vaccine effectiveness

Vaccine effectiveness and vaccine efficacy answer two different questions. The World Health Organization defines efficacy as what a controlled clinical trial measures, comparing how many vaccinated people developed the outcome of interest against how many in the placebo group did. Effectiveness is the same idea taken outside the trial: how well the vaccine performs in the real world, once it is being given to whoever turns up.

The two figures can differ, and the reason is the population. A trial recruits people who meet its entry criteria and follows them under study conditions. A national programme reaches every age, every underlying illness and every level of frailty, delivered by ordinary services with ordinary gaps in timing. WHO notes that trials cannot perfectly represent that diversity, which is why effectiveness has to be measured separately after rollout.

Effectiveness is usually estimated by comparing vaccinated and unvaccinated people within routine health records, so it carries the uncertainties of observational work rather than of a randomised trial. Estimates are reported with a range around them, and a programme's effectiveness can also change over time as the virus changes, as protection wanes, and as the people still coming forward for vaccination differ from those who came first.