Eurosurveillance - Volume 30, Issue 23, 12 June 2025

During the COVID-19 pandemic, non-pharmaceutical interventions (NPIs) such as social distancing, lockdowns and enhanced hygiene led to a decrease in respiratory pathogens. However, as NPIs were relaxed, a resurgence in several respiratory pathogens was observed including one local Chlamydia pneumoniae outbreak in Switzerland, prompting the need for a better understanding of C. pneumoniae epidemiology.

To assess temporal and geographical variations in C. pneumoniae detection before, during and after the COVID-19 pandemic.

Data on C. pneumoniae PCR detection ratios (number of positive tests/ total number of tests) across pre-pandemic (2018–2019), pandemic (2020–2022) and post-pandemic (2023) periods were collected via a global survey disseminated through various professional networks.

C. pneumoniae detection ratios were analysed across 28 sites (27 in Europe, one in Taiwan) in 2023 (Dataset A, n = 172,223 tests) and 20 sites from 2018 to 2023 (Dataset B, n = 693,106 tests). Twenty-seven sites were laboratories (hospital or clinical) and one a surveillance system (Denmark). A significant decrease in detection ratios was observed during the pandemic period (from 1.05% to 0.23%, p < 0.001). In 2023, detection ratios increased to 0.28% (p < 0.002). Notable regional variations were found, with statistically significant increases in detection ratios at six sites located in Switzerland and Slovenia, where ratios ranged from 0.52% to 3.25%.

The study highlights how NPIs influenced C. pneumoniae epidemiology, with reduced detection during the pandemic and partial resurgence afterwards. Regional variations suggest differing NPI impacts and underscore the need for continued surveillance.

Influenza A(H1N1)pdm09, A(H3N2) and B/Victoria viruses circulated in Europe in 2023/24, with A(H1N1)pdm09 dominance. First influenza infections in childhood may lead to different vaccine effectiveness (VE) in subsequent years.

The VEBIS primary care network estimated influenza VE in Europe using a multicentre test‐negative study.

Primary care practitioners collected information and specimens from patients consulting with acute respiratory infection. We estimated VE against influenza (sub)type and clade, by age group and by year of age for A(H1N1)pdm09, using logistic regression.

We included 29,958 patients, with 3,054, 1,053 and 311 influenza A(H1N1)pdm09, A(H3N2) and B cases, respectively. All-age VE against influenza A(H1N1)pdm09 was 52% (95% CI: 44–59). By year of age, VE was 27% (95% CI: −2 to 47) at 44 years with peaks at 72% (95% CI: 52–84) and 54% (95% CI: 41–64) among children and those 65 years and older, respectively. All-age A(H1N1)pdm09 VE against clade 5a.2a was 41% (95% CI: 24–54) and −11% (95% CI: −69 to 26) against clade 5a.2a.1. The A(H3N2) VE was 35% (95% CI: 20–48) among all ages and ranged between 34% and 40% by age group. All-age VE against clade 2a.3a.1 was 38% (95% CI: 1–62). All-age VE against B/Victoria was 83% (95% CI: 65–94), ranging between 70 and 92% by age group.

The 2023/24 VEBIS primary care VE against medically attended symptomatic influenza infection was high against influenza B/Victoria, but lower against influenza A(H1N1)pdm09 and A(H3N2). Clade- and age-specific effects may have played a role in the lower A(H1N1)pdm09 VE.

Carbapenemase-producing Enterobacterales (CPE) frequently cause nosocomial outbreaks. To investigate these, tracing focused on patients with related CPE strains and spatiotemporal contact (e.g. contact with each other in a room or on a ward during overlapping periods) has limitations. Moreover, as widely available molecular typing methods cannot detect plasmid-related transmissions, carbapenemase gene transfer across enteric bacteria through plasmids in hospitals remains poorly understood.

Because whole-genome sequencing (WGS), particularly long-read sequencing, can offer insights into bacterial relationships both at core-genome and plasmid levels, we tested its utility, using VIM-CPE as example, to investigate plasmid and CPE spread in a hospital beyond outbreaks.

We included inpatient episodes from 2018 to 2021 involving blaVIM-bearing CPE isolates. Short- and long-read WGS data were combined with patient movement information to identify genomically related hospital-acquired VIM-CPE and putative transmission routes.

Among 43 included inpatient episodes, 27 isolates were hospital-acquired, with 23 genomically related based on core-genome or plasmid analyses. For 14 of these 23 isolates, patient movement data supported suspected transmission events. Plasmid and core-genome level analyses revealed that most transmission events did not temporally concur, occurring over up to 33 months. Thus, conventional infection tracing methods focusing on concurrent spatiotemporal contact missed a substantial proportion of transmission events.

With our findings, we advocate for broader epidemiological investigations of temporal connections if genomic data suggest relatedness. We emphasise considering plasmid transfer alongside analyses of core-genome relatedness of bacteria beyond patient contact events to study CPE and resistance spread, and guide infection control strategies.