New infographic: What’s Vaccine Efficacy? Should we be comparing numbers? How does it differ from Vaccine Effectiveness? What does it mean on an individual level?

Vaccine 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.

But the definitions of primary efficacy endpoints varied between studies. Pfizer tested if any one symptom 7 days after vaccine, Moderna tested if any one symptom 14 days after vaccine, AZ varied between each trial site (!!!), J&J only counted if moderate to severe Covid-19.

So these trial efficacy endpoints cannot be pitted against each other. However, death due to Covid is absolute. So this is a number you can compare (somewhat) against trials.

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

Vaccine efficacy differs from vaccine effectiveness. Latter is from real-world data. Can be lower for many reasons.

What does this mean on an individual level? Well, not much. If vaccine efficacy is 95% at a population level, it doesn’t mean that your chance of getting Covid is 5% if exposed. Your personal risk is based on a number of variables and can be lower or higher than this.

But again, even if you do get a Covid-19 infection post-vaccination, you are most likely protected against having severe illness.

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