What measurement is typically used to assess vaccine efficacy?

The measurement used to assess vaccine efficacy is the percentage reduction in disease incidence among vaccinated individuals compared to unvaccinated individuals. This metric provides a clear and quantifiable way to evaluate how well a vaccine prevents disease compared to not being vaccinated.

The rationale behind this approach is that to determine efficacy, you need to know how many cases of the disease occurred in both groups over a defined period. By comparing the incidence rates, researchers can calculate the reduction in cases attributable to vaccination, which is fundamental to understanding how effective a vaccine is in preventing infection or disease.

On the other hand, simply counting the number of doses administered does not provide any insight into how well a vaccine works in preventing disease, since it does not consider the health outcomes of those vaccinated versus those unvaccinated. Similarly, while monitoring side effects is essential for safety and helps evaluate the vaccine's overall profile, it does not inform how effective the vaccine is at preventing the disease in the first place. Lastly, the total number of vaccines distributed is more related to supply and access rather than efficacy, as it does not involve evaluating health outcomes related to vaccination.