To gain insight into possible causes, general practitioners from the GP practices that serve as Nivel monitoring stations spent a short time period taking nose and throat swabs from patients with pneumonia and sending the samples to RIVM. RIVM examined the samples in the laboratory. During this process, multiple viruses and bacteria were detected that could have caused the increase in cases of pneumonia.

Other information sources in the Netherlands

RIVM also consulted other sources of information to identify any salient details that would indicate the presence of viruses and bacteria in the Netherlands. These sources are: the Infection Radar survey, data provided by laboratories, the database on antimicrobial resistance (ISIS-AR), and information on cases of pneumonia involving notifiable diseases. The information from those sources is outlined below. This corroborating data also shows that there was no single virus or bacteria that was responsible for the increase in pneumonia.

Information from Infection Radar 

Infection Radar is a weekly survey-based study tracking respiratory systems among people in the Netherlands. Survey participants with symptoms can take a self-sample of throat and nasal mucus using a cotton swab. Since October 2022, Infection Radar has asked 200 random participants with respiratory symptoms to take a sample each week. RIVM tests these samples to see which viruses and bacteria could be causing the respiratory symptoms. In this context, it is not possible to differentiate between people who have general respiratory symptoms and people with pneumonia. However, it was striking to note the incidence of Mycoplasma pneumoniae bacteria. The bacteria were not found between October 2022 and June 2023. Between July 2023 and January 2024, Mycoplasma pneumoniae were found 54 times – and 44 of those were in the period from November 2023 to January 2024. 

Laboratory data on viruses and bacteria

The weekly figures provided by Dutch laboratories (the virological updates) show that Mycoplasma pneumoniae bacteria were observed more often during this seasonal period for respiratory infections. The number of times the bacteria were detected increased from 24 in week 40 of 2023 to 186 in week 50. After a second peak in week 3 of 2024 with 181 detections, the trend went downward again. However, it is still high. The weekly number of reported cases was higher this winter than the maximum per week in these seasons since 2005. These weekly virological updates do not offer any information about the patients’ symptoms or age. That means that it is unclear whether these patients had pneumonia.

Figures from antimicrobial resistance database (ISIS-AR)

Laboratories regularly test patient samples, such as blood or mucus, at their doctor’s request. If bacteria are found, they are also checked for susceptibility to antimicrobials. The information about these bacteria cultures is compiled in the ISIS-AR system. Not all bacteria that could potentially cause pneumonia are included in this system; that includes Mycoplasma pneumoniae.

The 2023 figures from ISIS-AR show a higher incidence of the Haemophilus influenzae bacteria compared to other bacteria. There were too few samples from people under 18 years old to do reliable analyses.

Information about pneumonia and notifiable diseases

Some infectious diseases are subject to a mandatory reporting requirement: doctors must always notify the Municipal Public Health Services (GGDs) if they encounter cases of these diseases. The database on reported cases of notifiable diseases shows the following:

• In November and December 2023, there were more reported cases of serious infections caused by Haemophilus influenzae type b among children aged 0-5 years old than in the preceding years. However, these children did not have pneumonia. In January 2024 (based on data up to 17 January), there were no reported cases in people under 35 years old.
• In November and December 2023, invasive pneumococcal disease was somewhat higher among adolescents and young adults aged 15-25 years than in previous years. This trend continued in January 2024.
• The number of reported cases of serious infections involving Group A streptococcus rose in December 2023. This follows the expected trend at the start of the season when Group A Strep is common. The number of reported cases of pneumonia among young children with Group A Strep remains low.