efficacy and efficiency

We have been invited to see the distinction between these two words in the discussion around the various vaccines that have emerged as frontrunners of late.

 Efficacy is a French borrowing into English, ultimately from the Latin efficare which breaks down into the prefx  ef- or –ex  meaning ‘out, through’ and the verb facere  meaning ‘to make’.  

 Efficient comes from the same Latin origin but its path in English is a little different.

 With efficacy the emphasis is on getting the right results.

With efficiency the emphasis is on the smooth path to getting those results.

 So when it comes to judging vaccines we talk about the efficacy of the product which has been revealed in tests and trials.  The highest efficacy achieved so far is 95%.

 But these tests and trials are all conducted under closely controlled and monitored conditions.  What will happen when the vaccine has to go out into the world, survive all the conditions that it will experience there, survive the different levels of skill of the people administering it?

 If it can travel well in all sorts of circumstances and still produce adequate results, then it will have shown itself to be efficient.  The vaccine with the greatest efficacy may not necessarily have the greatest efficiency.  For example, the top runner at the moment needs to be stored at a very low temperature. Achieving that in the real world may be difficult and may decrease its efficiency.