This document discusses methods for accurately estimating the effectiveness of vaccines. It notes that simply comparing the average severity rates of unvaccinated people (1) and vaccinated people (2) at times t0 and t1 could overestimate effectiveness, because those most at risk of severe illness were likely vaccinated first. To reduce this bias, the document suggests separately estimating severity rates for age groups at t0.5 and t1, since age is a major risk factor. This would provide a more accurate assessment of the true vaccine impact by minimizing differences between unvaccinated and vaccinated groups over time.