Annual reporting on surveillance of acute respiratory infections in the Netherlands, winter 2023/2024 – Weekly mortality monitoring

Peaks in total deaths

The RIVM reports the total number of deaths weekly based on data from Statistics Netherlands (CBS). In the 2023/2024 season, there were two distinct peaks in these total numbers of deaths. The first peak occurred in the second half of the COVID-19 surge (week 51, 2023, with 3,904 deaths). The second peak was the highest, with 4,017 deaths in week 5 of 2024. This was at the beginning of the influenza epidemic, which started immediately after the COVID-19 surge. This peak is lower than the highest peaks in 2022/2023 (4,202 in week 51) and 2021/2022 (4,401 deaths in week 48). Previous seasons (2021/2022 and 2022/2023) also showed multiple peaks in mortality. In these seasons, the first peaks (during the COVID-19 surge) were higher than the second peaks (during the influenza epidemics).

Estimating excess mortality

Based on the total number of reported deaths, the RIVM estimates whether there is excess mortality. This means that more people die in a certain period than expected based on mortality rates in this period in previous years. Excess mortality often coincides with a significant event, such as an influenza epidemic, a heatwave, a cold spell, or during the COVID-19 pandemic. More background information on this mortality monitoring can be found in the background and method document on respiratory surveillance for 2023/2024.

Excess mortality in ten weeks during the 2023/2024 respiratory season

During the 2023/2024 respiratory season (week 40, 2023, to week 20, 2024), excess mortality was observed in ten of the 33 weeks. This occurred in weeks 44, 45, 49, 50, 51 of 2023, and weeks 1, 3, 4, 5, and 6 of 2024. From week 49, 2023, the excess mortality coincided with the COVID-19 surge and the subsequent influenza epidemic.

Lowest excess mortality since the start of the COVID-19 pandemic

The total excess mortality for the entire 2023/2024 respiratory season was 4,222. This was the lowest since the start of the COVID-19 pandemic. In the previous three respiratory seasons, the total excess mortality was 15,301 (2022/2023), 13,179 (2021/2022), and 7,121 (2020/2021). The excess mortality in 2023/2024 was also lower than in the last five pre-COVID-19 respiratory seasons (an average of 7,285 excess deaths from 2014/2015 – 2018/2019). During the 2023/2024 respiratory season, excess mortality was most observed in the age group 80 years and older.

Excess mortality during COVID-19 surge and influenza epidemic

The total excess mortality was 2,275 during the 12 weeks of the COVID-19 surge (week 34, 2023, to week 4, 2024). The total excess mortality was 2,050 during the nine weeks of the influenza epidemic (week 3 – week 11, 2024). This is lower than the excess mortality in the five influenza epidemics before the COVID-19 pandemic, where the average excess mortality was 6,424 cases per influenza season (2014/2015 – 2018/2019).

Figure 1. Observed and expected (‘baseline’) weekly number of deaths, July 2021 to May 19th 2024, by date of death (Source: Statistics Netherlands).

This figure shows the weekly number of deaths from July 2021 up to May 2024. The figure shows that the number of deaths highly exceeds the threshold during multiple periods. The highest peak in 2023/2024 occurred with 4,017 deaths in week 5 of 2024, which is 548 above expected baseline level. This peak is lower than the highest peak in 2022/2023 (4,202 in week 51 with 884 excess deaths) and in 2021/2022 (4,401 deaths in week 48, with 1,296 excess deaths).

Legend:

Blue shading: expected number of deaths (calculated using historical data in which extremes were excluded).
Black line: observed number of deaths per week.
Grey (vertical) shading: influenza epidemic weeks.
Orange shading: COVID-19 surge weeks in the Netherlands. The COVID-19 epidemic began in March 2020. The WHO declared the end of the pandemic on May 5, 2023.

Figure 2. Historical trends: observed and expected ('baseline') weekly number of deaths from 2013 to May 19th 2024, by date of death (Source: Statistics Netherlands).

This figure shows trends in weekly mortality numbers in the Netherlands from 2013 up to May 2024. The number of deaths shows a cyclical trend with the number of deaths being higher in the winter months than in the summer months. There was no influenza epidemic in 2020/2021.