What’s in a number? Let’s discuss vaccine efficacy.

What exactly is vaccine efficacy? Efficacy is a measure of how well a vaccine works in clinical trials – specifically, the percent reduction of disease by comparing two groups – one that receives the vaccine and one that doesn’t, under favourable and tightly controlled settings.

However, how one measures efficacy can vary significantly. Within the Covid-19 vaccine trials, there were several differences in the primary efficacy endpoints. For instance, some studies tested participants with any Covid-19 symptoms (even if one single symptom) whereas others only counted a case if it were a moderate to severe Covid-19 infection. In addition, these studies were also done at different time points, where variants may not have been circulating as much.

This means that we simply cannot compare primary efficacy numbers between trials. Instead, the numbers I would like you to focus on are those of hospitalization and death.

Here’s why:

1- there is no subjectivity on whether someone dies due to covid-19 or not. So this is a number we can compare against each trial

2- wouldn’t it be amazing if a vaccine can turn deadly Covid-19 infections to a much milder infection?

Bottom-line: ALL vaccines were nearly 100% effective in preventing severe illness, hospitalization, and deaths.

In the real-world, a whole host of factors can affect how well a vaccine works. This is known as the vaccine’s effectiveness. Often, vaccine effectiveness can be lower due to many reasons: individual factors such as age, immune response, comorbidities as well as environmental, behavioural factors.

The other aspect of vaccine efficacy has to do with what these numbers mean on an individual level. For instance, if a vaccine’s efficacy is 95%, it does NOT mean that your personal risk of getting Covid-19 infection is 5%. The numbers in the trials look at the efficacy on a population level. Whereas your personal risk will be dependent on a variety of factors and can be higher or lower. Again, important to remember that what we did see in the trials is that even if people did get Covid-19 post-vaccination, they still were protected from having severe illness.

So please do get the first vaccine that is offered to you!

And that’s the crux of the numbers debate!