Methodology: Deaths are reported to the competent civil registry office and recorded in a central database. Assuming a constant reporting flow, the FSO estimates the number of cases. The number of deaths normally expected is calculated on the basis of the development of case numbers in each age group over the previous five years. The seasonal distribution of deaths over the 52 weeks of the year is calculated on the basis of the median of each calendar week of the past ten years. Finally, a range is calculated for each expected value within which fluctuations must be considered random. The calculation of the expected number of deaths thus does not simply correspond to an average value, but takes into account the change in the population from year to year as well as random fluctuations.

In mortality monitoring, the "observed" figures can thus be compared with the "expected" figures. The FSO has been publishing these figures weekly since 11 May 2015. The monitoring covers all persons resident in Switzerland who have died in Switzerland.

The annual case numbers on influenza reported in the cause of death statistics of previous years are not suitable and not comparable for monitoring acute infectious diseases. In the cause of death statistics, a single diagnosis of the disease that led to death must be selected for each death. According to the rules of the World Health Organization, this is the underlying disease that was at the beginning of the course of the disease and not the last event that finally led to death. Even in the case of multiple illnesses, the tables published worldwide on the causes of death include only one underlying disease.