Vaccine efficacy

Vaccine efficacy is the percentage reduction in a specified outcome among vaccinated participants compared with a control group in a study. It depends on the outcome, population, follow-up period, and study design.

Relative risk reduction: The proportional decrease in risk in one group compared with another. Vaccine efficacy is commonly calculated as one minus the relative risk in vaccinated versus control groups.
Vaccine effectiveness: The reduction in a specified outcome associated with vaccination under real-world conditions. Effectiveness is measured outside controlled trials, where exposure, adherence, and populations differ.
Clinical endpoint: A measured health outcome used to assess the effect of an intervention in a study. Efficacy has meaning only relative to a defined endpoint, such as symptomatic disease or hospitalization.
Phase III trial: A large clinical study that evaluates an intervention's benefits and risks in a target population. Vaccine efficacy is often established in phase III trials before broad authorization.
Waning immunity: The decline of immune protection over time after infection or vaccination. Longer follow-up can reveal whether measured efficacy decreases after vaccination.
Randomized controlled trial: A study that assigns participants to interventions by chance and compares their outcomes. Random allocation helps make outcome differences interpretable as effects of vaccination.
Absolute risk reduction: The difference in outcome risk between an intervention group and a comparison group. Unlike relative efficacy, it depends directly on the control group's baseline risk.
Attack rate: The proportion of an at-risk population that develops a specified condition during a defined period. Trial attack rates provide the risks compared in common efficacy calculations.
Vaccine dose: A measured quantity of vaccine administered at one time. Dose-ranging studies compare immune responses and clinical outcomes to select a regimen.
Vaccine escape: Reduced recognition or control of a pathogen by immunity induced through vaccination or infection. Pathogen evolution can alter efficacy against particular variants or strains.

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