Differences in SARS-CoV-2 Clinical Manifestations and Disease Severity in Children and Adolescents by Infecting Variant

COVID-19 evolution in pediatric emergency department patients in Madrid, Spain: a single-center, retrospective study

INTRODUCTION

COVID-19, caused by the SARS-CoV-2 virus, had a significant global impact since its emergence in December 2019. In Spain, the pandemic caused multiple waves of infections, with variations in clinical presentation and predominant viral strains. This study analyzed the evolution of COVID-19 in pediatric patients in a pediatric emergency department in Madrid (Spain), focusing on the changes in clinical manifestations over time.

METHODS

This single-center, retrospective study was conducted from March 2020 to October 2024, including pediatric patients aged 0-18 years with confirmed SARS-CoV-2 infection. Data collected included demographic characteristics, clinical manifestations, and hospital admission rates. The sample was divided into groups based on the predominant circulating variant during specific periods.

RESULTS

A total of 1,949 confirmed COVID-19 cases were analyzed. The median age of the patients was 1.2 years (IQR: 0.4-7.4). The highest number of recorded cases was in infants aged 1-12 months (43.4%). Fever (77.6%) and respiratory symptoms (68.8%) were the most common clinical manifestations. A statistically significant association was observed between the presence of fever and the XBB variant (p < 0.015), the presence of upper respiratory symptoms and the XBB variant (p = 0.015), the presence of gastrointestinal symptoms and the Omicron BA.2 variant (p = 0.009), and the presence of laryngitis and the XBB variant (p < 0.001). The highest number of admissions was recorded in patients aged 1-12 months (61/133; 45.9%), followed by school-aged children aged 6-11 years (22/133; 16.5%).

CONCLUSION

The clinical presentation and age distribution of COVID-19 infections have changed over time. Infants aged 1-12 months were the most affected, a consistent trend across the pandemic. Fever remained the most common clinical manifestation throughout the study period, followed by upper respiratory symptoms.

Early, Robust Mucosal Secretory Immunoglobulin A but not Immunoglobulin G Response to Severe Acute Respiratory Syndrome Coronavirus 2 Spike in Oral Fluid Is Associated With Faster Viral Clearance and Coronavirus Disease 2019 Symptom Resolution

BACKGROUND

Efforts are underway to support the development of novel mucosal coronavirus disease 2019 (COVID-19) vaccines. However, there is limited consensus about the complementary role of mucosal immunity in disease progression and how to evaluate immunogenicity of mucosal vaccines. This study investigated the role of oral mucosal antibody responses in viral clearance and COVID-19 symptom duration.

METHODS

Participants with polymerase chain reaction (PCR)-confirmed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection provided oral fluid for testing with SARS-CoV-2 antibody multiplex assays, nasal swabs for reverse-transcription PCR, and symptom information at up to 8 follow-ups from April 2020 to February 2022.

RESULTS

High and moderate oral fluid anti-spike (S) secretory IgA (SIgA) postinfection was associated with significantly faster viral clearance and symptom resolution across age groups with effect sizes equivalent to prior COVID-19 vaccine immunity at the time of infection. Those with high and moderate anti-S SIgA cleared the virus 14 (95% confidence interval [CI], 10-18) days and recovered 9-10 (95% CI, 6-14) days earlier. Delayed and higher anti-S IgG was associated with significantly longer time to clearance and recovery. Experiencing symptoms >4 weeks was associated with lower anti-receptor-binding domain SIgA 15-30 days after infection onset (P < .001).

CONCLUSIONS

Robust mucosal SIgA early postinfection appears to support faster clearance of SARS-CoV-2 and recovery from COVID-19 symptoms. This research underscores the importance of harmonizing mucosal immune response assays to evaluate new mucosal vaccines.

SARS-CoV-2 Disease Severity and Cycle Threshold Values in Children Infected during Pre-Delta, Delta, and Omicron Periods, Colorado, USA, 2021-2022

In adults, viral load and disease severity can differ by SARS-CoV-2 variant, patterns less understood in children. We evaluated symptomatology, cycle threshold (Ct) values, and SARS-CoV-2 variants among 2,299 pediatric SARS-CoV-2 patients (0-21 years of age) in Colorado, USA, to determine whether children infected with Delta or Omicron had different symptom severity or Ct values than during earlier variants. Children infected during the Delta and Omicron periods had lower Ct values than those infected during pre-Delta, and children <1 year of age had lower Ct values than older children. Hospitalized symptomatic children had lower Ct values than asymptomatic patients. Compared with pre-Delta, more children infected during Delta and Omicron were symptomatic (75.4% pre-Delta, 95.3% Delta, 99.5% Omicron), admitted to intensive care (18.8% pre-Delta, 39.5% Delta, 22.9% Omicron), or received oxygen support (42.0% pre-Delta, 66.3% Delta, 62.3% Omicron). Our data reinforce the need to include children, especially younger children, in pathogen surveillance efforts.

The Effect of Age and Comorbidities: Children vs. Adults in Their Response to SARS-CoV-2 Infection

While children have experienced less severe coronavirus disease (COVID-19) after SARS-CoV-2 infection than adults, the cause of this remains unclear. The objective of this study was to describe the humoral immune response to COVID-19 in child vs. adult household contacts, and to identify predictors of the response over time. In this prospective cohort study, children with a positive SARS-CoV-2 polymerase chain reaction (PCR) test (index case) were recruited along with their adult household contacts. Serum IgG antibodies against SARS-CoV-2 S1/S2 spike proteins were compared between children and adults at 6 and 12 months after infection. A total of 91 participants (37 adults and 54 children) from 36 families were enrolled. Overall, 78 (85.7%) participants were seropositive for anti-S1/S2 IgG antibody at 6 months following infection; this was higher in children than in adults (92.6% vs. 75.7%) (p = 0.05). Significant predictors of a lack of SARS-CoV-2 seropositivity were age ≥ 25 vs. < 12 years (odds ratio [OR] = 0.23, p = 0.04), presence of comorbidities (vs. none, adjusted OR = 0.23, p = 0.03), and immunosuppression (vs. immunocompetent, adjusted OR = 0.17, p = 0.02).

Myeloid-derived suppressor cells and T cell populations in children with Multisystem Inflammatory Syndrome

BACKGROUND

Multisystem inflammatory syndrome in children (MIS-C) represents a hyperinflammatory state that can result in multi-organ dysfunction and death. Myeloid-derived suppressor cells (MDSC) are an immunosuppressive cell population that expands under inflammatory conditions and suppresses T cell function. We hypothesized that MDSC would be increased in children with MIS-C and that MDSC expansion would be associated with T cell lymphopenia.

METHODS

We conducted a prospective, observational study. Initial blood samples were collected within 48 h of admission. Age-matched healthy controls underwent sampling once. MDSC and T cell populations were identified by flow cytometric methods.

RESULTS

We enrolled 22 children with MIS-C (12 ICU, 10 ward) and 21 healthy controls (HC). Children with MIS-C demonstrated significantly higher MDSC compared to HC, and MDSC expansion persisted for >3 weeks in the ICU group. Children with MIS-C admitted to the ICU demonstrated significantly lower absolute numbers of T cells and natural killer cells. There were no significant associations between MDSC and cardiac dysfunction, duration of hospitalization, or vasoactive inotrope score.

CONCLUSIONS

Our study suggests that children critically ill with MIS-C have expansion of MDSC and associated decreased T cell and NK cell populations. Our results did not demonstrate associations between MDSC and clinical outcomes.

IMPACT

Multisystem inflammatory syndrome in children (MIS-C) is a dysregulated immune response occurring several weeks after SARS-CoV-2 infection that can result in multi-organ dysfunction and death. Children severely ill with MIS-C demonstrated increased myeloid-derived suppressor cells and decreased absolute numbers of CD4+ and CD8 + T cells and NK cells compared to healthy controls. There was no significant association between MDSC numbers and clinical outcomes; including cardiac dysfunction, length of stay, or requirement of vasoactive support, in children with MIS-C.

Characteristics of SARS-CoV-2 Omicron BA.5 variants in Shanghai after ending the zero-COVID policy in December 2022: a clinical and genomic analysis

Introduction

An unprecedented surge of Omicron infections appeared nationwide in China in December 2022 after the adjustment of the COVID-19 response policy. Here, we report the clinical and genomic characteristics of SARS-CoV-2 infections among children in Shanghai during this outbreak.

Methods

A total of 64 children with symptomatic COVID-19 were enrolled. SARS-CoV-2 whole genome sequences were obtained using next-generation sequencing (NGS) technology. Patient demographics and clinical characteristics were compared between variants. Phylogenetic tree, mutation spectrum, and the impact of unique mutations on SARS-CoV-2 proteins were analysed in silico.

Results

The genomic monitoring revealed that the emerging BA.5.2.48 and BF.7.14 were the dominant variants. The BA.5.2.48 infections were more frequently observed to experience vomiting/diarrhea and less frequently present cough compared to the BF.7.14 infections among patients without comorbidities in the study. The high-frequency unique non-synonymous mutations were present in BA.5.2.48 (N:Q241K) and BF.7.14 (nsp2:V94L, nsp12:L247F, S:C1243F, ORF7a:H47Y) with respect to their parental lineages. Of these mutations, S:C1243F, nsp12:L247F, and ORF7a:H47Y protein were predicted to have a deleterious effect on the protein function. Besides, nsp2:V94L and nsp12:L247F were predicted to destabilize the proteins.

Discussion

Further in vitro to in vivo studies are needed to verify the role of these specific mutations in viral fitness. In addition, continuous genomic monitoring and clinical manifestation assessments of the emerging variants will still be crucial for the effective responses to the ongoing COVID-19 pandemic.

Differences in Clinical Presentation of COVID-19 in Children Hospitalized During Domination of Early (BA.1, BA.2) and Late (BA.5, BA.2.75, BQ.1 and XBB.1.5) SARS-CoV-2 Omicron Subvariants

This study analyzed data for 1098 children: 575 diagnosed with COVID-19 between January and June 2022 (early Omicron) and 523 hospitalized from July 2022 to April 2023 (late Omicron). New Omicron subvariants lead to similar recovery rates without deaths and acute respiratory distress syndrome in children as BA.1 and BA.2, however, they more often cause fever and croup. Children suffering from comorbidities, presenting with pulmonary lesions and older, may be prone to a more severe consequences of COVID-19 in terms of the currently dominating Omicron subvariants.

Clinical Manifestations Laboratory Tests Abdominal Ultrasonic Findings and In-hospital Prognosis of COVID-19 in 185 Pediatric Cases in a Tertiary Center

BACKGROUND

Despite the COVID-19 pandemic, there is little information about the different clinical aspects of COVID-19 in children. In this study, we assessed the clinical manifestations, outcome, ultrasound, and laboratory findings of pediatric COVID-19.

METHODS

This retrospective study was conducted on 185 children with definitive diagnosis of COVID-19 between 2021 and 2022. The patients' information was retrieved from hospital records.

RESULTS

The average age of the patients was 5.18 ± 4.55 years, and 61.1% were male. The most frequent clinical manifestation was fever (81.1%) followed by cough (31.9%), vomiting (20.0%), and diarrhea (20.0%). Mesenteric lymphadenitis was common on ultrasound and found in 60% of cases. In-hospital death was identified in 3.8% of cases. The mean length of hospital stay was 8.5 days. Mandating intensive care unit (ICU) stay was found in 19.5% and 5.9% of cases were intubated. Acute respiratory distress syndrome (ARDS), lower arterial oxygen saturation, higher white blood cell (WBC) count, and higher C-reactive protein (CRP) were the main determinants of death. Lower age, respiratory distress, early onset of clinical manifestations, lower arterial oxygen saturation, lower serum hemoglobin (Hb) level, and higher CRP level could predict requiring ICU admission.

CONCLUSION

We recommend close monitoring on CRP, serum Hb level, WBC count, and arterial level of oxygenation as clinical indicators for potential progression to critical illness and severe disease. Mesenteric lymphadenitis is a common sonographic finding in pediatric COVID-19 which can cause abdominal pain. Ultrasound is helpful to avoid unnecessary surgical interventions in COVID-19.

Review of pediatric emergency care and the COVID-19 pandemic

The coronavirus disease 2019 (COVID-19) pandemic posed new challenges in health care delivery for patients of all ages. These included inadequate personal protective equipment, workforce shortages, and unknowns related to a novel virus. Children have been uniquely impacted by COVID-19, both from the system of care and socially. In the initial surges of COVID-19, a decrease in pediatric emergency department (ED) volume and a concomitant increase in critically ill adult patients resulted in re-deployment of pediatric workforce to care for adult patients. Later in the pandemic, a surge in the number of critically ill children was attributed to multisystem inflammatory syndrome in children. This was an unexpected complication of COVID-19 and further challenged the health care system. This article reviews the impact of COVID-19 on the entire pediatric emergency care continuum, factors affecting ED care of children with COVID-19 infection, including availability of vaccines and therapeutics approved for children, and pediatric emergency medicine workforce innovations and/or strategies. Furthermore, it provides guidance to emergency preparedness for optimal delivery of care in future health-related crises.

Clinical phenotypes and outcomes in children with multisystem inflammatory syndrome across SARS-CoV-2 variant eras: a multinational study from the 4CE consortium

Background

Multisystem inflammatory syndrome in children (MIS-C) is a severe complication of SARS-CoV-2 infection. It remains unclear how MIS-C phenotypes vary across SARS-CoV-2 variants. We aimed to investigate clinical characteristics and outcomes of MIS-C across SARS-CoV-2 eras.

Methods

We performed a multicentre observational retrospective study including seven paediatric hospitals in four countries (France, Spain, U.K., and U.S.). All consecutive confirmed patients with MIS-C hospitalised between February 1st, 2020, and May 31st, 2022, were included. Electronic Health Records (EHR) data were used to calculate pooled risk differences (RD) and effect sizes (ES) at site level, using Alpha as reference. Meta-analysis was used to pool data across sites.

Findings

Of 598 patients with MIS-C (61% male, 39% female; mean age 9.7 years [SD 4.5]), 383 (64%) were admitted in the Alpha era, 111 (19%) in the Delta era, and 104 (17%) in the Omicron era. Compared with patients admitted in the Alpha era, those admitted in the Delta era were younger (ES -1.18 years [95% CI -2.05, -0.32]), had fewer respiratory symptoms (RD -0.15 [95% CI -0.33, -0.04]), less frequent non-cardiogenic shock or systemic inflammatory response syndrome (SIRS) (RD -0.35 [95% CI -0.64, -0.07]), lower lymphocyte count (ES -0.16 × 109/uL [95% CI -0.30, -0.01]), lower C-reactive protein (ES -28.5 mg/L [95% CI -46.3, -10.7]), and lower troponin (ES -0.14 ng/mL [95% CI -0.26, -0.03]). Patients admitted in the Omicron versus Alpha eras were younger (ES -1.6 years [95% CI -2.5, -0.8]), had less frequent SIRS (RD -0.18 [95% CI -0.30, -0.05]), lower lymphocyte count (ES -0.39 × 109/uL [95% CI -0.52, -0.25]), lower troponin (ES -0.16 ng/mL [95% CI -0.30, -0.01]) and less frequently received anticoagulation therapy (RD -0.19 [95% CI -0.37, -0.04]). Length of hospitalization was shorter in the Delta versus Alpha eras (-1.3 days [95% CI -2.3, -0.4]).

Interpretation

Our study suggested that MIS-C clinical phenotypes varied across SARS-CoV-2 eras, with patients in Delta and Omicron eras being younger and less sick. EHR data can be effectively leveraged to identify rare complications of pandemic diseases and their variation over time.

Funding

None.

Classification of SARS-CoV-2 Pediatric Hospitalizations: Delta vs Omicron Variant

OBJECTIVES

To classify COVID-19 pediatric hospitalizations based on reason for admission ("for COVID" or "with COVID") and to compare disease severity and age between the delta and omicron variant predominant time frames.

METHODS

This was a retrospective study in patients aged ≤18 years who were hospitalized at a large, tertiary care pediatric hospital for COVID-19 from July 2021 to February 2022. Disease severity was determined based on length of stay, PICU admission, and Remdesivir receipt. A χ2 analysis and a Cochran-Mantel-Haenszel test were used to analyze relationships between presumed variant type with admission reason, disease severity, and age.

RESULTS

There were 492 patients included with more admissions during the omicron variant predominant time frame (n = 307). More patients were admitted "for COVID" during the delta variant predominant time frame compared with the omicron variant predominant time frame (P < .001) with interrater reliability testing indicating substantial agreement between reviewers. PICU admissions (P < .001), receipt of remdesivir (P < .001), and length of stay (P < .001) were greater during the delta variant predominant time frame. Hospitalizations "for COVID" were more common in patients aged <5 years during both the delta (odds ratio, 2.6; 95% confidence interval, 1.2-5.7) and omicron (odds ratio, 5.7; 95% confidence interval, 3.4-9.4) predominant time frames compared with older patients.

CONCLUSIONS

There was a higher proportion of symptomatic COVID-19 hospitalizations with higher disease severity during the delta variant predominance, indicating greater disease severity during earlier periods of the pandemic despite lower hospitalization rates.

Acute COVID-19 in unvaccinated children without a history of previous infection during the delta and omicron periods

BACKGROUND

The Omicron variant has been the predominant severe acute respiratory syndrome coronavirus 2 variant circulating in Korea since January 2022. This study evaluated and compared the clinical characteristics of children with coronavirus disease 2019 (COVID-19) between the Delta and Omicron periods.

METHODS

The medical records of children aged < 12 years diagnosed with acute COVID-19 (<2 weeks of symptom onset) at seven university-affiliated hospitals were retrospectively reviewed. Children with a previous history of COVID-19 or vaccination were excluded. The clinical characteristics of the included children during the Delta (1 August 2021 to 15 January 2022) and Omicron (16 January to 30 June 2022) periods were compared.

RESULTS

Among the 515 children included in the study, 36 (7.0%) and 479 (93.0%) were diagnosed with COVID-19 during the Delta and Omicron periods, respectively. A total of 142 (27.6%) were hospitalized, and the hospitalization rate was higher during the Delta period than the Omicron period (91.7% vs. 22.8%, p < 0.001). The incidence of fever (p = 0.009), vomiting (p = 0.031), and seizures (p = 0.007) was higher during the Omicron period, whereas the incidence of rhinorrhea (p = 0.027) was higher during the Delta period. Clinical severity and outcomes were comparable between the two periods. During the Omicron period, 6.4% of the hospitalized children received oxygen therapy and 1.8% received intensive care.

CONCLUSION

The incidence of fever and seizures was higher during the Omicron period in pediatric patients without a history of vaccination or previous COVID-19. However, the clinical severity was similar during both periods.

Clinical features of COVID-19 in Italian outpatient children and adolescents during Parental, Delta, and Omicron waves: a prospective, observational, cohort study

Introduction

COVID-19 features changed with the Omicron variant of SARS-CoV-2 in adults. This study aims to describe COVID-19 symptoms in children and adolescents during the Parental, Delta, and Omicron eras.

Methods

A single-centre, prospective observational study was conducted on individuals aged 0-20 years attending the University Hospital of Padua (Italy) from April 2020 to December 2022. COVID-19 cases were defined by positive SARS-CoV-2 molecular detection and/or serology; patient/family symptoms and virological positivity were considered to determine the infection onset. Variables were summarized and compared using appropriate tests of descriptive statistics.

Results

A total of 509 cases [46% female, median age eight years (IQR: 4-12)] were studied. Three-hundred-eighty-seven (76%), 52 (10%), and 70 (14%) subjects experienced COVID-19 during the Parental, Delta, and Omicron waves, respectively. All subjects developed an asymptomatic/mild COVID-19. Overall, the most frequent symptoms were fever (47%) and rhinitis (21%), which showed a significant increasing incidence from the Parental to Omicron waves (p < 0.001). Conversely, diarrhea was most common during the pre-Omicron eras (p = 0.03). Stratifying symptoms according to the age group, fever, rhinitis, and skin rashes were observed more frequently among infants/toddlers; conversely, fatigue was more common in children older than five years. The duration of symptoms was similar across different SARS-CoV-2 variants of concern (VOCs); conversely, the number of symptoms varied according to the age group (p < 0.0001).

Discussion

This study showed differences in COVID-19 clinical presentation among infants, children, and adolescents and confirmed Omicron infection is more likely to be associated with upper respiratory symptoms. However, further population-based studies are needed to support these findings. In addition, active surveillance will play a crucial role in assessing the disease severity of future VOCs.

Infection-induced seroconversion and seroprevalence of SARS-CoV-2 among a cohort of children and youth in Montreal, Canada

The EnCORE study is a prospective serology study of SARS-CoV-2 in a cohort of children from Montreal, Canada. Based on data from our fourth round of data collection (May-October 2022), we estimated SARS-CoV-2 seroprevalence and seroconversion. Using multivariable regression, we identified factors associated with seroconversion. Our results show that previously seronegative children were approximately 9-12 times more likely to seroconvert during the early Omicron-dominant period compared to pre-Omicron rounds. Unlike the pre-Omicron rounds, the adjusted rate of seroconversion among 2- to 4-year-olds was higher than older age groups. As seen previously, higher seroconversion rates were associated with ethnic/racial minority status.

The Influence of SARS-CoV-2 Variants B.1.1.7 and B.1.617.2 on a Different Clinical Course and Severity of COVID-19 in Children Hospitalized in 2021 Compared With 2020

BACKGROUND

This study aimed to analyze the differences in the epidemiologic and clinical characteristics of coronavirus disease 2019 (COVID-19) in children hospitalized in 2021, when the severe acute respiratory syndrome coronavirus 2 variants B.1.1.7 (alpha) and B.1.617.2 (delta) dominated, compared with 2020.

METHODS

In this multicenter study based on the pediatric part of the national SARSTer register (SARSTer-PED), we included 2771 children (0-18 years) with COVID-19 diagnosed between March 1, 2020, and December 31, 2021, from 14 Polish inpatient centers. An electronic questionnaire, which addressed epidemiologic and clinical data, was used.

RESULTS

Children hospitalized in 2021 were younger compared with those reported in 2020 (mean 4.1 vs. 6.8 years, P = 0 .01). Underlying comorbidities were reported in 22% of the patients. The clinical course was usually mild (70%). A significant difference in the clinical course assessment between 2020 and 2021 was found, with more asymptomatic patients in 2020 and more severely ill children in 2021. In total, 5% of patients were severely or critically ill, including <3% of the participants in 2020 and 7% in 2021. The calculated mortality rate was 0.1% in general and 0.2% in 2021.

CONCLUSION

Infections with severe acute respiratory syndrome coronavirus 2 variants alpha and delta lead to a more severe course of COVID-19 with more pronounced clinical presentation and higher fatality rates than infection with an original strain. Most of the children requiring hospitalization due to COVID-19 do not have underlying comorbidities.

The impact of RSV/SARS-CoV-2 co-infection on clinical disease and viral replication: insights from a BALB/c mouse model

RSV and SARS-CoV-2 are prone to co-infection with other respiratory viruses. In this study, we use RSV/SARS-CoV-2 co-infection to evaluate changes to clinical disease and viral replication in vivo. To consider the severity of RSV infection, effect of sequential infection, and the impact of infection timing, mice were co-infected with varying doses and timing. Compared with a single infection of RSV or SARS-CoV-2, the co-infection of RSV/SARS-CoV-2 and the primary infection of RSV followed by SARS-CoV-2 results in protection from SARS-CoV-2-induced clinical disease and reduces SARS-CoV-2 replication. Co-infection also augmented RSV replication at early timepoints with only the low dose. Additionally, the sequential infection of RSV followed by SARS-CoV-2 led to improved RSV clearance regardless of viral load. However, SARS-CoV-2 infection followed by RSV results in enhanced SARS-CoV-2-induced disease while protecting from RSV-induced disease. SARS-CoV-2/RSV sequential infection also reduced RSV replication in the lung tissue, regardless of viral load. Collectively, these data suggest that RSV and SARS-CoV-2 co-infection may afford protection from or enhancement of disease based on variation in infection timing, viral infection order, and/or viral dose. In the pediatric population, understanding these infection dynamics will be critical to treat patients and mitigate disease outcomes.

Clinical Course and Severity of COVID-19 in 940 Infants with and without Comorbidities Hospitalized in 2020 and 2021: The Results of the National Multicenter Database SARSTer-PED

This study aimed to analyze the differences in severity and clinical characteristics of COVID-19 in infants hospitalized in Poland in 2021, when the dominance of variants of concern (VOCs) alpha and delta was reported, compared to 2020, when original (wild) SARS-CoV-2 was dominant (III-IV vs. I-II waves of the pandemic, respectively). In addition, the influence of the presence of comorbidities on the clinical course of COVID-19 in infants was studied. This multicenter study, based on the pediatric part of the national SARSTer database (SARSTer-PED), included 940 infants with COVID-19 diagnosed between March 1, 2020, and December 31, 2021, from 13 Polish inpatient centers. An electronic questionnaire, which addressed epidemiological and clinical data, was used. The number of hospitalized infants was significantly higher in 2021 than in 2020 (651 vs. 289, respectively). The analysis showed similar lengths of infant hospitalization in 2020 and 2021, but significantly more children were hospitalized for more than 7 days in 2020 (p < 0.009). In both analyzed periods, the most common route of infection for infants was household contact. There was an increase in the percentage of comorbidities, especially prematurity, in children hospitalized in 2021 compared to 2020. Among the clinical manifestations, fever was predominant among children hospitalized in 2021 and 2020. Cough, runny nose, and loss of appetite were significantly more frequently observed in 2021 (p < 0.0001). Severe and critical conditions were significantly more common among children with comorbidities. More infants were hospitalized during the period of VOCs dominance, especially the delta variant, compared to the period of wild strain dominance, even though indications for hospitalization did not include asymptomatic patients during that period. The course of COVID-19 was mostly mild, characterized mainly by fever and respiratory symptoms. Comorbidities, particularly from the cardiovascular system and prematurity, were associated with a more severe course of the disease in infants.