To monitor mortality in Switzerland, the number of deaths in a given year are estimated based on the trends seen in the previous five years; distribution across individual weeks is estimated on the basis of the median value for each individual calendar week of the previous ten years. Separate estimates are carried out for those under the age of 65 and for those aged above 65. The mortality monitoring figures are based on the daily civil registry notifications, which are sent to the FSO for its Vital Statistics (BEVNAT) by the civil registry offices. The processing of notifications takes time. A sufficiently large percentage (> 85%) of deaths are usually registered after nine days. This allows an estimate to be made of the actual number of deaths based on a wide data base. The level of excess mortality is calculated based on the difference between the estimated and expected number of deaths. It is therefore an estimate.
Deaths are reported to the relevant civil registry office and registered in a centralised database (BEVNAT). Assuming a constant reporting flow, the FSO estimates the final figures expected for the current period. The number of weekly deaths normally expected in the current year is calculated based on the development of the number of deaths in the previous five years for the under-65 age group and the over-65 age group. Seasonal fluctuations in mortality are also taken into account. Lastly, a range is calculated for each expected value within which fluctuations must be considered random. Therefore, the calculation of expected deaths is not just an average value, but also takes into account changes in the population from year to year as well as random fluctuations.
The methodology used at national level has been extended to the major regions and all cantons, by making an individual extrapolation for each canton or major region and calculating the expected values according to the same principles. The projections and expected values for Switzerland as a whole are based on the major regions.
The mortality monitoring is based on the day-to-day reporting of deaths from the electronic civil register (computerised civil status register, Infostar). Deaths must be confirmed by a doctor’s death certificate and reported to the cantonal civil registry office concerned.
The monitoring includes all deaths occurring in Switzerland of people resident in Switzerland. It does not include the deaths of Swiss residents occurring abroad. The regional mortality monitoring by major regions and cantons is based on the deceased person's current place of residence.
The number of deaths from the mortality monitoring calculated for the current year are based on the daily civil status reports, which are sent to the FSO for its Vital Statistics by the communes (civil registry offices). After nine days there is usually a sufficiently large database. The reporting flow to the civil registry offices is assumed to be constant. After 40 days, 97.5% of all deaths have been reported and numbers can be shown without the need for extrapolation. The remaining 2.5% of reports arrive within the course of the year and are once received are included in the figures. With the conclusion of the statistical year for BEVNAT in the following year, the number of deaths within the mortality monitoring are also definitive.
Calculations for mortality monitoring are made in four stages. (1) At the start of each year, extrapolation weights for reporting delays are calculated. (2) To calculate the expected number of weekly deaths for the current year, the numbers registered in the excess mortality periods in previous years with Covid-19 related excess mortality are replaced by the expected numbers. (3) The weekly expected number of deaths for the current year is estimated on this basis and an upper and lower limit (range) is calculated. (4) Every Tuesday, the current number of deaths are compared with the expected figures.
(1) Number of deaths (extrapolation)
Based on the deaths of the previous year, this distribution is used to calculate weighting factors to correct the current year’s number of deaths in respect of reporting delays. Separate weighting factors are calculated for each canton and major region to take into account individual reporting characteristics.
(2) Excess mortality (COVID-19 pandemic)
During the COVID-19 pandemic, excess mortality was observed primarily for the 65 years and above age group. This excess mortality varied regionally and over time. In order not to overestimate the mortality in this age group for the affected regions and periods in the following years, the number of deaths in the respective regions and periods is corrected. For this purpose, if the number of deaths observed among the 65 years and above age group during the periods of excess mortality was more than one standard deviation above the expected value, then it is replaced with the expected value.
(3) Upper / lower limit for expected numbers
The number of deaths to be expected annually is estimated separately for the group aged 0-64 years and for the group aged 65 and over. Here, for the regression analysis and prediction of the expected values, additional subcategories of the two age groups are formed, which are later added up again. This is done based on the number of deaths from the previous 5 years, taking into account random fluctuations in trends by smoothing. This takes into account the development of the population over time. Furthermore, the expected number of deaths is estimated for each week of the current year. This is based on the weekly number of deaths from the previous 10 years, again smoothing out random fluctuations. This takes into account seasonal fluctuations in mortality. Both forecasts are combined by scaling the weekly forecasts so that the sum of expected deaths equals that calculated from the forecast for the current year. For the expected weekly deaths, the statistical assumption of a Poisson distribution is made. This makes it possible to determine upper and lower limits of the predictions within which actual deaths can normally be expected (in weeks without exceptionally increased mortality) in 99 out of 100 weeks.
(4) Weekly analysis
In the weekly evaluation on Tuesdays, the daily deaths already registered by then are multiplied by a projection factor so as to estimate the final number of daily deaths available only later due to reporting delays. This results in the time series shown as the number of deaths (extrapolation) for the whole of Switzerland as well as for the major regions and cantons. The expected numbers from step (3) are added with their range.