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. 2025 Apr;97(4):e70293.
doi: 10.1002/jmv.70293.

Virus Monitoring in Denmark: A Community-Based Self-Sampling System to Surveil Respiratory Viruses and Associated Symptoms

Tine Graakjær Larsen  1   2 , Jonathan R Ginty  1 , Randi Jessen  3   4 , Ruben Bjerregaard Nielsen  3   5 , Anders Jensen  6 , Amanda Bolt Botnen  7 , Bartlomiej Wilkowski  6 , Camilla Rasmussen  8 , Dalila Lepirica Brødbæk  6 , Daniel Steven Shaw  3   4 , Elias Benameur  8 , Morten Rasmussen  7 , Ramona Trebbien  7 , Shila Mortensen  7 , Stefan S Olsen  8 , Steven Chong  6 , Tyra Grove Krause  9 , Uffe Vest Schneider  4   7   10 , Anne-Marie Vangsted  3   5 , Nina Ruth Steenhard  3   4 , Frederik Trier Møller  1
Affiliations

Virus Monitoring in Denmark: A Community-Based Self-Sampling System to Surveil Respiratory Viruses and Associated Symptoms

Tine Graakjær Larsen et al. J Med Virol. 2025 Apr.

Abstract

This study presents findings captured in the first 1.5 years of the Virus Monitoring in Denmark (VMD) surveillance system. It describes trends in respiratory viruses, related symptoms, and participant demographics and behaviors. VMD used self-swabbing and self-reported symptoms to monitor respiratory viruses in the general population. Participants were recruited via digital invitations to a representative sample of the population or through workplaces. Symptomatic participants could self-swab and register their samples and report their symptoms via a dedicated smartphone web app. With 30 627 participants and 12 642 samples analyzed, VMD had a broad demographic representation. SARS-CoV-2 was the most frequently detected virus, with positivity rates peaking at over 50% in late 2023. Participants commonly self-swabbed because of fever, cough, and rhinorrhea, with influenza A linked to the highest median number of symptoms. Participants only provided samples after reaching a specific symptom threshold, and participation affected the health-seeking behaviors and work attendance of a few individuals. VMD continuously provided real-time insights into respiratory virus trends and symptomatology in the general non-healthcare-seeking population. Its accessibility - available to anyone with a Danish identification number, a smartphone, and an invitation - highlights its potential as a pandemic preparedness tool.

Keywords: eHealth; pandemic preparedness; public health surveillance; respiratory tract infections; self‐testing; symptom assessment.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
Overview of participation in Virus Monitoring in Denmark. (A) CONSORT diagram of participant recruitment. (B) Cumulated number of participants per week (line) and number of samples analyzed per week (bars). The vertical red lines indicate the calendar weeks where Digital Post invitations were sent out. Data from Week 24, 2023, to Week 46, 2024.
Figure 2
Figure 2
Weekly distribution of viruses in Virus Monitoring in Denmark during the study period. Left panel: Percentage of positive samples. Right panel: Incidence pr. 100.000 with a logarithm scale based on the Digital Post cohort. Data from Week 24, 2023, to Week 46, 2024. The dotted line represents the expansion of the RT‐PCR panel, and the dashed line represents the week from which samples were reanalyzed with the extended RT‐PCR panel.
Figure 3
Figure 3
(A) Prevalence of symptoms in participants infected with each virus, data from Week 40, 2023, to Week 46, 2024. Only responses from participants who have reported at least one symptom are included. Error bars represent a 95% confidence interval. (B) Distribution of the number of symptoms reported per positive sample, stratified by virus and ordered by median symptom count, from highest to lowest. n indicates the number of symptomatic individuals per virus.
Figure 4
Figure 4
Top panel: prevalence of symptoms over time. Only symptomatic individuals who perform a sample from Week 40, 2023, to Week 46, 2024. Bottom panel: number of each symptom reported among individuals with a positive test at the time of sampling (Baseline), symptoms that were only reported at the time of sampling but were not present after 7–28 days (Gone at Follow‐up), and those that were only reported at 7–28 days after the sample, but not at the time of sampling (New at Follow‐up). Includes only individuals who answered both symptom questionnaires.
Figure 5
Figure 5
Top panel shows the weekly incidence rate of SARS‐CoV‐2 in VMD (red line, left y‐axis) and the weekly concentration of SARS‐CoV‐2 RNA in wastewater surveillance (blue line, right y‐axis), with the peak of both curves in the same level, both on logarithmic scales. Bottom panel shows SARS‐CoV‐2 variant composition in VMD samples during the study period. Data from Week 40, 2023, to Week 46, 2024.
Figure 6
Figure 6
Prevalence of individual symptoms in participants infected with different SARS‐CoV‐2 subvariants. Only responses from participants who have reported at least one symptom are included. Error bars represent 95% confidence. Data from Week 24, 2023, to Week 46, 2024.
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