Abstract
The COVID-19 pandemic had a profound global impact, marked by high morbidity and mortality rates. It also placed substantial strain on epidemiological understanding, complicating efforts to measure the pandemic's scale and speed of transmission, as well as the implementation of mitigation and control strategies. Throughout the pandemic, key epidemiological concepts—such as the basic reproductive number, serial interval, incidence rate, fatality rate, and effective reproductive number—came under intense scrutiny. New concepts, like incidence moments, were even developed to better assess the scale of the outbreak and support temporal forecasting.
In this analysis, we trace the chronological relationship between the progression of the pandemic and the application of these key epidemiological concepts. We briefly examine their historical development, definitions, significance, interconnections, and how they informed both non-pharmaceutical interventions and, later, pharmaceutical measures with the introduction of vaccines. We show that the reproductive number proved to be a particularly valuable tool, guiding the design of epidemiological interventions and helping to define conditions for herd immunity and the vaccination threshold. In contrast, crude case fatality rates were often misleading, requiring adjustments to account for the time lag between infection and death. While incidence rates and reproductive numbers were essential for understanding disease burden and transmission dynamics, they were unsufficent in forecast future case counts. The concept of herd immunity was also critical, but its implications were frequently misunderstood. It was necessary to clarify that reaching the herd immunity threshold does not abruptly end an epidemic, and to highlight its dependence on vaccination thresholds and vaccine effectiveness.