5. Magnitude and dynamics
Daily and cumulative death curves. Daily mortality data by COVID-19 are particularly valuable for understanding the dynamics: in the short term to act and in the longer term to prevent and organize. What is the magnitude of the epidemic, what is its tempo? Are there similarities or differences between countries? Analysis of daily number of deaths allows us to assess the levels and rates of change.
Cross-country comparisons. The cross-country analysis of the progression of deaths by COVID-19 from day to day until mid-April allows identifying at the beginning of a COVID-19 epidemic an exponential increase in the number of deaths at the same daily rate. Then, after the implementation of physical distancing measures and/or lockdown and closure of businesses, the curves begin a plateau around the 30th Day of the epidemics, with variations depending on the country: the onset of this plateau is sometimes subject to marked fluctuations, with variations in the duration or in the level. A decrease in the number of daily deaths is then perceptible (situation of Italy and Spain and, to a lesser extent for the moment, France). The exception is South Korea, where the number of deaths stabilised very early on.
Daily death curves are highly fluctuating: these variations are most likely the result of information feedback in health monitoring systems that is not instantaneous or continuous; this point is all the more crucial as the feedback systems vary from one country to another but also within the same country, with recording times that can vary according to the place of death (hospital, medico-social institutions, at home). This effect is clearly illustrated in the figure below, by the fall in deaths published at weekends (the diamond signs on the curves indicate Sundays), followed by an increase reflecting the delayed publication of these deaths. It is important to analyse these trends carefully, for example by "smoothing out" these peaks and troughs or by preferring to interpret trends over longer periods.
Figure 5.1 Total and daily deaths by COVID-19 published in different countries (logarithmic scale) according to the day since the beginning of the epidemic Sundays are marked by dia.mond-shaped dots (until 23 April 2020).