What does Covid-19 vaccine ‘Efficacy’ mean?

COVID-19 vaccines have been developed within less than a year after the virus was first detected. More than 80 are in clinical development and 182 are in preclinical development.  Several vaccines that have become available have shown different levels of efficacy.

Pfizer and Moderna have announced efficacy rates of 94-95%. AstraZeneca  has reported 79% efficacy and India’s Covaxin has announced 81% efficacy

So what does vaccine efficacy mean? 

Vaccine efficacy is the percentage reduction in disease in a group of people who received a vaccination in a clinical trial.

All the vaccines undergo clinical trials.  When a new vaccine is being tested, one group called a test group will be given that vaccine while the other group called a placebo group would be offered a placebo (a harmless inactive substance that looks like the vaccine that is being tested)

If a vaccine has, for example, 85% efficacy, an 85% reduction in new cases of the disease in the vaccine group compared with the placebo group.

Efficacy vs Effectiveness

When a vaccine is given outside a clinical trial, it is referred to as vaccine ‘effectiveness’. Compared with efficacy, effectiveness takes into account all the complexities of the real world, outside of a controlled clinical trial setting

It is said that currently, several vaccines are highly effective in real-world settings. However, one still needs to assess the full consequences for each vaccine’s effectiveness against newly identified variants