibodies? One of the final steps in the immune process is manufacturing those antibodies.
Based on studies done by vaccine manufacturers, about 14 days after the second dose, our bodies have made enough antibodies to recognize and fight off SARS-CoV-2, so the two-week regimen should give you a thorough considered to have been vaccinated.
An important statistic that needs to be mentioned is that both mRNA vaccines have been shown to prevent hospitalizations and deaths. This means that even in the rare case of a vaccinated person developing COVID-19, they are much less likely to be hospitalized or die if they were not vaccinated.
How do the COVID-19 vaccines compare to others?
Another question I have been asked is how is the effectiveness of COVID-19 vaccines compared to vaccines made for other diseases. Well, the MMR vaccine is 97 percent effective against measles and rubella and 88 percent effective against mumps. The effectiveness of the DTaP vaccine (diphtheria, tetanus, acellular pertussis) is between 80-85 percent. The effectiveness of flu vaccines ranges between 10-60 percent depending on the year the vaccines protect against each year and the actual strains that cause influenza and influenza-like illnesses.
All of these numbers reflect a reduction in disease rates between vaccinated and unvaccinated people. So the next time you hear that a vaccine is 95 percent effective, it doesn’t mean that 5 percent of people who get the vaccine will get the disease; This means that vaccinated people are 95 percent less likely to develop the disease than those who are not vaccinated.
Do you have questions about COVID-19 vaccines? Email us at ca‑[email protected] and vaccine experts will answer questions in upcoming articles.