Understanding Vaccine Efficacy and Effectiveness

Many different endpoints are used in vaccine research to define efficacy depending on the pathogen, consequences of infection and transmission dynamics.

Commonly, outcome data from Phase III randomised controlled trials (RCTs) are presented as a proportional reduction in disease between vaccinated and control subjects to calculate a vaccine-attributable reduction. Outcomes may include:

If you need a text version of the image above, this is available as a PDF.

Such RCTs represent ‘best case scenarios’ of vaccine efficacy under ideal conditions in particular populations and provide key data necessary for vaccine licensure. However, vaccine efficacy does not always predict vaccine effectiveness – the protection attributable to vaccines administered in real-life conditions. For example, the effectiveness of rotavirus vaccines in children in low and middle-income settings was lower than the efficacy observed in children in high-income countries.

Equally, RCTs conducted in a particular age group or geographical setting may not predict effectiveness if more widely deployed. For example, alternative vaccine platforms or the addition of adjuvants are required for adequate immunogenicity of influenza vaccines in older age groups. For this reason, prospective studies of vaccine effectiveness in ‘real-world’ scenarios post-licensure are routinely needed.

In the current pandemic, it is crucial that we find out if a vaccine against COVID-19 is successful. We will discuss how this is done in the next few steps.