Vaccine studies: why allocate groups at random?

In a randomised trial, each participant’s group is assigned by chance: this is random allocation. The intervention chosen therefore does not depend on the participant’s or professional’s preference for that participant.

This method aims to distribute factors that may influence the outcome, both known and unknown, between groups. It helps comparison without guaranteeing perfectly identical groups or a well-conducted trial.

An observational study may compare people who had already chosen or received different vaccinations. Their age, health or medical history can also differ: these factors can influence the result independently of the vaccinations compared.

This is confounding. Statistical adjustments seek to account for it, but HAS notes that residual bias can persist in observational cohorts. A large number of participants alone does not remove it.

Randomised does not necessarily mean ‘vaccine versus placebo’. The flu trials described in the current official document use a placebo or another vaccine, depending on the study. Identify both group allocation and the comparator.

When reading a comparison, look for how groups were formed, which factors were accounted for and which limitations remain acknowledged. Discuss these points with a professional before relating the result to current campaign recommendations.