Challenges in Estimating Time-Varying Epidemic Severity Rates from Aggregate Data

Severity rates like the case-fatality rate and hospitalization-fatality rate are key metrics in public health. To guide decision-making in response to changes like new variants or vaccines, it is imperative to understand how these rates shift in real time. In practice, time-varying severity rates are typically estimated using a ratio of aggregate counts. We demonstrate that these estimators are capable of exhibiting large statistical biases, with concerning implications for public health practice, as they may fail to detect heightened risks or falsely signal nonexistent surges. We supplement our mathematical analyses with experimental results on real and simulated COVID-19 data. Finally, we discuss strategies to mitigate this bias, drawing connections with effective reproduction number ( R t ) estimation.