How is the effectiveness of a vaccine typically measured?

The effectiveness of a vaccine is typically measured by the percentage reduction in disease incidence among vaccinated individuals. This approach evaluates how much the vaccination reduces the risk of developing the disease compared to those who are unvaccinated. This measurement is crucial because it provides a clear understanding of how well the vaccine protects the population from illness. It involves comparing the incidence rate of the disease in the vaccinated group to that in an unvaccinated group or within the broader population, providing insights into the real-world impact of vaccination efforts.

Other metrics, such as the number of doses administered or reported side effects, do not directly reflect the vaccine's ability to prevent disease. The number of doses might indicate a vaccination program's outreach but does not account for individual or population-level effectiveness in preventing illness. Similarly, while the duration of immunity can relate to a vaccine’s performance, it does not measure the effectiveness in preventing disease in the population. Thus, the percentage reduction in disease incidence remains the gold standard for assessing vaccine effectiveness.