Steen Ethelberg

Salmonella Enteritidis is typically transmitted to humans via consumption of eggs or poultry meat. In July 2023, Denmark and Austria concurrently reported two outbreaks of genetically distinct S. Enteritidis sequence type (ST) 11. In both countries, cases were identified as part of the outbreaks using core-genome Multilocus Sequence Typing (cgMLST) between 1 ^st of January and 1 ^st of October, 2023. Outbreak investigations were conducted and included interviews with cases, food tracing, and, using cgMLST, comparisons of human, food, and environmental isolates. In the EU, 207 cases across 11 countries were found. Patient food histories pointed towards chicken, often sold as kebab meat. Extensive traceback investigations from affected restaurants and wholesalers in Austria and Denmark identified chicken meat produced in Poland as the vehicle of the outbreaks. Due to the death of one patient in Austria, the food safety authorities initiated more food safety controls for this food product, which led to food recalls. Recalls of specific chicken products from Poland were also conducted in Denmark. Despite national interventions and regulations targeting poultry producers, the EU remains collectively vulnerable to large-scale food contamination events originating in a single member state. Effective international and cross-sector collaboration was crucial to identifying the vehicle of these outbreaks.

BACKGROUND: Because of climate change, the frequency of extreme weather events, including heavy rainfall and flooding, is increasing in Europe. It is important to understand how such events affect infectious disease incidence to inform preparedness and response activities. AIM: We aimed to gather evidence of associations between heavy rainfall and flooding and infectious disease incidence in humans in Europe, as well as report preparedness and response measures. METHODS: In this rapid review, PubMed, Embase and the Cochrane Library were searched using a combination of keywords relevant to infectious diseases as well as to heavy rainfall and flooding to identify published literature and preprints. Articles published between 1 January 2018 and 15 June 2025 were eligible for inclusion. RESULTS: From 897 records identified, 14 articles were included in the review. In particular, outbreaks of campylobacteriosis and cases of leptospirosis were reported following heavy rainfall and flooding events in Europe. Public health communication on awareness and hygienic practices was among the most commonly reported preparedness and response measure, while structural measures such as sanitary infrastructure and One Health surveillance systems were suggested. CONCLUSION: While heavy rainfall and flooding have been associated with outbreaks of infectious diseases in different parts of Europe, relatively few studies investigating these associations were identified in Europe despite wide search criteria, highlighting the need for further research to strengthen European preparedness and response measures.

In recent years, wastewater-based surveillance (WBS), also referred to as wastewater environmental surveillance (WES), has developed into an important and promising tool for monitoring pathogens, antimicrobial resistance (AMR) and a wide range of chemical substances. To further develop, strengthen and institutionalise WBS capacity across Europe, public health organisations from 26 countries have joined forces in the European Joint Action project “EU-Wastewater Integrated Surveillance for Public Health” (EU-WISH). Half-way through the project implementation period, an International Workshop and Consortium Meeting was held at University College Dublin, Dublin, Ireland, which connected EU-WISH partners and external stakeholders. A series of thematic breakout-sessions held at this workshop provided reflections on progress and developments in the rapidly growing WBS field. Here, we position the EU-WISH project in the WBS landscape and summarise key insights, current EU-WISH activities, and continued challenges. This open letter aims to set direction for and inform the future development of WBS in Europe, while maintaining a global outlook. Topics presented and discussed include: prioritisation of targets for surveillance; cross-border comparisons of WBS data in a landscape of multiple analysis set-ups; integration and institutionalisation within the field of WBS; sustainable training and capacity building; and networks, actors and the need for continued cooperation. The derived insights underscore the need for sustained EU-level coordination, institutionalised WBS programmes and integration of WBS into public health systems across Europe.

Background: Respiratory syncytial virus (RSV) is a major cause of hospitalisation among young infants worldwide. Denmark implemented a national maternal RSV vaccination programme in October 2025. In this nationwide register-based cohort study we estimate the vaccine effectiveness against RSV hospitalisation among infants aged 0-6 months following the first maternal RSV vaccination programme and evaluate the population-level impact.Methods; We included all live births between November 1, 2025 to March 31, 2026 and compared rates of RSV-related hospitalisation occurring before May 1, 2026 among infants to vaccinated and unvaccinated mothers using Cox regression with calendar time as the underlying timescale adjusted for potential confounders. Population-level impact was estimated using Danish RSV surveillance data.Findings: Of the 22,593 infants included, 16,416 (72.7%) were born to vaccinated mothers. During the study, 66 and 104 infants were hospitalised for RSV in the vaccinated and unvaccinated group, respectively. The adjusted vaccine effectiveness against RSV hospitalisation was 77.9% (95% CI 69.5-84.0) overall, higher among infants aged 0-90 days (82.6%, 95% CI 74.9-88.0) than among infants aged 91-180 days (56.8%, 95% CI 18.4-77.2). The population impact analysis showed a 70.4% reduction in RSV hospitalisations among infants aged 0-90 days.Interpretation: Maternal RSV vaccination provided substantial protection against RSV hospitalisation, particularly during the first months of life. A considerable reduction in RSV hospitalisations was observed at the population level. These findings support national programmes as an effective strategy to reduce severe RSV burden in early infancy and highlight the importance of successful programme implementation and high maternal vaccination coverage for maximising population-level benefit.

Background The extent and burden of post-acute cognitive dysfunctions following SARS-CoV-2 infection is uncertain. Methods 25,485 SARS-CoV-2 test-positive and 25,032 test-negative individuals were repeatedly asked to score symptoms of subjective cognitive deficits 2 to 18 months after test using the “Cognitive complaints in bipolar disorder rating assessment” (COBRA) tool. Poisson mixed-effects models were used to estimate Score Ratios (SRs) by comparing scores between test-positive and test-negative individuals. Results At each follow-up point, test-positive individuals have low but slightly higher mean COBRA scores compared with test-negatives. For the combined 2–18 months period, COBRA scores among test-positive individuals are 11% higher than corresponding scores among test-negatives (SR_2-18mth = 1.11 (95% CI; 1.09–1.13)). Of effect modifiers explored, being hospitalized with a positive SARS-CoV-2 test particularly elevates COBRA scores (SR_2-18mth = 1.38 (95% CI; 1.24–1.54)). Conclusion In the general population of SARS-CoV-2 infected individuals, self-reported post-acute scores of cognitive dysfunctions are low and only slightly higher than corresponding scores among test-negatives. Higher COBRA scores among hospitalized SARS-CoV-2 test positives corroborate with long-term cognitive impairment being most pronounced among those with severe SARS-CoV-2 infection.

Background: The global, national and subnational geographical and temporal distribution of reported Legionnaires’ disease is extremely heterogeneous, and it is unknown to what extent this accurately reflects the variation in true disease incidence. We studied how diagnostic testing intensity varied across Danish municipalities between 2014 and 2022, how it influenced epidemiological surveillance, and how testing-adjusted reporting can be used to improve the study of disease incidence and its determinants. Methods and findings: We used data from the clinical Danish Microbiology Database and the Epidemiological Surveillance System and considered a tentative causal model of how Legionella infections give rise to the observed Legionnaires’ disease surveillance data. We fitted spatio-temporal models using an approximate Bayesian inference for latent Gaussian models (INLA), providing probabilistic estimates. These allowed us to identify areas of increased risk and spatio-temporal interaction. Our assessment of the Legionnaires’ disease testing intensity in Denmark showed considerable spatio-temporal variation across the country. The estimated municipal annual testing intensity ranged from 128 to 2,446 persons receiving at least one Legionella urinary antigen or PCR test per 100,000 inhabitants. The median increased between 2014 and 2022 steadily from 275 to 620 tested persons per 100,000 inhabitants, reflecting an upward trend. The proportion of tested persons with age over 70 increased from 44.8% to 56.4%. Increasing testing intensity leads to higher case reporting, until testing intensity reaches approximately 1000–1200 tested persons per 100 000 inhabitants. The estimated municipal annual testing-adjusted case reporting ranged from 1.4 to 12.0 per 100 000 inhabitants. The median fluctuated over the study period between 2.5 (range 1.4–6.2) in 2014 and 5.2 (range 1.5–11.7) in 2022 with a flat overall time effect. Conclusions: We obtained estimates of the spatio-temporal variation of Legionnaires’ disease among Danish municipalities. We quantified the positive effect of testing intensity on Legionnaires’ disease reporting and found a threshold of annually testing slightly over 1% of the population above which the yield of new cases does not further increase. Despite limitations and possible bias, our study of testing-adjusted case reporting suggests that no substantial increase in Legionnaires’ disease has occurred over the nine-year study period. Instead, case ascertainment by physicians has improved considerably through increased Legionella testing, particularly in elderly patients. Insight in the variation of testing intensity and its effect on Legionnaires’ disease reporting can be used to improve guidance for Legionnaires’ disease diagnosis, to better study determinants of Legionnaires’ disease, and ultimately to improve Legionnaires’ disease prevention and control.

BACKGROUND: Influenza infection is a recognised trigger of acute myocardial infarction (AMI) and stroke, but whether influenza vaccination modifies this risk remains unclear. AIM: We aimed to quantify the short-term cardiovascular risk after laboratory-confirmed influenza infection and assess whether vaccination attenuates it. METHODS: We conducted a nationwide self-controlled case series study using Danish health registries (2014–2025) and included individuals aged ≥ 40 years with a first-ever hospital admission for AMI or stroke within ± 365 days of a PCR-confirmed influenza infection. We defined days 1–7 after specimen date as the risk period and excluded a 14-day pre-exposure period to reduce reverse causality, where influenza testing might be prompted by cardiovascular disease symptoms. We linked testing, hospitalisation, vaccination and mortality data deterministically via unique personal identifiers. We estimated incidence rate ratios (IRRs) and 95% confidence intervals (CI) with conditional Poisson regression. RESULTS: Among 1,221 individuals with a first-ever AMI (n = 429; 35%) or stroke (n = 792; 65%), median age was 75 years (interquartile range: 66–82); 561 (46%) were female. After calendar-month adjustment, the IRR for cardiovascular events during the risk period was 3.5 (95% CI: 2.6–4.7), higher for AMI (IRR = 4.7; 95% CI: 3.1–7.4) than stroke (IRR = 2.9; 95% CI: 2.0–4.2). Prior influenza vaccination during the same influenza season, recorded in 610 (50%) episodes, reduced the excess risk of AMI or stroke associated with influenza infection (interaction p = 0.020). CONCLUSIONS: Influenza infection conferred a transiently increased risk of first-time AMI and stroke. Vaccination substantially attenuated this risk, supporting its role in preventing cardiovascular complications after breakthrough infection.

Early in the COVID-19 pandemic, Denmark launched COVIDmeter, a national participatory surveillance platform collecting real-time, self-reported symptoms from a community cohort, aimed to support early signal detection of COVID-like illness. This study describes the community cohort, the reported symptoms among persons testing positive and evaluates COVIDmeter’s performance in detecting trends compared to other established surveillance indicators. A total of 143000 individuals registered as participants, of whom 98% completed at least one weekly questionnaire, resulting in approximately 5.8 million responses over the period from March 2020 to March 2023. Of those who tested positive, the most commonly reported symptoms overall were headache, fatigue, muscle or body aches, cough and fever. Trends in COVID-like illness followed similar patterns to other indicators, with COVID-like illness peaks often preceding increases in incidence and hospital admissions, suggesting early detection potential. The study demonstrated that participatory surveillance can serve as an early detection tool for tracking infection trends, particularly in the early stages of a pandemic. While subject to limitations such as selection bias and self-reporting inaccuracies and participatory symptom surveillance proved to be a rapid, scalable and cost-effective complement to traditional surveillance independent of virus testing, this highlights its relevance for future pandemic preparedness.

Notifications of Salmonella Strathcona infections increased in Europe in 2023 prompting a multi-country outbreak investigation. We aimed to describe the epidemiology of S . Strathcona infections in 17 European countries 2011–2024, investigate the genetic relatedness of S . Strathcona isolates and identify the vehicle. Cases were persons residing in the study area and with a laboratory-confirmed S . Strathcona infection 2011–2024. Confirmed cases had a S . Strathcona isolate clustering with the outbreak reference strain in core genome multilocus sequence typing (cgMLST) within 7 allelic differences (AD) and possible cases within 8–13 AD. Probable cases had an epidemiological link to a confirmed case and non-outbreak cases had an isolate > 13 AD from the outbreak reference strain. Since 2011, 662 S . Strathcona infections have been identified: 469 confirmed, 161 probable, 13 possible and 19 non-outbreak cases. Median age of the cases was 34 years (IQR: 19–58 years) and 306 (47.5%) were notified in 2023–2024. Most sequenced isolates (469/496; 94.5%) were highly genetically related (≤ 7 AD) over time and across countries, compatible with a common source. Epidemiological and traceback investigations identified small tomatoes from Sicily as the suspect food vehicle. Stringent control measures at the source are needed to stop the contamination and prevent future cases.