This international retrospective Franco-Swiss study focuses on community-acquired pneumonia (CAP) caused by Chlamydia pneumoniae.

The working hypothesis is that the prevalence of C. pneumoniae pneumonia is overestimated by the medical community.

The primary objective is to determine the true prevalence of C. pneumoniae infections among patients with pneumonia who underwent simplex or multiplex PCR testing.

Secondary objectives include;

1. Outpatient management
2. Hospitalization in a medical ward
3. Admission to the intensive care unit
4. In-hospital mortality
5. Radiological presentation of patients with community-acquired pneumonia (CAP) caused by Chlamydia pneumoniae
6. Prevalence of viral or bacterial co-infections associated with C. pneumoniae CAP

This international retrospective Franco-Swiss study focuses on community-acquired pneumonia (CAP) caused by Chlamydia pneumoniae. The working hypothesis is that the prevalence of C. pneumoniae pneumonia is overestimated by the medical community. The primary objective is to determine the true prevalence of C. pneumoniae infections among patients with pneumonia who underwent simplex or multiplex PCR testing. Secondary objectives include;1)Outpatient management2)Hospitalization in a medical ward3)Admission to the intensive care unit4) In-hospital mortality5) Radiological presentation of patients with community-acquired pneumonia (CAP) caused by Chlamydia pneumoniae 6)Prevalence of viral or bacterial co-infections associated with C. pneumoniae CAP

Detailed Description We have all learned and continue to teach that community-acquired pneumonia (CAP) is essentially represented by three bacteria: Legionella pneumophila, Mycoplasma pneumoniae, and Chlamydia pneumoniae. While the recent Mycoplasma outbreak in France and the prevalence of severe Legionella infections in intensive care units confirm the real impact of these pathogens in CAP, the low number of Chlamydia pneumoniae cases documented by PCR raises questions.Over the past twenty years, the development of highly sensitive molecular tests (specific PCR or multiplex PCR targeting intracellular respiratory pathogens) has made it possible to precisely detect Chlamydia pneumoniae and to confirm or rule out its presence in the respiratory tract of patients with pneumonia.We are therefore conducting a retrospective study between France and Switzerland to determine, on the one hand, the number of CAP cases due to Chlamydia pneumoniae confirmed by PCR, and on the other hand, to better characterize the phenotype of these patients.

Inclusion Criteria:

• Positive simplex or multiplex PCR for Chlamydia pneumoniae on an upper or lower respiratory tract specimen between January 2015 and December 2025.
• Availability of clinical data allowing confirmation of pneumonia diagnosis.

Exclusion Criteria:

• Patient opposition to the use of medical data for research purposes.
• Absence of pneumonia diagnosis.
• Insufficient clinical data for analysis.

The study population consists of pediatric and adult patients from participating French and Swiss centers who had a positive simplex or multiplex PCR result for Chlamydia pneumoniae on an upper or lower respiratory tract specimen between January 2015 and December 2025. Eligible patients must have a documented diagnosis of pneumonia and available clinical data allowing characterization of disease presentation, management, radiological findings, co-infections, and outcomes. Patients who objected to the use of their medical data for research purposes or who did not have pneumonia are excluded.