Infections are not only a major cause of short- and long-term morbidity and mortality after solid organ transplantation (SOT) [1, 2], but also an important cause of hospital admission and graft failure [3–5]. The timeline of infections in the first year following SOT was first published more than 25 years ago: at this point, three different periods (0–1, 1–6 and >6 months) associated with different infectious risks were described [6]. A recent analysis of the Swiss Transplant Cohort Study (STCS) highlighted the substantial burden of infections in adult SOT recipients, with 55% experiencing at least one event during the first post-transplant year, predominantly bacterial [7]. However, these results cannot be directly extrapolated to children, whose susceptibility to infection is shaped by developmental and environmental factors: (1) immune system immaturity, (2) limited prior pathogen exposure and adaptive immunity, (3) possible incomplete immunization, and (4) higher exposure in daycare and school settings. In children, infection remains the most common cause of early death after SOT and continues to affect the quality of life for several years thereafter [2, 3, 5, 6]. To the best of our knowledge, no studies have systematically described the incidence and types of infectious complications in a large cohort of pediatric SOT recipients. Reports have generally focused on a single pathogen or on infections in a single organ transplant population. In a cohort of more than 2000 pediatric liver transplant recipients, more than half presented a serious post-transplant infection [8]. Similarly, infection is deemed responsible of 20% of deaths after pediatric kidney transplantation [9]. Therefore, a comprehensive multicenter analysis of post–pediatric SOT infections across various organ types can offer valuable insight into the overall infection burden and facilitate comparisons between transplant groups.

This study aimed to characterize infection incidence during the first post-transplant year in pediatric SOT recipients within the STCS nationwide cohort and to identify associations between infection, age, organ type, and rejection episodes [10].

This nationwide study of pediatric SOT during the first year after transplantation highlights several findings: (1) more than half of patients experienced at least one infectious event; (2) incidence of infection was highest in younger recipients; (3) liver and lung transplantation, as well as male sex were independently associated with an increased risk of infections (5) viral infections accounted for the largest proportion of infections; (6) bacterial infections predominated during the first 3 months, with a large proportion due to Gram-negative organisms (6) distinct organ-specific microbiological patterns were observed (7) and the burden of fungal infection was considerable, with 3 of 22 fungal infections resulting in death.

To our knowledge, this is the first comprehensive and standardized characterization of the infectious burden within the first year after pediatric SOT in a nationwide cohort. Most studies have focused on a single type of infection or on severe infections [20, 21], a single type of transplanted organ [8, 22], or data collection was restricted to the immediate early post-SOT period [23]. While organ-specific studies are essential, our objective is complementary: to provide an overall view of the infectious burden across pediatric SOT and enable comparisons between transplant groups, that share underlying immunosuppressive mechanisms. Our data showed that approximately 50% of pediatric SOT recipients presented at least one infection during the first year after transplant, in line with previous reports after pediatric liver transplantation [8]. Notably, one fifth of patients experienced >3 infections. Nearly half of infections required hospitalization. The challenge of invasive diagnostic and therapeutic procedures in children (venous access, biopsies of the infected sites,…), the impact on quality of life, and the economic burden of these infections were not analyzed in this study but are likely to be considerable [24].

At an incidence rate of 1.36 infection, the incidence of infection in our cohort was very similar to adult SOT recipients using the same design, definitions, and study population [7]. However, infection patterns differed substantially between children and adults, supporting the need for age-specific preventive and management strategies.

Infections were more frequent in younger patients, likely to immune immaturity and more frequent behavioural exposure. Young children lack antigen-specific adaptive responses and therefore rely on stronger but less specific innate responses [25, 26]. Lung transplant recipients had the highest incidence of infections, likely attributable to intense immunosuppression, airway exposure to pathogens and impaired mucociliary clearance [27–29]. Liver transplant recipients also had a high incidence of infections, despite a lower immunosuppression [8], and even after adjusting for age. This may reflect the liver immunologic role, cirrhosis-related immune defects [30–35] and the role of basiliximab induction. Male sex remained an independent risk factor, consistent with known sex-based immune differences and possible behavioral factors, including treatment non-adherence [36–47].

More than half of infections were viral, contrasting with the adult study where bacterial infections predominated [7]. Viral infections were even more frequent in younger children and occurred year-round, reflecting transitions from donor-derived and immunosuppression-related risk to community exposure after return to school or daycare. Respiratory and gastrointestinal viruses were most common, mirroring pediatric viral epidemiology [48–50], followed by herpesviruses, reflecting their burden after pediatric SOT [51–54]. These findings emphasize the need for heightened vigilance for viral infections in routine pediatric follow-up, early testing for respiratory pathogens in order to avoid unnecessary prolonged empiric antibiotherapy, importance of immunization in the community for vaccine-preventable diseases immunization and infection control measures such as masks, social distancing and hand hygiene whenever possible in case of sustained viral circulation in schools and daycare.

Bacterial infections predominated in the early months, reflecting intensified immunosuppression and post-operative complications. Like in adult SOT recipients [7], E. coli was the most frequent cause of bacterial infections, of which 24% were MDR. [12]. The high prevalence of Gram-negative pathogens in this cohort supports the adaptation of empirical antibacterial therapy toward broader-spectrum coverage in cases of suspected bacterial infection. These findings also support the importance of early postoperative surveillance cultures in febrile patients and emphasize the need for close multidisciplinary coordination between transplant teams and community pediatricians to ensure timely detection and rapid referral for suspected severe infections.

Moreover, our findings reveal clear organ-specific microbiological patterns. Kidney infections were largely driven by enteric Gram-negative bacteria, consistent with a urinary or digestive origin, whereas liver infections displayed a mixed and frequently polymicrobial profile, supporting a biliary or intra-abdominal source [55, 56]. Lung infections were more heterogeneous and included opportunistic pathogens, suggesting a role for host-related factors such as immunosuppression. In contrast, heart infections were predominantly caused by Gram-positive bacteria, reflecting typical endovascular infections associated with skin flora and biofilm formation.

Viral infections also exhibited organ-specific distributions, with latent viruses predominating in the kidney and a broader, more diverse spectrum in the liver, possibly reflecting younger age and primary exposure to common viruses. Lung infections were mainly driven by respiratory viruses, whereas cardiac involvement remained rare and nonspecific.

Although the number of reported fungal infections remained low, the outcome was fatal in 13% of cases. This raises the question of whether the indications for antifungal prophylaxis in SOT recipients, rarely applied in our cohort, should be expanded, and of the clinical situations in which screening for fungal colonization should be considered.

Less than a quarter of infections in our dataset were caused by opportunistic pathogens. This is probably overestimated given the fact that some viral pathogens considered as opportunistic by the STCS infectious diseases working group might indeed be opportunistic in the adult setting, but might simply reflect primary community infection in children (i.e., VZV, adenovirus). Overall, the relatively low burden of opportunistic infections likely reflects effective prophylaxis, underscoring adherence to standardized protocols and ongoing education of families and healthcare providers. Also, only 6% of infections were attributed to vaccine-preventable pathogens. While this may suggest a potential contribution of the pre-transplant immunization to post-transplant protection, this could not be verified in our cohort as individual immunization charts were not available. [14]. These findings nonetheless support the importance of ensuring up to date vaccination before transplantation and post-transplant catch-up when feasible.

No significant association was found between rejection episodes and infections. This may reflect limited power and pathogen-specific differences in the risk of rejection [57]. In this cohort, steroid-free regimens were not associated with a reduced risk of infection. However, this finding may be subject to selection bias, as most patients received steroid-based therapy. Contrary to our initial hypothesis, recipients of deceased-donor kidney transplants did not exhibit a higher infection rate, as the number of infections per patient was greater in the living-donor group.

Limitations of our study include the sample size limiting conclusion of subgroup analyses, possible reporting bias toward severe infections (as 60% required hospitalization), and underreporting of milder community-acquired cases. The absence of a pediatric control group limits comparison with the general population, and the results may not be generalized to other settings with different epidemiology or clinical practices. In addition, detailed data on immunosuppressive drug dosing and levels, treatment of rejection were unavailable restricting analysis of their association with infection. As with all registry-based studies, our findings may also be affected by loss to follow-up, missing or incomplete data, and variability in data reporting across centers.

In conclusion, this study provides a comprehensive overview of the infectious burden in the first year following pediatric SOT. More than half of patients experienced at least one clinically relevant infection, with viral infections predominated highlighting the unique vulnerabilities of pediatric recipients. The higher risk of infection was for liver and lung transplant recipients. These findings underscore the need for age and organ specific prevention and management strategies, including optimized vaccination, early diagnostics, and tailored antimicrobial approaches.