Pandemics, Severity, and Context—Some Loose Ends

Simonsen, Lone; Viboud, Cecile

Early robust measurement of clinical severity is challenging in an emerging threat scenario. As the authors point out, the case fatality rate of the 2009 pandemic was at first overestimated, likely due to selection bias in the study of early case series. On the other hand, the first pandemic wave in the summer of 1918 was mild, and early measurement would have resulted in substantial underestimation of the serious main pandemic wave that autumn. The first type of measurement issue is being addressed; the second is impossible to gauge. Perhaps just knowing that this is possible, that a scenario goes from mild to severe, is worth remembering. On the upside, a mild herald wave provides time to mount a vaccine response before the main pandemic impact occurs.