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In fact there are two components of vaccine protection - direct and indirect effect.

 

Direct effect is the prevention of infection by the vaccine in a vaccinated individual

 

Indirect effect is the effect of the vaccine at the population level. With increasing coverage, there is a fall in the number of cases and thus sources of transmission in a given population and thus a reduction in the risk of infection in the community - so called herd immunity

 

Measures of VE (i.e. RCT) are usually randomised at an individual level to eliminate the indirect effect and thus measure only the direct effect of the vaccine, whereas effectiveness trials measure both direct and indirect effects with the unit of allocation  groups of individuals