Annual reporting on surveillance of acute respiratory infections in the Netherlands, winter 2023/2024 - Acute respiratory infections, influenza-like illness, bronchiolitis in children, and pneumonia

This page describes the surveillance of respiratory infections and their causes in the Netherlands during the respiratory season 2023/2024.

On this page, you can find information about the surveillance of:

acute respiratory infections (ARI), including influenza-like illness (ILI);
bronchiolitis in children;
pneumonia (limited to community-acquired pneumonia).

Since respiratory infections occur mainly in winter, the data are presented for a respiratory season or a respiratory year. A respiratory season runs from week 40 of one year to week 20 of the following year. A respiratory year runs from week 40 of one year to week 39 of the following year. In this retrospect the data from the respiratory year 2023/2024 is limited to the respiratory season.

Acute respiratory infections (ARI) and influenza-like illness (ILI)

Acute respiratory infections (ARI) and influenza-like illness (ILI) are clinical syndromes that can be caused by a range of different viruses and bacteria. The definition for ILI falls within the ARI syndrome but is more specifically focused on symptoms that may indicate an influenza virus infection, such as fever. For more details on the definition of ARI and ILI, see the background and method document on respiratory surveillance for 2023/2024.

Peaks

The incidence of ARI and ILI in the general population, as reported in Infectieradar, peaked at the end of 2023. The weekly number of ARI general practitioner (GP) visits and the ILI incidence at GPs peaked at the beginning of 2024. These peaks corresponded with peaks in the circulation of SARS-CoV-2, influenza virus, RSV, and other respiratory pathogens. The ILI threshold of 5.6 per 10,000 inhabitants per week was exceeded in week 50 of 2023 and in weeks 3 to 6 of 2024.

Viruses in people with respiratory complaints

During the respiratory season of 2023/2024, among Infectieradar participants with self-reported ARI who sent a self-test specimen to the RIVM, rhinovirus was the most commonly found across all age groups. During the peak incidence of ARI complaints in Infectieradar (week 50, 2023), SARS-CoV-2 was the most found virus in those self-test specimens.
In specimens from GP sentinel surveillance, from both with ARI (including ILI) patients and ILI patients, influenza virus A(H1N1)pdm09, rhinovirus, and SARS-CoV-2 were most commonly found during the respiratory season 2023/2024.
In the weeks of the peak of ILI and ARI consultations in GP sentinel surveillance, influenza virus was mainly found in sampled patients . Over the entire respiratory season 2023/2024, influenza virus A(H1N1)pdm09 and SARS-CoV-2 were most commonly found in older age groups (45 years and older), while rhinovirus was more commonly found in younger age groups (under 15 years). Although influenza virus and RSV were more frequently found in the specimens from GP sentinel surveillance than in the self-test specimens from Infectieradar, the trends over time in the occurrence of these pathogens were similar during this respiratory season.

Number of GP visits comparable to last year and pre-COVID-19 pandemic

During the respiratory season in 2023/2024, the total number of GP visits due to ARI was comparable to the previous season 2022/2023 and the five pre-COVID-19 seasons (2014/2015 to 2018/2019). The weekly number of ARI GP visits and the ILI incidence at GPs in 2023/2024 was highest in the youngest age group (0-4 years). This was also the case in previous seasons. In 2023/2024, the peak in the number of ARI GP visits among zero to four-year-olds occurred earlier than the peaks in older age groups. This is related to the fact that different viruses are more common in children than in adults. These viruses, such as RSV, may have a different seasonal pattern than viruses more common in adults, such as influenza virus. The total number of ILI patients in nursing homes during the respiratory season 2023/2024 was higher than in the previous four seasons and the five pre-COVID-19 seasons.

Footnote: The line in the graph represents the total number of tested specimens. A specimen can be positive for multiple viruses. Therefore, the stacked percentages in the graph may indicate a higher percentage of positive specimens than the actual percentage of positive specimens.

Footnote: Starting from this report, the GP ICPC code for COVID-19, R83.03, has been added to the definition of ARI from the 2022/2023 season onwards. Therefore, the reported numbers of ARI GP consultations in the respiratory season 2022/2023 are slightly higher than reported last year.

The black line shows the average number of GP consultations per week for the respiratory years 2014/2015 – 2018/2019, while the other lines show the weekly GP consultations of the subsequent respiratory years.

The black line shows the average ILI incidence for the respiratory years 2014/2015 – 2018/2019, while the other lines show the ILI incidence of the subsequent respiratory years.

Bronchiolitis in children in primary care

Bronchiolitis is an inflammation of the smaller lower airways (bronchioles). It is particularly common in young children under the age of 2 and is often caused by RS-virus (RSV). In primary care, there is one ICPC code for acute bronchitis/bronchiolitis combined. More information about the definition of bronchiolitis can be found in the background and method document on respiratory surveillance for 2023/2024.

Early Peak

The weekly number of GP consultations for acute bronchitis/bronchiolitis in children under 5 years followed the same trend as the RSV-specific surveillance sources during the respiratory season of 2023/2024, with a peak at the end of 2023. The peak, in weeks 46 and 47 of 2023, occurred earlier this respiratory season than in the previous four seasons (2019/2020 to 2022/2023) and the previous five pre-COVID-19 seasons (2014/2015 to 2018/2019) (see figure below).

Community-acquired pneumonia (CAP) in primary care

During the respiratory season of 2023/2024, 318 per 10,000 inhabitants consulted their GP for pneumonia. This is based on data from Nivel Primary Care Database. This was higher than in the previous four seasons (2019/2020 to 2022/2023) and the last 5 pre-COVID-19 seasons (2014/2015 – 2018/2019). In the summer of 2023, the number of pneumonia consultations increased, reaching a peak in week 6 of 2024 (14 GP visits per 10,000 inhabitants). This peak was later in time and higher than in the 2022/2023 season. The timing of the peak was similar to that in the last 5 pre-COVID-19 seasons, but the number of GP visits was higher in the 2023/2024 season. The weekly number of GP visits for pneumonia in 2023/2024 was highest among patients aged 65 and older. This is consistent with the previous four seasons and the last five pre-COVID-19 seasons.

Increase in pneumonia among 5- to 45-year-olds

The total number of GP consultations for pneumonia in 2023/2024 was higher than the average number in the nine preceding seasons. This was true for all age groups, but particularly for the age groups 5-14 years and 15-44 years, where the number of visits in 2023/2024 was significantly increased.

Mycoplasma pneumoniae

Other countries in Europe and beyond (including China) also reported an increase in pneumonia cases in 2023/2024, with the bacterium Mycoplasma pneumoniae identified as one of the main causative agents. Since the cause of pneumonia in the Netherlands is often unknown, it was unclear what caused the increase in GP visits. However, an unusually high number of M. pneumoniae detections was reported in the weekly virological surveillance. There was no information available on the clinical background and age of these patients. Therefore, the RIVM and Nivel conducted a follow-up study on the possible causative pathogens, as an extension of the GP sentinel surveillance.

Multiple viruses and bacteria

The results indicated that the bacterium M. pneumoniae likely played a significant role in the increase among older children and young adults, but various other viruses and bacteria also played a role. The peak in pneumonia cases has passed, but at the end of the reporting period (week 20 of 2024), the number of GP consultations for pneumonia was still higher than in the previous and pre-COVID-19 seasons.