Asymptomatic and Mild SARS-CoV-2 Infections Elicit Lower Immune Activation and Higher Specific Neutralizing Antibodies in Children Than in Adults

SARS-CoV-2 Infection and Postacute Risk of Non-Coronavirus Disease 2019 Infectious Disease Hospitalizations: A Nationwide Cohort Study of Danish Adults Aged ≥50 Years

BACKGROUND

Reports suggest that the potential long-lasting health consequences of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection may involve persistent dysregulation of some immune populations, but the potential clinical implications are unknown. We investigated the associated risk of hospitalization due to non-coronavirus disease 2019 (COVID-19) infectious diseases following the postacute phase of SARS-CoV-2 infection.

METHODS

By cross-linking data from the comprehensive Danish test and surveillance system for COVID-19 together with nationwide healthcare and demographic registers, we established a study cohort of 2 430 694 individuals aged ≥50 years, from 1 January 2021 to 10 December 2022, with no evidence of SARS-CoV-2 infection prior to study entry. Using Poisson regression, we compared the outcome rates of non-COVID-19 infectious disease hospitalizations following the acute phase of (a first) SARS-CoV-2 infection (defined as ≥29 days since the day of infection) in recovered individuals with rates among SARS-CoV-2-uninfected individuals.

RESULTS

Among 2 430 694 included individuals (mean age, 66.8 [standard deviation, 11.3] years), 930 071 acquired SARS-CoV-2 infection during follow-up totaling 4 519 913 person-years. The postacute phase of SARS-CoV-2 infection was associated with an incidence rate ratio (IRR) of 0.90 (95% confidence interval [CI]: .88-.92) for any infectious disease hospitalization. Findings (IRR [95% CI]) were similar for upper respiratory tract (1.08 [.97-1.20]), lower respiratory tract (0.90 [.87-.93]), influenza (1.04 [.94-1.15]), gastrointestinal (1.28 [.78-2.09]), skin (0.98 [.93-1.03]), urinary tract (1.01 [.96-1.08]), certain invasive bacterial (0.96 [.91-1.01]), and other (0.96 [.92-1.00]) infectious disease hospitalizations and in subgroups.

CONCLUSIONS

Our study does not support an increased susceptibility to non-COVID-19 infectious disease hospitalization following SARS-CoV-2 infection.

Proteomic profiling identifies biomarkers of COVID-19 severity

SARS-CoV-2 infection remains a major public health concern, particularly for the aged and those individuals with co-morbidities at risk for developing severe COVID-19. Understanding the pathogenesis and biomarkers associated with responses to SARS-CoV-2 infection remain critical components in developing effective therapeutic approaches, especially in cases of severe and long-COVID-19. In this study blood plasma protein expression was compared in subjects with mild, moderate, and severe COVID-19 disease. Evaluation of an inflammatory protein panel confirms upregulation of proteins including TNFβ, IL-6, IL-8, IL-12, already associated with severe cytokine storm and progression to severe COVID-19. Importantly, we identify several proteins not yet associated with COVID-19 disease, including mesothelin (MSLN), that are expressed at significantly higher levels in severe COVID-19 subjects. In addition, we find a subset of markers associated with T-cell and dendritic cell responses to viral infection that are significantly higher in mild cases and decrease in expression as severity of COVID-19 increases, suggesting that an immediate and effective activation of T-cells is critical in modulating disease progression. Together, our findings identify new targets for further investigation as therapeutic approaches for the treatment of SARS-CoV-2 infection and prevention of complications of severe COVID-19.

Stronger and durable SARS-CoV-2 immune response to mRNA vaccines in 5-11 years old children with prior COVID-19

BACKGROUND AND OBJECTIVES

mRNA vaccines elicit a durable humoral response to SARS-CoV-2 in adults, whereas evidence in children is scarce. This study aimed to assess the early and long-term immune response to the mRNA vaccine in children with or without previous SARS-CoV-2 infection.

METHODS

In a multicentre prospective observational study, we profiled the immune response to the Pfizer BioNTech (BNT162b2) vaccine in 5-11-year-old children attending the University Pediatric Hospital of Padua and Bambino-Gesù Hospital in Rome (Italy) from December-2021 to February-2023. Blood samples were collected pre-, 1-, and 6-months after vaccination. Neutralizing antibodies (NAbs) and anti-spike-receptor-binding-domain (anti-S-RBD) IgG titers were analyzed through Plaque Reduction Neutralization Test (PRNT) and chemiluminescent immune-enzymatic assay (CLIA), respectively. Immune cell phenotypes were analyzed by flow cytometry.

RESULTS

Sixty children (26 [43 %] female, median age = 8 years [IQR = 7-10.7]) were enrolled in the study, including 46 children with a laboratory-confirmed previous COVID-19 (SARS-CoV-2-recovered) and 14 SARS-CoV-2-naïve participants defined as the absence of antigen-specific antibodies before vaccination. SARS-CoV-2-recovered participants recorded higher anti-S-RBD IgG and Wild-type and Omicron BA.2 NAbs titers than SARS-CoV-2-naïve participants at both 1- and 6-months after vaccination. Antibody titers correlated with T (Tregs) and B (Bregs) regulatory cell frequencies in SARS-CoV-2-recovered children. Both SARS-CoV-2-recovered and SARS-CoV-2-naïve participants decreased antibody titers by approximately 100 to 250 % from 1 to 6 months. While children with immunocompromising underlying conditions developed immune responses comparable to those of healthy children, solid organ transplant recipients exhibited lower levels of NAbs and anti-S-RBD IgG titers, as well as reduced frequencies of Tregs and Bregs.

CONCLUSIONS

mRNA vaccination triggered a higher production of specific anti-SARS-CoV-2 antibodies along with increased levels of regulatory cells in children with previous SARS-CoV-2 infection up to the following 6 months. These findings provide insights into boosting pre-existing immunity.

Differences in the Clinical Manifestations and Host Immune Responses to SARS-CoV-2 Variants in Children Compared to Adults

The COVID-19 pandemic challenged the medical field to rapidly identify and implement new approaches to the diagnosis, treatment and prevention of SARS-CoV-2 infections. The scientific community also needed to rapidly initiate basic, translational, clinical and epidemiological studies to understand the pathophysiology of this new family of viruses, which continues to evolve with the emergence of new genetic variants. One of the earliest clinical observations that provided a framework for the research was the finding that, in contrast to most other respiratory viruses, children developed less severe acute and post-acute disease compared to adults. Although the clinical manifestations of SARS-CoV-2 infection changed with each new wave of the pandemic, which was dominated by evolving viral variants, the differences in severity between children and adults persisted. Comparative immunologic studies have shown that children mount a more vigorous local innate response characterized by the activation of interferon pathways and recruitment of innate cells to the mucosa, which may mitigate against the hyperinflammatory adaptive response and systemic cytokine release that likely contributed to more severe outcomes including acute respiratory distress syndrome in adults. In this review, the clinical manifestations and immunologic responses in children during the different waves of COVID-19 are discussed.

Cohort profile: evaluation of immune response and household transmission of SARS-CoV-2 in Costa Rica: the RESPIRA study

PURPOSE

The RESPIRA cohort aims to describe the nature, magnitude, time course and efficacy of the immune response to SARS-CoV-2 infection and vaccination, population prevalence, and household transmission of COVID-19.

PARTICIPANTS

From November 2020, we selected age-stratified random samples of COVID-19 cases from Costa Rica confirmed by PCR. For each case, two population-based controls, matched on age, sex and census tract were recruited, supplemented with hospitalised cases and household contacts. Participants were interviewed and blood and saliva collected for antibodies and PCR tests. Participants will be followed for 2 years to assess antibody response and infection incidence.

FINDINGS TO DATE

Recruitment included 3860 individuals: 1150 COVID-19 cases, 1999 population controls and 719 household contacts from 304 index cases. The age and regional distribution of cases was as planned, including four age strata, 30% rural and 70% urban. The control cohort had similar sex, age and regional distribution as the cases according to the study design. Among the 1999 controls recruited, 6.8% reported at enrolment having had COVID-19 and an additional 12.5% had antibodies against SARS-CoV-2. Compliance with visits and specimens has been close to 70% during the first 18 months of follow-up. During the study, national vaccination was implemented and nearly 90% of our cohort participants were vaccinated during follow-up.

FUTURE PLANS

RESPIRA will enable multiple analyses, including population prevalence of infection, clinical, behavioural, immunological and genetic risk factors for SARS-CoV-2 acquisition and severity, and determinants of household transmission. We are conducting retrospective and prospective assessment of antibody levels, their determinants and their protective efficacy after infection and vaccination, the impact of long-COVID and a series of ancillary studies. Follow-up continues with bimonthly saliva collection for PCR testing and biannual blood collection for immune response analyses. Follow-up will be completed in early 2024.

TRIAL REGISTRATION NUMBER

NCT04537338.

Comparative study showed that children faced a 78% higher risk of new-onset conditions after they had COVID-19

AIM

Children have largely been unaffected by severe COVID-19 compared to adults, but data suggest that they may have experienced new conditions after developing the disease. We compared outcomes in children who had experienced COVID-19 and healthy controls.

METHODS

A retrospective nested cohort study assessed the incidence rate of new-onset conditions after COVID-19 in children aged 0-14 years. Data were retrieved from an Italian paediatric primary care database linked to Veneto Region registries. Exposed children with a positive nasopharyngeal swab were matched 1:1 with unexposed children who had tested negative. Conditional Cox regression was fitted to estimate the adjusted hazard ratios (aHR) and 95% confidence intervals (CI) for the exposure and outcome associations after adjusting for covariates.

RESULTS

We compared 1656 exposed and 1656 unexposed children from 1 February 2020 to 30 November 2021. The overall excess risk for new-onset conditions after COVID-19 was 78% higher in the exposed than unexposed children. We found significantly higher risks for some new conditions in exposed children, including mental health issues (aHR 1.8, 95% CI 1.1-3.0) and neurological problems (aHR 2.4, 95% CI 1.4-4.1).

CONCLUSION

Exposed children had a 78% higher risk of developing new conditions of interest after COVID-19 than unexposed children.

Psychiatric disorders during pregnancy in asymptomatic and mildly symptomatic SARS-CoV-2 positive women: Prevalence and effect on outcome

The effect of psychiatric comorbidity on pregnancy outcome among SARS-CoV-2 positive women with asymptomatic and mildly symptomatic infections remains largely unknown. We reviewed the electronic medical records of all pregnant women who received care at Mayo Health System and tested positive for SARS-CoV-2 (RT-PCR) from March 2020 through October 2021. Among 789 patients, 34.2% (n = 270) had psychiatric comorbidity. Of those with psychiatric comrobidity, 62.2% (n = 168) had depression prior to pregnancy, and 5.2% (n = 14) reported new-onset depression during pregnancy. Before pregnancy, 65.6% (n = 177) had anxiety, and 4.4% (n = 12) developed anxiety during pregnancy Thirteen percent of SARS-CoV-2 positive pregnant women (n = 108) received psychotropic medication during pregnancy. In addition, 6.7% (n = 18) and 10.7% (n = 29) of pregnant women with psychiatric comorbidity had documented nicotine, cannabis and/ or illicit substance use during and prior to pregnancy, respectively. We depicted a significantly higher risk for cesarean delivery [35.6% vs. 24.9%) in asymptomatic and mildly symptomatic SARS-CoV-2 positive pregnant women with psychiatric comorbidity. In conclusion, the prevalence rates of depression, anxiety, and prescribed antidepressant medications during pregnancy among asymptomatic and mildly symptomatic SARS-CoV-2 infected women were substantially higher than average, which negatively impacted pregnancy and neonatal outcomes.

The Immunological Profile of SARS-CoV-2 Infection in Children Is Linked to Clinical Severity and Age

Coronavirus disease 19 (COVID-19) is clinically less severe in children, even if the wide variety and degree of severity of symptoms reported in children pose a still-unresolved challenge for clinicians. We performed an in-depth analysis of the immunological profiles of 18 hospitalized SARS-CoV-2-infected children, whose results were compared to those obtained from 13 age- and sex-matched healthy controls (HC). The patients were categorized as paucisymptomatic/moderate (55.6%) or severe/critical (44.5%) according to established diagnostic criteria and further stratified into the categories of infants (1–12 months), children (1–12 years), and adolescents (>12 years). We assessed SARS-CoV-2-specific RBD antibodies (Ab), neutralizing antibodies (nAb), and circulating cytokines/chemokines in the plasma, and the SARS-CoV-2-specific immune response was measured in PBMCs by gene expression and secretome analyses. Our results showed peculiar circulating cytokine/chemokine profiles among patients sharing a similar clinical phenotype. A cluster of patients consisting of infants with severe symptoms presented hyperinflammatory profiles, together with extremely polarized antibody profiles. In a second cluster consisting of paucisymptomatic patients, a less pronounced increase in the level of inflammatory cytokines, together with an association between the selected cytokines and humoral responses, was observed. A third cluster, again consisting of paucisymptomatic patients, showed a circulating cytokine/chemokine profile which overlapped with that of the HC. The SARS-CoV-2-stimulated production of pro-inflammatory proteins, T lymphocyte activation, and migration-specific proteins, were significantly increased in SARS-CoV-2-infected children compared to the HC. Our findings suggest that immune response activation in the course of SARS-CoV-2 infection in children is directly correlated with clinical severity and, to a lesser extent, age.

Regulatory T Cells (Tregs) and COVID-19: Unveiling the Mechanisms, and Therapeutic Potentialities with a Special Focus on Long COVID

The COVID-19 pandemic has caused havoc all around the world. The causative agent of COVID-19 is the novel form of the coronavirus (CoV) named SARS-CoV-2, which results in immune system disruption, increased inflammation, and acute respiratory distress syndrome (ARDS). T cells have been important components of the immune system, which decide the fate of the COVID-19 disease. Recent studies have reported an important subset of T cells known as regulatory T cells (Tregs), which possess immunosuppressive and immunoregulatory properties and play a crucial role in the prognosis of COVID-19 disease. Recent studies have shown that COVID-19 patients have considerably fewer Tregs than the general population. Such a decrement may have an impact on COVID-19 patients in a number of ways, including diminishing the effect of inflammatory inhibition, creating an inequality in the Treg/Th17 percentage, and raising the chance of respiratory failure. Having fewer Tregs may enhance the likelihood of long COVID development in addition to contributing to the disease's poor prognosis. Additionally, tissue-resident Tregs provide tissue repair in addition to immunosuppressive and immunoregulatory activities, which may aid in the recovery of COVID-19 patients. The severity of the illness is also linked to abnormalities in the Tregs' phenotype, such as reduced expression of FoxP3 and other immunosuppressive cytokines, including IL-10 and TGF-beta. Hence, in this review, we summarize the immunosuppressive mechanisms and their possible roles in the prognosis of COVID-19 disease. Furthermore, the perturbations in Tregs have been associated with disease severity. The roles of Tregs are also explained in the long COVID. This review also discusses the potential therapeutic roles of Tregs in the management of patients with COVID-19.

Understanding COVID-19 in children: immune determinants and post-infection conditions

Coronavirus disease 2019 in children presents with milder clinical manifestations than in adults. On the other hand, the presence of a wide range of inflammatory manifestations, including multisystem inflammatory syndrome in children (MIS-C), in the period after infection suggests a particular susceptibility of some children toward severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Both protective factors that prevent evolution to severe forms and risk factors for post-infectious conditions are likely to be found in age-related differences in the immune system. The prompt innate response with type I IFN production and the generation of neutralizing antibodies play a crucial role in containing the infection. The greater number of naive and regulatory cells in children helps to avoid the cytokine storm while the causes of the intense inflammatory response in MIS-C need to be elucidated. This review aims to analyze the main results of the recent literature assessing immune response to SARS-CoV-2 over the pediatric age group. We summarized such observations by dividing them into innate and acquired immunity, then reporting how altered immune responses can determine post-infectious conditions. IMPACT: The main immune markers of acute SARS-CoV-2 infection in children are summarized in this review. This paper reports a broad overview of age-related differences in the immune response to SARS-CoV-2 and emerging post-infection conditions. A summary of currently available therapies for the pediatric age group is provided.

Immune profiling of SARS-CoV-2 epitopes in asymptomatic and symptomatic pediatric and adult patients

BACKGROUND

The infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has unpredictable manifestations of coronavirus disease (COVID-19) and variable clinical course with some patients being asymptomatic whereas others experiencing severe respiratory distress, or even death. We aimed to evaluate the immunoglobulin G (IgG) response towards linear peptides on a peptide array containing sequences from SARS-CoV-2, Middle East respiratory syndrome-related coronavirus (MERS) and common-cold coronaviruses 229E, OC43, NL63 and HKU1 antigens, in order to identify immunological indicators of disease outcome in SARS-CoV-2 infected patients.

METHODS

We included in the study 79 subjects, comprising 19 pediatric and 30 adult SARS-CoV-2 infected patients with increasing disease severity, from mild to critical illness, and 30 uninfected subjects who were vaccinated with one dose of SARS-CoV-2 spike mRNA BNT162b2 vaccine. Serum samples were analyzed by a peptide microarray containing 5828 overlapping 15-mer synthetic peptides corresponding to the full SARS-CoV-2 proteome and selected linear epitopes of spike (S), envelope (E) and membrane (M) glycoproteins as well as nucleoprotein (N) of MERS, SARS and coronaviruses 229E, OC43, NL63 and HKU1 (isolates 1, 2 and 5).

RESULTS

All patients exhibited high IgG reactivity against the central region and C-terminus peptides of both SARS-CoV-2 N and S proteins. Setting the threshold value for serum reactivity above 25,000 units, 100% and 81% of patients with severe disease, 36% and 29% of subjects with mild symptoms, and 8% and 17% of children younger than 8-years reacted against N and S proteins, respectively. Overall, the total number of peptides in the SARS-CoV-2 proteome targeted by serum samples was much higher in children compared to adults. Notably, we revealed a differential antibody response to SARS-CoV-2 peptides of M protein between adults, mainly reacting against the C-terminus epitopes, and children, who were highly responsive to the N-terminus of M protein. In addition, IgG signals against NS7B, NS8 and ORF10 peptides were found elevated mainly among adults with mild (63%) symptoms. Antibodies towards S and N proteins of other coronaviruses (MERS, 229E, OC43, NL63 and HKU1) were detected in all groups without a significant correlation with SARS-CoV-2 antibody levels.

CONCLUSIONS

Overall, our results showed that antibodies elicited by specific linear epitopes of SARS-CoV-2 proteome are age dependent and related to COVID-19 clinical severity. Cross-reaction of antibodies to epitopes of other human coronaviruses was evident in all patients with distinct profiles between children and adult patients. Several SARS-CoV-2 peptides identified in this study are of particular interest for the development of vaccines and diagnostic tests to predict the clinical outcome of SARS-CoV-2 infection.

The Notch1/CD22 signaling axis disrupts Treg function in SARS-CoV-2-associated multisystem inflammatory syndrome in children

Multisystem inflammatory syndrome in children (MIS-C) evolves in some pediatric patients following acute infection with SARS-CoV-2 by hitherto unknown mechanisms. Whereas acute-COVID-19 severity and outcomes were previously correlated with Notch4 expression on Tregs, here, we show that Tregs in MIS-C were destabilized through a Notch1-dependent mechanism. Genetic analysis revealed that patients with MIS-C had enrichment of rare deleterious variants affecting inflammation and autoimmunity pathways, including dominant-negative mutations in the Notch1 regulators NUMB and NUMBL leading to Notch1 upregulation. Notch1 signaling in Tregs induced CD22, leading to their destabilization in a mTORC1-dependent manner and to the promotion of systemic inflammation. These results identify a Notch1/CD22 signaling axis that disrupts Treg function in MIS-C and point to distinct immune checkpoints controlled by individual Treg Notch receptors that shape the inflammatory outcome in SARS-CoV-2 infection.

A scoping review of regulatory T cell dynamics in convalescent COVID-19 patients - indications for their potential involvement in the development of Long COVID?

Background

Recovery from coronavirus disease 2019 (COVID-19) can be impaired by the persistence of symptoms or new-onset health complications, commonly referred to as Long COVID. In a subset of patients, Long COVID is associated with immune system perturbations of unknown etiology, which could be related to compromised immunoregulatory mechanisms.

Objective

The objective of this scoping review was to summarize the existing literature regarding the frequency and functionality of Tregs in convalescent COVID-19 patients and to explore indications for their potential involvement in the development of Long COVID.

Design

A systematic search of studies investigating Tregs during COVID-19 convalescence was conducted on MEDLINE (via Pubmed) and Web of Science.

Results

The literature search yielded 17 relevant studies, of which three included a distinct cohort of patients with Long COVID. The reviewed studies suggest that the Treg population of COVID-19 patients can reconstitute quantitatively and functionally during recovery. However, the comparison between recovered and seronegative controls revealed that an infection-induced dysregulation of the Treg compartment can be sustained for at least several months. The small number of studies investigating Tregs in Long COVID allowed no firm conclusions to be drawn about their involvement in the syndrome's etiology. Yet, even almost one year post-infection Long COVID patients exhibit significantly altered proportions of Tregs within the CD4+ T cell population.

Conclusions

Persistent alterations in cell frequency in Long COVID patients indicate that Treg dysregulation might be linked to immune system-associated sequelae. Future studies should aim to address the association of Treg adaptations with different symptom clusters and blood parameters beyond the sole quantification of cell frequencies while adhering to consensualized phenotyping strategies.

Long-term Immune Response to SARS-CoV-2 Infection Among Children and Adults After Mild Infection

IMPORTANCE

Understanding the long-term immune response against SARS-CoV-2 infection in children is crucial to optimize vaccination strategies. Although it is known that SARS-CoV-2 antibodies may persist in adults 12 months after infection, data are limited in the pediatric population.

OBJECTIVE

To examine long-term anti-SARS-CoV-2 spike receptor-binding domain (S-RBD) IgG kinetics in children after SARS-CoV-2 infection.

DESIGN, SETTING, AND PARTICIPANTS

In this single-center, prospective cohort study, patients were enrolled consecutively from April 1, 2020, to August 31, 2021, at the COVID-19 Family Cluster Follow-up Clinic, Department of Women's and Children's Health, University Hospital of Padua. A cohort of 252 COVID-19 family clusters underwent serologic follow-up at 1 to 4, 5 to 10, and more than 10 months after infection with quantification of anti-S-RBD IgG by chemiluminescent immunoassay.

EXPOSURES

SARS-CoV-2 infection.

RESULTS

Among 902 study participants, 697 had confirmed SARS-CoV-2 infection, including 351 children or older siblings (mean [SD] age, 8.6 [5.1] years) and 346 parents (mean [SD] age, 42.5 [7.1] years). Among 697 cases, 674 (96.7%) were asymptomatic or mild. Children had significantly higher S-RBD IgG titers than older patients across all follow-up time points, with an overall median S-RBD IgG titer in patients younger than 3 years 5-fold higher than adults (304.8 [IQR, 139.0-516.6] kBAU/L vs 55.6 [24.2-136.0] kBAU/L, P < .001). Longitudinal analysis of 56 study participants sampled at least twice during follow-up demonstrated the persistence of antibodies up to 10 months from infection in all age classes, despite a progressive decline over time.

CONCLUSIONS AND RELEVANCE

In this cohort study of Italian children and adults following SARS-CoV-2 infection different kinetics of SARS-CoV-2 antibodies were found across several age classes of individuals with asymptomatic or mild COVID-19, which could help in optimizing COVID-19 vaccination strategies and prevention policies. This work provides further evidence of sustained immune response in children up to 1 year after primary SARS-CoV-2 infection.

Extracellular ATP and Imbalance of CD4+ T Cell Compartment in Pediatric COVID-19

Severe COVID-19 in children is rare, but the reasons underlying are unclear. Profound alterations in T cell responses have been well characterized in the course of adult severe COVID-19, but little is known about the T cell function in children with COVID-19. Here, we made three major observations in a cohort of symptomatic children with acute COVID-19: 1) a reduced frequency of circulating FoxP3+ regulatory T cells, 2) the prevalence of a TH17 polarizing microenvironment characterized by high plasma levels of IL-6, IL-23, and IL17A, and an increased frequency of CD4+ T cells expressing ROR-γt, the master regulator of TH17 development, and 3) high plasma levels of ATP together with an increased expression of the P2X7 receptor. Moreover, that plasma levels of ATP displayed an inverse correlation with the frequency of regulatory T cells but a positive correlation with the frequency of CD4+ T cells positive for the expression of ROR-γt. Collectively, our data indicate an imbalance in CD4+ T cell profiles during pediatric COVID-19 that might favor the course of inflammatory processes. This finding also suggests a possible role for the extracellular ATP in the acquisition of an inflammatory signature by the T cell compartment offering a novel understanding of the involved mechanisms.

COVID-19 in children: From afterthought to unknown

Children were initially considered unsusceptible to severe COVID-19. Our knowledge after two years has changed dramatically, but there are still many unknowns. Here, we report the current knowledge about why children generally experience a milder COVID-19 course and highlight research questions about pediatric infection that require answers.

Children and Adults With Mild COVID-19: Dynamics of the Memory T Cell Response up to 10 Months

Background

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has led to considerable morbidity/mortality worldwide, but most infections, especially among children, have a mild course. However, it remains largely unknown whether infected children develop cellular immune memory.

Methods

To determine whether a memory T cell response is being developed, we performed a longitudinal assessment of the SARS-CoV-2-specific T cell response by IFN-γ ELISPOT and activation marker analyses of peripheral blood samples from unvaccinated children and adults with mild-to-moderate COVID-19.

Results

Upon stimulation of PBMCs with heat-inactivated SARS-CoV-2 or overlapping peptides of spike (S-SARS-CoV-2) and nucleocapsid proteins, we found S-SARS-CoV-2-specific IFN-γ T cell responses in infected children (83%) and adults (100%) that were absent in unexposed controls. Frequencies of SARS-CoV-2-specific T cells were higher in infected adults, especially in those with moderate symptoms, compared to infected children. The S-SARS-CoV-2 IFN-γ T cell response correlated with S1-SARS-CoV-2-specific serum antibody concentrations. Predominantly, effector memory CD4+ T cells of a Th1 phenotype were activated upon exposure to SARS-CoV-2 antigens. Frequencies of SARS-CoV-2-specific T cells were significantly reduced at 10 months after symptom onset, while S1-SARS-CoV-2-specific IgG concentrations were still detectable in 90% of all children and adults.

Conclusions

Our data indicate that an antigen-specific T cell and antibody response is developed after mild SARS-CoV-2 infection in children and adults. It remains to be elucidated to what extent this SARS-CoV-2-specific response can contribute to an effective recall response after reinfection.