Molecular Characterization of Human Adenoviruses Before and After a Severe HAdV‐7 Military Outbreak in Finland in 2024

ABSTRACT

Background

In 2024, a severe outbreak caused by human adenovirus type 7d (HAdV‐7) occurred in Finnish military garrisons. Molecular typing was implemented to support outbreak control. The aim of this study was to determine adenovirus types circulating in Finnish garrisons during 2013–2023 by retrospective surveillance and to review earlier findings to better understand factors contributing to the 2024 outbreak.

Methods

Adenovirus‐positive respiratory samples collected through routine surveillance (2013–2023) and enhanced surveillance (2024–2025) were typed using type‐specific PCR assays targeting types 3, 4, 7, 11, 14, 16, and 21. Whole‐genome sequencing was performed on 11 selected samples. Index case interviews were conducted among conscripts infected with HAdV‐7 in late 2023.

Results

During 2013–2023, among 497 typed samples, HAdV‐4 was most common (n = 446; 89.7%), followed by HAdV‐3 (n = 25; 5%). Other types detected were HAdV‐21 (n = 11; 2.2%), HAdV‐7 (n = 6; 1.2%), and HAdV‐11 (n = 1; 0.2%). HAdV‐4 was predominant in 2013–2017 and HAdV‐3 in 2018–2019. Between February 2024 and June 2025, 1692 adenovirus‐positive respiratory samples were typed. The adenovirus types detected were HAdV‐4 (n = 1133; 67%), HAdV‐7 (n = 440; 26%), co‐detection of HAdV‐4 and ‐7 (n = 70; 4.1%), and HAdV‐14 (n = 1; 0.1%). Epidemiological investigation and whole‐genome sequencing data revealed that the adenovirus associated with the 2024 outbreak (HAdV‐7d) was already circulating in Finnish garrisons in 2023.

Conclusion

The severity of the 2024 outbreak may reflect limited prior circulation of HAdV‐7 in Finland. Although HAdV‐7 continued to circulate among conscripts in 2025, the severity of the epidemic subsided, possibly due to effective preventative measures guided by active surveillance.

1. Introduction

Adenoviruses are classified under the family Adenoviridae and divided into six genera. Adenoviruses of the genus Mastadenovirus infect mammals, including humans [1]. Human adenoviruses are classified into seven species (A–G) including 51 recognized serotypes and over 100 genetically distinct genotypes. Specific human adenovirus types are linked to distinct clinical manifestations. Adenovirus types 3, 4, 7, 14, and 21 are primarily associated with respiratory tract infections and 8, 19, and 37 with conjunctivitis, whereas types 40 and 41 are known to cause gastrointestinal infections [2]. Adenovirus outbreaks affecting the respiratory tract typically occur during the winter and spring months, with most infections affecting children under 5 years of age. Most cases present with mild symptoms, but severe and even fatal outcomes have been reported, although subclinical infections are also common [3]. Adenovirus infection elicits type‐specific neutralizing antibodies that can protect against reinfection, with immunity persisting for several years [4].

Adenoviruses are well‐recognized causes of outbreaks in crowded environments, where close contact and shared living conditions facilitate rapid transmission. Adenovirus types 4 and 7 have repeatedly been implicated in severe outbreaks on university campuses [5], type 3 adenoviruses have been responsible for outbreaks in nurseries and schools [6, 7], and types 4, 7, and 14 have been identified among outbreaks in prisons, law enforcement training centers, and elderly care facilities [8, 9, 10]. Perhaps the most consistently documented examples come from military garrisons, where outbreaks, particularly those involving types 4, 7, 14, and 21, are widely described in literature from around the world [11, 12, 13, 14, 15, 16].

All Finnish male citizens aged from 18 to 60 are obligated to take part in the military defense of Finland, and women may apply for voluntary service. Respiratory virus outbreaks occur in Finnish military garrisons annually, and several garrisons participate in the national respiratory virus surveillance program coordinated by the National Institute for Health and Welfare (THL). Adenovirus outbreaks tend to appear in Finnish garrisons in February and March, after the January arrival of new conscripts [17]. Between February and June 2024, there was an exceptionally severe outbreak of adenovirus type 7d in Finland, which particularly affected military conscripts and their close contacts. During the outbreak, 129 patients with adenovirus infection were hospitalized, 30 were admitted to ICU, and 10 patients required ECMO treatment. Six deaths were reported among civilians. Of the hospitalized patients, 75 (58%) were conscripts and 54 (42%) were civilians [13]. Adenovirus serotypes 1, 2, 3, and 4 have previously been detected from respiratory samples collected from Finnish conscripts during 2008–2012 [16].

The aim of this study was to determine, by laboratory analysis, the adenovirus types circulating in Finnish garrisons between 2013 and 2023 and to compare these findings with previously reported adenovirus circulation before this period to better understand the epidemiological factors contributing to the severe outbreak in 2024. Furthermore, the study sought to characterize adenovirus types detected after the outbreak and to assess whether adenovirus type 7 continues to circulate in Finnish garrison environment.

2. Materials and Methods

2.1. Sample Collection

As part of this study, two sample sets were examined. The first one comprises samples collected from garrisons as part of the routine national influenza and other respiratory virus surveillance in Finland between the years 2013 and 2023. The second sample set is composed of adenovirus‐positive respiratory samples collected during enhanced adenovirus surveillance that started in February 2024 and continued in 2025.

2.1.1. Respiratory Virus Surveillance 2013–2023

As part of the Finnish national respiratory surveillance program, 2580 respiratory samples were collected for laboratory testing between years 2013 and 2023 from Finnish military garrisons. The number of participating garrisons ranged from 6 to 23 units, and the annual number of submitted samples ranged between 97 and 472 (2013: n = 472; 2014: n = 349; 2015: n = 354; 2016: n = 219; 2017: n = 101; 2018: n = 122; 2019: n = 98; 2020: n = 132; 2021: n = 389; 2022: n = 247; 2023: n = 97). Samples were collected from conscripts exhibiting symptoms of acute respiratory illness (ARI) or influenza‐like illness (ILI), using the WHO case definitions. Sample materials included nasopharyngeal swabs, nasopharyngeal aspirates, bronchoalveolar lavage specimens, and sputum. Samples were transported from garrisons all over Finland to the virology laboratory of THL, located in Helsinki, by mail or courier services and analyzed upon arrival, after which the samples were stored at −70°C. During the 11‐year surveillance period, respiratory samples were tested with PCR panels. The composition of viral targets in the panels has varied across years. Across all panels, targets included influenza A and B, RSV (with types A and B introduced in 2023), rhinovirus, enterovirus D68 (introduced into the panel in 2015), seasonal coronaviruses (229E, HKU1, NL63, OC43), SARS‐CoV‐2 (introduced into the panel in 2020), and adenovirus.

2.1.2. Enhanced Adenovirus Surveillance 2024–2025

During the period of enhanced adenovirus surveillance from February 2024 to June 2025, a total of 1891 respiratory samples initially tested positive for adenovirus nucleic acid were collected from five clinical laboratories across the country for molecular characterization. These samples were collected from Finnish conscripts, representing 18 garrisons. Sample materials included nasopharyngeal swabs and bronchoalveolar lavage specimens. The samples were transported to the virology laboratory of THL by mail or courier services. Molecular typing of adenovirus was performed upon arrival of the samples, after which the samples were stored at −70°C.

2.2. Nucleic Acid Extraction

Nucleic acids were extracted from 300 μL of the original clinical sample according to the manufacturer's instructions using the Chemagic Viral DNA/RNA 300 kit H96 and the PerkinElmer Chemagic 360 instrument (PerkinElmer, Shelton, USA) and the QIAGEN RNeasy Mini kit (QIAGEN, Venlo, Netherlands). The elution volume used for the purified nucleic acids was 50 μL.

2.3. Molecular Typing of Adenoviruses

Adenovirus nucleic acid was determined from all samples using a probe‐based PCR assay specific for 51 distinguished HAdV prototypes [18]. Human adenovirus type was determined from the samples using a probe‐based real‐time PCR assay panel. The panel utilizes assays specific to adenovirus types 3, 4, 7, 11, 14, 16, and 21 [19]. The thermal cycling profile consisted of 15 min at 95°C, followed by 45 cycles of 30 s at 95°C, 30 s at 55°C, and 30 s at 72°C, using the 2× qScript XLT One‐Step RT‐qPCR ToughMix kit and Bio‐Rad CFX thermal cycler (Bio‐Rad, California, USA). Samples were analyzed according to the testing algorithm described in Figure 1.

FIGURE 1.

All samples were analyzed using PCR assays targeting 51 adenovirus prototypes (Adeno‐pan), as well as specific assays for adenovirus type 7 (HAdV‐7) and type 4 (HAdV‐4). If either HAdV‐4 or HAdV‐7 was detected, no further testing was performed, and the result was reported as HAdV‐4 or HAdV‐7 positive, respectively. If no specific type was identified and the sample tested negative in the Adeno‐pan assay, the result was reported as negative. However, if the sample was Adenovirus‐pan positive but no specific type was initially identified, additional PCR assays were performed to detect adenovirus types 3, 11, 14, 16, and 21. If one of these types was detected, the result was reported accordingly (e.g., HAdV‐3, ‐11, ‐14, ‐16, or ‐21 positive). If no type was identified in this second round of testing, the result was classified as undetermined adenovirus.

2.4. Sequencing and Phylogenetic Analysis

Whole‐genome sequencing libraries were produced using the Illumina Respiratory Virus Enrichment Kit, which is a hybrid‐capture method intended for enrichment of common respiratory virus targets, according to the manufacturer's manual (#20044311; # 20040537, Illumina https://emea.illumina.com/products/by‐type/sequencing‐kits/library‐prep‐kits/respiratory‐virus‐oligo‐panel.html). Target enrichment libraries were sequenced on an Illumina MiSeq instrument (Illumina, California, USA). Reads were initially run through Kraken 2 (v. 2.1.2) with database Std‐8 (v. 12.1.2024), and viral reads were extracted by KrakenTools (v. 1.2) [20]. Contigs were assembled by MEGAHit (v. 1.2.9), producing about 35‐kb contigs. Based on the BLAST results, follow‐up samples were mapped to reference MT113942.1 (hAdV‐B7) and references KX868498.2, KY996451.1, and MF002042.1 (hAdV‐E4) with Bowtie 2 (v. 2.4.4). Mapped reads were sorted and consensus sequences were created with Samtools (v. 1.19.2) [21].

To assess the phylogenetic relationships of HAdV‐7 viruses observed in 2023–2024 and HAdV‐4 viruses observed between 2013 and 2017, separate maximum likelihood (ML) trees were generated for each type using complete genome sequences. Reference sequences were selected from BLAST search results with high sequence identities, representing different geographic origins and outbreaks, and retrieved from GenBank (https://www.ncbi.nlm.nih.gov/nucleotide/). The sequences were aligned with MAFFT (v. 7.535) [22], and a tree based on the ML method using the HKY + F + I model and 1000 ultrafast bootstrap (UFBoot) replicates was constructed with IQ‐TREE (v.2.3.6). The tree was visualized with iTOL (https://itol.embl.de/). Pairwise distances were calculated using the p‐distance model with pairwise deletion of gaps and ambiguous positions with MEGA12 version 12.0.10.

2.5. Data Analysis

Data processing was performed using R version 4.5.0 (2025‐04‐11; “How About a Twenty‐Six”) within the RStudio integrated development environment version 2025.05.1 + 513 (“Mariposa Orchid”). To ensure sample independence, all specimens originating from the same individual were excluded from the analysis as duplicates, irrespective of the time elapsed between their collection.

3. Results

3.1. Respiratory Virus Surveillance 2013–2023

Of the 2580 samples initially screened using real‐time PCR, adenovirus nucleic acid was detected from 503 (19.5%) samples. Over the study period, the monthly mean for adenovirus‐positive samples ranged from 0.2 to 10.4, with the highest monthly mean for positive samples in March (Figure 2). A clear seasonal pattern was not observed every year during the study period. No adenovirus‐positive samples were collected for 33 months between May 2020 and January 2023 when the COVID‐19 pandemic disrupted respiratory virus surveillance (Figure S1).

FIGURE 2.

Bars indicate the mean monthly number of samples tested for adenovirus and samples positive for adenovirus from samples collected in Finnish garrisons, across the 11‐year study period. The line indicates the monthly adenovirus positivity rate. The positivity rate was calculated from the total number of positive samples divided by the total number of samples for each calendar month across the study period (2013–2023).

Of the 503 adenovirus‐positive samples, 497 samples collected from conscripts were subjected to molecular typing of human adenoviruses. HAdV‐4 was the most common type observed, detected in 446 (89.7%) samples during the study period 2013–2023. Other types detected were HAdV‐3 in 25 (5%) samples, HAdV‐21 in 11 (2.2%) samples, HAdV‐7 in six (1.2%) samples, and HAdV‐11 in one (0.2%) sample. Adenovirus type remained undetermined in eight (1.6%) samples.

A clear variance in adenovirus type distribution was observed across the years. The most common adenovirus type detected in samples collected from conscripts between years 2013 and 2017 was HAdV‐4. HAdV‐3 was the only type detected in years 2018 and 2019. Adenovirus was not detected in the 636 respiratory samples obtained from garrisons during the years 2021–2022, which were affected by the COVID‐19 pandemic. In 2023, HAdV‐7 was the most common type observed, with all detections made between August and November (Figure 3).

FIGURE 3.

Yearly detections of adenovirus types in Finnish garrisons between the years 2013 and 2023. Bars indicate the number of detections for each adenovirus type per year.

3.2. Enhanced Surveillance February 2024–June 2025

A total of 1891 initially adenovirus PCR‐positive respiratory samples were collected from conscripts for molecular typing. One hundred thirty‐five samples were duplicate samples from the same subject, and these samples were excluded from the analysis. Sixty‐four samples that were negative for adenovirus in PCR tests at THL were also excluded. For further analysis, 1692 valid samples remained. Among the 1692 valid samples collected from conscripts between February 2024 and June 2025, HAdV‐7 was detected in 440 (26%) samples, whereas HAdV‐4 was detected in 1133 (67%) samples. Across the entire surveillance period, 70 (4.1%) samples showed co‐detection of both types, most of which occurred in autumn 2024. HAdV‐14 was detected in one sample (0.1%). Adenovirus type remained undetermined in 48 (2.8%) samples. During spring 2024 (weeks 2024/01–2024/25), HAdV‐7 predominated, accounting for most of the typed samples. Autumn 2024 (weeks 2024/28–2024/51) was characterized by a transition phase when co‐circulation of both virus types was observed, before shifting to HAdV‐4 dominance in spring 2025 (weeks 2025/01–2025/25) (Figure 4).

FIGURE 4.

Adenovirus types detected in samples collected from Finnish garrisons during epidemiological weeks 2024/06–2025/27. Bars indicate the number of detections for each adenovirus type.

Adenovirus cases were observed in the spring of 2024 in 12 garrisons across the country. Adenovirus cases increased again in most garrisons in the autumn of 2024. Between weeks 2024/28 and 2024/51 in nine garrisons, HAdV‐4 was the most common observed adenovirus type. In contrast, HAdV‐7 continued to be the most frequent type observed in three garrisons, located in the northern and central parts of Finland. In all garrisons where more than 10 samples were collected during the study period 2024–2025, adenovirus cases were observed in the spring of 2025. HAdV‐4 was the most common type detected in all garrisons between weeks 2025/01 and 2025/25; however, sporadic HAdV‐7 cases were observed in two garrisons.

3.3. Phylogenetic Analysis

Two HAdV‐7 type‐specific real‐time PCR‐positive respiratory samples collected from military conscripts in late 2023 underwent whole‐genome sequencing (PX734071 and PX778751). Phylogenetic analysis demonstrated that the selected HAdV‐7 reference sequences associated with the Finnish 2024 outbreak (PQ600596 and PQ600589) [13] and those generated in this study were genetically identical, showing 100% pairwise nucleotide identity across the ~35‐kb genome. Among global published reference strains, the highest nucleotide similarity (> 99.997% whole‐genome nucleotide identity) and close phylogenetic clustering were observed with a sequence detected in England in 2023 (PX491659). Similar levels of sequence identity were observed with strains reported from England in 2025 (PX491651 and PX491691) and with a sequence detected through campus surveillance in the United States in 2023 (PP591776) [23]. In phylogenetic analysis, the outbreak sequences also clustered closely with HAdV‐7 strains detected during an outbreak in a children's hospital in China in 2019 (MW815977) [24] and with strains reported from a university campus in the United States in 2018 (MN307146 and PP591776) [5], all three sharing > 99.99% nucleotide identity with the outbreak sequences.

Additionally, nine HAdV‐4 samples collected between 2013 and 2017 were sequenced. Comparative phylogenetic analysis of historical HAdV‐4 sequences and sequences generated during this study revealed three distinct clusters, suggesting independent introductions, rather than continuous circulation of these HAdV‐4 strains into the Finnish garrison environment (Figure 5).

FIGURE 5.

Maximum likelihood phylogenetic trees were constructed from complete genome sequences of Finnish adenoviruses and closely related global reference strains. Reference sequences were selected based on high identity in BLAST searches. Alignments were generated with MAFFT, and trees were inferred in IQ‐TREE using the HKY + F + I model with 1000 bootstrap replicates. Trees were visualized with iTOL. Samples sequenced as part of this study are highlighted in blue. Panel (A) shows two HAdV‐7 whole‐genome sequences from samples collected in autumn 2023 along with Finnish 2024 outbreak sequences and reference strains. Adenovirus 7 vaccine strain sequence was chosen as the root of the tree. Panel (B) shows nine HAdV‐4 whole‐genome sequences from samples collected in Finland between 2013 and 2017 with global reference strains. All sequences from Finland cluster with reference strains from phylogroup II and reference strains belonging to phylogroup I were used as an outgroup to root the tree.

3.4. Index Case Interviews

Retrospectively, six cases of HAdV‐7 were identified by molecular typing of adenovirus PCR‐positive respiratory samples collected from military conscripts in late summer and autumn 2023. These conscripts had been sampled due to respiratory symptoms and had been performing their military service in three different garrisons in different parts of Finland. Five of them were interviewed regarding their symptoms. All of them reported high fever associated with various respiratory symptoms, such as cough and sore throat. Some also had gastrointestinal symptoms. Most of them reported several other conscripts around the same time having signs of respiratory infection with similar symptoms in the same barrack room, or elsewhere in the garrison. None of these conscripts needed hospital treatment because of their condition, but some needed bed rest for a few days.

4. Discussion

Historical evidence suggests that HAdV‐7 has circulated in the Finnish garrison environment before 2023. Archival samples collected by the National Public Health Institute (the predecessor of THL) during the 1960s and 1970s, recently digitized for publication [25], include several notifications of samples positive for HAdV‐7, some of which originated from garrison hospitals. Specifically, HAdV‐7‐positive samples were collected from Finnish garrisons in the years 1969–1972, 1975, and 1977 (data not shown). Outbreaks associated with HAdV‐7 have also been documented in Finland outside military settings before 2023. In 1962, an outbreak caused by HAdV‐7 was reported at the Helsinki University Eye Hospital [26]. Additionally, between 1967 and 1968, HAdV‐7 was reported in children with severe adenovirus pneumonia [27]. It is evident that HAdV‐7 has been circulating in Finland, both in garrisons and among the general population, prior to the 2024 outbreak, but the source of reintroduction of the virus into Finnish garrisons remains unclear. During interviews with index cases, there was no mention of recent travel history abroad or contacts with foreign civilians or military personnel prior to falling ill.

Molecular typing data from 2008 to 2012 and from 2013 to 2023 indicate that HAdV‐4 has been the predominant adenovirus type associated with outbreaks in Finnish garrisons [16]. In certain years, specifically 2010, 2018, and 2019, HAdV‐3 emerged as the dominant type within garrisons. In 2010, HAdV‐3 was also the most frequently detected type in the civilian population [28], suggesting a link between the circulation in military and civilian settings. A comparable pattern was observed during the serious outbreak in 2024 and the following enhanced adenovirus surveillance, when HAdV‐7 cases were observed concurrently in the general population and the garrison environment [13].

Examination of samples collected between 2013 and 2023 revealed the first cases of HAdV‐7 in Finnish garrisons in 2023, with the earliest case identified in late August. That year, HAdV‐7 was observed in three garrisons, where it was the most common adenovirus type detected. It remains unknown how widely HAdV‐7 was circulating in Finnish garrisons at the time, as garrison participation in surveillance in 2023 was low and only eight adenovirus‐positive samples were typed. Based on epidemiological data and phylogenetic analyses (Figure 5), it appears that the adenovirus type (HAdV‐7d) associated with the severe outbreak documented in 2024 was already circulating in Finnish garrisons at the end of 2023. Healthcare officials at the time were not aware of adenovirus cases with exceptionally severe clinical outcomes. The reason for medical outcomes appearing to be milder among conscripts during the fall of 2023, as compared to the 2024 outbreak, remains unclear. It is possible that the incidence of adenovirus infections in 2023 remained below the threshold at which severe outcomes typically emerge. A dose–response effect may also have contributed to the observed increase in disease severity between 2023 and 2024. Although this concept has not been extensively studied in the context of adenovirus infections, it has recently gained attention as a possible factor influencing disease outcomes in respiratory illnesses [29]. Due to the increasing numbers of cases and escalating amount of potential viral inoculum, as well as the duration of exposure, infections in immunologically naïve individuals to this adenovirus strain may have been more severe during the spring of 2024, resulting in more hospitalizations.

There were also several cases where both HAdV‐4 and HAdV‐7 were detected simultaneously from the same respiratory sample. It is difficult to distinguish whether the HAdV‐7 detected in patients with coinfections in the fall of 2024 is genomic relics of a previous infection, subclinical infections, or indeed new clinically relevant HAdV‐7 infections, as adenovirus nucleic acid can persist in nasopharyngeal excretions for over 6 months [30].

Following the severe outbreak in 2024, adenovirus cases involving HAdV‐7 were observed in several Finnish garrisons. During the autumn of 2024, HAdV‐7 was the predominant adenovirus type identified in five garrisons, whereas HAdV‐4 was most frequently detected in seven other garrisons. These findings suggest geographical variation in the prevalence of HAdV‐7, and continued or renewed surveillance could be valuable for guiding preventive measures. By June 2025, HAdV‐4 was the only adenovirus type detected in garrison surveillance. As previously noted, enhanced adenovirus surveillance is ongoing in Finland, and detections of HAdV‐7 in the autumn of 2025 indicate that this type remains present in Finnish garrisons for the time being (data not shown).

During spring 2024, several non‐pharmaceutical interventions were implemented to control the adenovirus outbreak in Finnish garrisons. These measures included reinforcement of hand hygiene, with an emphasis on washing hands with soap and water, and promotion of proper cough hygiene. In certain situations, symptomatic individuals were segregated from asymptomatic individuals, and cohorting of affected units was applied to limit transmission. Environmental control measures were strengthened through intensified cleaning of facilities. In select high‐risk situations, such as during transportation of conscripts by bus, the use of surgical masks was mandated. During and after the outbreak, molecular genotyping of adenovirus was used to guide targeted implementation of these measures. The rapid turnaround time of PCR‐based molecular typing, compared to sequencing, enabled early implementation of interventive measures in garrisons where HAdV‐7 cases were observed. Although HAdV‐7 has continued to circulate in Finnish garrisons after the outbreak, the associated clinical impact among conscripts has been milder. While the effectiveness of the implemented measures in limiting HAdV‐7 transmission was not scientifically evaluated, observations from epidemiologists and clinicians involved suggest that these interventions contributed to a reduced HAdV‐7‐associated disease burden in Finnish garrisons.

The reintroduction of HAdV‐7 into the garrison environment raises questions regarding the possible need for vaccination. The adenovirus vaccine currently used by the US military provides protection against both HAdV‐4 and HAdV‐7. Considerations for its use in Finland would include ensuring protection of unvaccinated conscripts or their close contacts from exposure to virus shed by recipients of the live, non‐attenuated vaccine in Finland, where military service is mandatory for men, but vaccination is not.

This study has several limitations. The number of samples analyzed between 2013 and 2023 was limited during some years, particularly during the COVID‐19 pandemic. Participation in the surveillance program varied between garrisons, potentially affecting representativeness. Molecular analyses could not be performed for all samples initially reported as adenovirus positive by clinical laboratories, possibly due to issues related to sample collection, transport, or storage. In addition, molecular typing alone cannot distinguish between active infection and residual viral DNA from previous infections, which is relevant when multiple adenovirus types are detected in the same sample. Another significant limitation was incomplete metadata on patient garrison affiliation and military status during enhanced surveillance in 2024–2025. These limitations should be considered when interpreting the observed trends in adenovirus circulation and type distribution.

In conclusion, the severity of the adenovirus outbreak in 2024 may be attributable to the absence of prior circulation of this adenovirus type in Finnish garrisons, at least since 2008. The introduction of adenovirus type 7d into a population lacking immunity to this specific type likely contributed to the number of hospitalizations among otherwise healthy conscripts. Non‐pharmaceutical preventative strategies were implemented in Finnish garrisons during the severe outbreak in 2024 and have been maintained thereafter. Deploying these methods in a military setting is more straightforward than in civilian settings; however, such measures are disruptive to military training. Continued surveillance, utilizing molecular genotyping, was used in targeting the implementation of preventative means to garrisons where they were most needed to balance the well‐being of conscripts with the maintenance of effective training programs.

Author Contributions

Niko Tervo: writing – review and editing, writing – original draft, formal analysis, data curation, methodology, visualization, validation. Marjaana Pitkäpaasi: data curation, methodology, investigation, writing – review and editing, writing – original draft. Pilvi Hepo‐oja: formal analysis, software, data curation, writing – review and editing, methodology, visualization. Hanna Jarva: writing – review and editing, data curation, resources. Anna Katz: writing – review and editing, writing – original draft. Erika Lindh: writing – original draft, writing – review and editing, validation, visualization, methodology, data curation, formal analysis, software. Richard Lundell: data curation, resources, writing – review and editing, writing – original draft, investigation, formal analysis. Minna Paloniemi: writing – review and editing, data curation, resources. Laura Savolainen: writing – review and editing, data curation, resources. Carita Savolainen‐Kopra: project administration, data curation, resources, conceptualization, writing – original draft, writing – review and editing, formal analysis. Niina Ikonen: conceptualization, supervision, resources, writing – original draft, writing – review and editing, project administration, formal analysis, visualization, methodology, validation. Simo Nikkari: conceptualization, supervision, resources, writing – original draft, writing – review and editing, project administration, formal analysis, visualization.

Funding

The authors have nothing to report.

Ethics Statement

Data for this study were collected as part of an outbreak investigation conducted by THL. For this purpose, no ethical approval is required according to the Communicable Diseases Act (Tartuntatautilaki | 1227/2016 | Legislation | Finlex).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Figure S1: Monthly adenovirus sample counts and positive sample counts collected from Finnish military garrisons for years 2013 and 2023. Bars indicate the number of samples analyzed for the presence of adenovirus nucleic acid. The line indicates the number of adenovirus‐positive samples.

Data Availability Statement

Sequence data associated with this study have been deposited in GenBank under Accession Numbers PX734071, PX778751, and PX778756–PX778764.