Is efficacy and effectiveness of the COVID19 vaccine the same thing?

2 minute read

Both efficacy and effectiveness sound similar. They have similar meanings with subtle yet important differences.

Efficacy: the ability to produce a desired or intended result
Effectiveness: degree to which something is successful in producing a desired result
Oxford Languages

In clinical trials to study efficacy, the vaccine is administered to a test group (A) of people and a placebo is given to a control group (B) of people comparable to the test group.

Efficacy of the Vaccine

Efficacy is the ability to achieve results under ideal or controlled conditions. So, the vaccine’s efficacy is the % reduction in disease incidence in a vaccinated group (A) compared to an unvaccinated group (B) under optimal conditions.

The vaccines are still in trial phase. This means today we have more data about the efficacy, that is the ability of the vaccine to prevent spread of the SARS-CoV-2 virus.

Moderna’s experimental COVID-19 vaccine has a 94% efficacy against symptomatic infection based on final results of its late-stage clinical trial. Pfizer showed a similar efficacy. AstraZeneca and Sputnik vaccines claim a 90% efficacy in their trials.

These are high numbers and it is certainly a positive sign.

So is the vaccine with the highest efficacy also the most effective?

Effectiveness of the Vaccine

Effectiveness refers to the outcome given the vaccine’s efficacy and all other conditions. Imagine the management of all resources for best performance, with minimal waste. The vaccine effectiveness is the ability of a vaccine to prevent the disease in the ‘real world’, as WHO puts it.

“It’s very standard to see lower effectiveness once a vaccine is rolled out to the population compared to the efficacy measured in a trial.” Dr Bruce Y. Lee

The real effectiveness comes from protecting 7 billion people. The planning and execution of the wide scale distribution of the vaccine will govern how effective the vaccine is.

So how do I interpret the vaccine news coming out?

Questions to answer

How easily can the vaccine be stored, distributed and delivered? For example, the Pfizer vaccine needs to be stored in ultra cold freezers. Moderna claims a longer shelf-life for their vaccines.

We also need more data on people post-vaccination. Are there any side effects? Do the results vary across demographics? How long will the protection last? Is the effect short-lived?

More Questions in the real world

How many doses need to be administered and when? What is the cost and who will bear it? Can the pharmaceutical company and network handle the supply needs?

Are countries and Governments well prepared to handle the procurement and delivery logistics?

Some of these answers will help us move past the efficacy numbers.

Meanwhile, continue to wash your hands and wear masks.


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