How is vaccine efficacy calculated?

November 1st 2021 - The 'real world' efficacy analysis, performed weekly by the Istituto Superiore di Sanità – ISS (Italian National Institute of Health), is based on the pandemic data collected daily on the population and compares the frequency of the various events (infection, illness, illness with hospitalization, illness with hospitalization in intensive care, death) between the vaccinated and the unvaccinated. This means that, unlike the results of efficacy trials carried out in the laboratory, these analyses take into account complexity due to a number of factors determined by the real context in which vaccination takes place. This includes information such as the presence of variants or changes in behavior, which make their interpretation more robust.

Two different reports are derived from the analysis:

• Each week an estimate is made based on a comparison of the incidences of infections, hospitalizations and deaths between vaccinated and unvaccinated, on the basis of data from the beginning of the pandemic. The results are available in the 'extended report' published weekly by the Institute.
• Periodically the “Impact of vaccination” report is published, in which efficacy is instead calculated by comparing the incidences among the vaccinated with those of the same sample in the period between 0 and 14 days after the first dose.

Results are shared with the competent authorities. For example, the most recent vaccine data analysis has shown that vaccine efficacy in preventing any symptomatic or asymptomatic diagnosis of COVID-19 in fully vaccinated persons decreased from 89%, during the epidemic phase with alpha variant prevalent, to 76% during the epidemic phase with delta variant prevalent.