Significant rise of Chlamydia pneumoniae infection in 2024 in Marseille, France

We detected in October 2024 an abnormally high number of Chlamydia pneumoniae diagnoses through real-time surveillance of infections in Southeastern France, which followed significant increases in Mycoplasma pneumoniae and Bordetella pertussis diagnoses. Therefore, we retrospectively analyzed C. pneumoniae quantitative polymerase chain reaction (qPCR) results performed on respiratory samples collected between 2018 and 2024 in our center and described features of these infections.

C. pneumoniae qPCR was part from a multiplex syndromic panel or an in-house simplex qPCR assay. Next-generation sequencing was performed directly on available respiratory sample residues using Oxford Nanopore/Illumina technologies.

We observed a 19-fold increase of C. pneumoniae qPCR positivity in 2024 vs 2018-2023. Five (0.02%) of 25,255 respiratory samples were positive during 2018-2022, five (0.12%) of 4294 in 2023, and 37 (0.64%) of 5795 in 2024 (21 during September to October). Cases were mostly in children, followed by young adults. The highest incidence was in children aged 11-15 years (eight of 1075, 0.7%) and 6-10 years (eight of 1669, 0.5%). We obtained four (near) full-length C. pneumoniae genomes. They were of serotype ST16 and those the most closely related with each other, apart from the six other ST16 genomes from GenBank, suggesting an epidemic spread in our area.

The present findings warrant a close monitoring of diagnoses of C. pneumoniae infections at the local and country scales and to implement genomic surveillance and characterize drug resistance for diagnosed cases.