Noticed many people misinterpreting efficacy as effectiveness while talking or commenting around Pfizer/Moderna vaccines for Covid-19

Efficacy is basically the performance of a treatment under an "ideal and controlled environment" like clinical trials (1/4)

Effectiveness is generally measured once the treatment or vaccine in this case is approved for use of general public, i.e "real-world conditions". Important parameters are tracked which include vaccine performance for different groups (age, ethnic background etc.) (2)

..duration of protection (duration of immunity and effectiveness against evolving virus strains), the balance of benefit against harms. Cost effectiveness is also an important consideration, based on comparisons with other vaccine and treatment options (3)

So, efficacy of over 90% is definitely encouraging although there is no measure of effectiveness as of now & can be measured once the Covid-19 vaccines are out. Historically, most of the vaccines have effectiveness in the range of 40-70% but do save millions of lives annually (4)