How effective are the COVID-19 vaccines? How long does the immunity to COVID-19 last after getting the vaccine? If a vaccine has 95% efficacy, what happens to the people who are the other 5%? Do they have no immunity at all?

Question: 

How effective are the COVID-19 vaccines? How long does the immunity to COVID-19 last after getting the vaccine? If a vaccine has 95% efficacy, what happens to the people who are the other 5%? Do they have no immunity at all?

Answer: 

How our immune system works

Our immune systems don’t work like a light switch that goes simply on or off. Think of it more like a dimmer switch. With dimmer switches, the switch can go all the way up, but it can also go somewhere in between fully bright and completely off. If you stopped the switch in the middle, it’s not off - you can still see, but it is dim. 

The immune system is in many ways similar to the dimmer switch. When you are vaccinated, the switch to make antibodies in your body is slowly turned up and more and more antibodies get made. Antibodies are what give you immunity.

A vaccine with 95% efficacy means that in the clinical trials, 95% of people had full protection after getting the vaccine. This doesn’t mean that the other 5% did not get any protection. It just means that their immune system “dimmer switch” got partially there, and that without the vaccine there would have been zero protection. Their body made antibodies, but not enough to be considered fully protected.

However, when it comes to COVID-19 we also don’t know the number of antibodies needed to be considered fully protected. We are currently using our best estimate, based on what we know about our bodies. This is not new. We have seen this with other vaccines before, such as the MMR vaccine. With the MMR vaccine, studies later revealed that when we gave everyone two doses one month apart almost 100% of people were actually fully protected.

In fact, studies in the real world - including the recently published UK study on 7.5 million older adults - shows that there is comparable effectiveness of the AstraZeneca (COVISHIELD) COVID-19 vaccine to the Pfizer mRNA COVID-19 vaccine. All of the COVID-19 vaccines that have been approved in Canada provide excellent protection against severe disease and death from COVID-19.

Clinical Trial Results

Clinical trials are not all the same, and cannot be compared to each other. 

In the clinical trials, both mRNA vaccines were about 95 percent effective in preventing COVID-19 after receiving two doses. Protection, beginning 14 days after one dose of either vaccine, is greater than 90 percent. The duration of protection is not yet known for one or two doses of the vaccine but is being actively monitored. In general, some vaccines provide lifelong immunity, while others such as tetanus only work for 10 years.

In the clinical trials, the AstraZeneca COVID-19 vaccine was about 62% effective in preventing symptomatic disease starting 2 weeks after the second dose. However, AstraZeneca confirms 100% protection against severe disease, hospitalization and death in the primary analysis of their Phase III trials. 

In the clinical trials, the Johnson & Johnson (Janssen) vaccine was about 66% effective in preventing moderate to severe COVID-19 and 100% effective in preventing COVID-19–related hospitalization and death after receiving one dose.

Do these clinical trial statistics mean that one vaccine is more effective than the others? No. These were not head to head trials, and so cannot be directly compared. There is no need to worry about differences in either effectiveness or safety of these vaccines when your turn to be immunized comes up.

Long Term Immunity

We don't currently know how long immunity from the COVID-19 vaccines lasts, but the research is ongoing. COVID-19 vaccine trial participants continue to be monitored so we will learn more. However, we do know that right now we can safely reduce the risks of COVID-19 disease by getting vaccinated.

The degree to which these vaccines protect against COVID-19 one or two years after vaccination will be determined in the future, and public health experts will recommend whether booster shots are needed as we get more information.

Date last updated: 
Monday, Mar 15, 2021
Date last reviewed: 
Friday, Mar 12, 2021