vaccine efficacy

Vaccine efficacy is a number produced by a controlled clinical trial. The World Health Organization defines it by what the trial counts: how many people who got the vaccine developed the outcome of interest, usually the disease, compared with how many people in the comparison group developed the same outcome. If a vaccine has an efficacy of 80 percent, the vaccinated group had an 80 percent lower risk of developing the disease than the comparison group.

The figure is a reduction in risk between two groups, not a level of protection for any one person, and it is not a percentage of people protected. It always carries a hidden second half: efficacy against what. In most trials the comparison group receives a dummy injection, so the figure describes the vaccine against nothing. Where the comparison group receives an existing licensed vaccine instead, the result is a relative vaccine efficacy, and it describes only the gap between the two vaccines. A relative efficacy of 26 percent against another vaccine means about a quarter fewer cases than that vaccine produced, in a group that was already partly protected by it. It does not mean a quarter of infections were prevented outright.

Efficacy is also distinct from effectiveness. Efficacy is what a trial measures under controlled conditions in a selected group of volunteers; effectiveness is how well the vaccine works afterwards in the real world, in populations that are larger and more varied than any trial.