Blinded vaccine studies: who knows which vaccine was received?

In a blinded study, certain people do not know which intervention was received. The HAS methodological guide explains that this aims to limit bias in measuring or classifying outcomes, particularly when their assessment has a subjective component.

The participant may be blinded: knowing the assigned group can influence how a result is reported or assessed. The guide also identifies the professional administering the intervention, the outcome assessor and the statistician; they do not necessarily all have the same information.

An assessor can, for example, evaluate an outcome without knowing the received intervention, even if other people know it. HAS therefore distinguishes complete, partial and assessment-only blinding. Read the actual description of the roles to understand which information was hidden.

‘Double blind’ usually describes a study in which the patient and clinician do not know the received intervention. A ‘single blind’ label alone does not identify who was kept unaware. The protocol needs to specify who knew what.

The HAS report validated in April 2025 describes a flu study conducted in the United States during the 2013-2014 season as single blind. This is a historical example of a method: the term provides neither an outcome of the current campaign nor a guarantee of individual benefit.

When reading a study result, ask who did not know the received vaccine and how the outcome was assessed. Random allocation and blinding have different functions; one does not automatically establish the other. Blinding does not remove the need to check the study’s other limitations.