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Expected mortality in case there was no corona epidemic has been estimated based on the number of deaths in previous weeks, adjusted for seasonal effects. For weeks 11 through 20, the assumption is that the weekly death count is equal to weeks 3 through 10. The first few weeks of 2020 are not taken into account in these assumptions, because there was a brief influenza epidemic in weeks 1 and 2 which may have impacted mortality. The number of influenza-related deaths is estimated to have been relatively minor in subsequent weeks. As for seasonal effects, figures have been adjusted for the average weekly temperature. The assumption is that every one degree Celsius above the average weekly temperature in weeks 3 through 10 results in one percent decline in mortality. As for week 11 to 14 inclusive, the seasonal adjustment was particularly minor due to the minor temperature difference relative to week 3 through 10. In weeks 15 and 15 the weather was warmer and there were several percentage points of adjustment.
Excess mortality is the difference between the observed mortality and the level that could be expected in case there was no COVID-19 epidemic, based on estimates as per the method explained above. Excess mortality will be determined more precisely at a later stage, based on information from death certificates and in consultation with RIVM.