Comparative Study of T-Cell Repertoires after COVID-19 Immunization with Homologous or Heterologous Vaccine Booster.

Sequencing of the T-cell repertoire is an innovative method to assess the cellular responses after immunization. The purpose of this study was to compare T-cell repertoires after COVID-19 immunization with homologous (HOB) and heterologous (HEB) boosting. The study included 20 participants with a median age of 27.5 (IQR:23) years, who were vaccinated with one dose of the Ad26.COV2.S vaccine and were boosted with either Ad26.COV2.S (n = 10) or BNT162b2 (n = 10) vaccine. Analysis of the T-cell receptor beta locus (TCRß) sequencing one month after the booster dose identified that the HEB compared to the HOB group exhibited a higher number of both total and COVID-19-related functional T-cell rearrangements [mean of total productive rearrangements (TPRs): 63151.8 (SD ± 18441.5) vs. 34915.4 (SD ± 11121.6), p = 0.001 and COVID-19-TPRs: 522.5 (SD ± 178.0) vs. 298.3 (SD ± 101.1), p = 0.003]. A comparison between the HOB and HEB groups detected no statistically significant differences regarding T-cell Simpson clonality [0.021 (IQR:0.014) vs. 0.019 (IQR:0.007)], richness [8734.5 (IQR:973.3) vs. 8724 (IQR:383.7)] and T-cell fraction [0.19 (IQR:0.08) vs. 0.18 (IQR:0.08)]. HEB also exhibited a substantially elevated humoral immune response one month after the booster dose compared to HOB [median antibody titer (IQR): 10115.0 U/mL (6993.0) vs. 1781.0 U/mL (1314.0), p = 0.001]. T-cell repertoire sequencing indicated that HEB had increased SARS-CoV-2-related T-cell rearrangements, which was in accordance with higher humoral responses and possibly conferring longer protection. Data from the present study indicate that the administration of different COVID-19 vaccines as a booster may provide better protection.

Higher COVID-19 pneumonia risk associated with anti-IFN-α than with anti-IFN-ω auto-Abs in children.

We found that 19 (10.4%) of 183 unvaccinated children hospitalized for COVID-19 pneumonia had autoantibodies (auto-Abs) neutralizing type I IFNs (IFN-α2 in 10 patients: IFN-α2 only in three, IFN-α2 plus IFN-ω in five, and IFN-α2, IFN-ω plus IFN-ß in two; IFN-ω only in nine patients). Seven children (3.8%) had Abs neutralizing at least 10 ng/ml of one IFN, whereas the other 12 (6.6%) had Abs neutralizing only 100 pg/ml. The auto-Abs neutralized both unglycosylated and glycosylated IFNs. We also detected auto-Abs neutralizing 100 pg/ml IFN-α2 in 4 of 2,267 uninfected children (0.2%) and auto-Abs neutralizing IFN-ω in 45 children (2%). The odds ratios (ORs) for life-threatening COVID-19 pneumonia were, therefore, higher for auto-Abs neutralizing IFN-α2 only (OR [95% CI] = 67.6 [5.7-9,196.6]) than for auto-Abs neutralizing IFN-ω only (OR [95% CI] = 2.6 [1.2-5.3]). ORs were also higher for auto-Abs neutralizing high concentrations (OR [95% CI] = 12.9 [4.6-35.9]) than for those neutralizing low concentrations (OR [95% CI] = 5.5 [3.1-9.6]) of IFN-ω and/or IFN-α2.

The Soluble Urokinase Plasminogen Activator Receptor as a Severity Biomarker in Children With Acute COVID-19 or Multisystem Inflammatory Syndrome.

BACKGROUND: Elevated soluble urokinase plasminogen activator receptor (suPAR) has been associated with a poor prognosis in serious infections. The aim of this study was to evaluate the clinical value of suPAR in children with acute coronavirus disease 2019 (COVID-19) or multisystem inflammatory syndrome (MIS-C). METHODS: Serum suPAR was measured using the suPARnostic AUTO Flex enzyme-linked immunosorbent assay in hospitalized children with COVID-19, MIS-C, bacterial pneumonia, and healthy controls. RESULTS: A total of 211 children with a mean (±SD) age of 6.9 ± 4.96 years were tested; with COVID-19: 59 (28%), MIS-C: 36 (17%), pneumonia: 78 (37%) and healthy controls: 38 (18%). In the acute phase, the levels of suPAR (mean ± SD) were: MIS-C: 8.11 ± 2.80 ng/mL, COVID-19: 4.91 ± 1.90 ng/mL, pneumonia: 4.25 ± 1.44 ng/mL and controls: 2.09 ± 0.47 ng/mL ( P < 0.001). Children with acute COVID-19 and a severe or moderate clinical presentation had higher values than those with mild symptoms: 5.79 ± 1.58 versus 5.40 ± 1.94 versus 3.19 ± 0.73 ng/mL, respectively ( P < 0.001). In the MIS-C group, children hospitalized in the intensive care unit and in need of mechanical ventilation had higher suPAR than those who were not admitted to an intensive care unit: 9.32 ± 3.06 versus 7.13 ± 2.19 ng/mL, respectively ( P = 0.023). In children with COVID-19 or MIS-C, a correlation was detected between suPAR values and length of hospitalization ( rs = 0.418, P < 0.001). CONCLUSIONS: The findings suggest that suPAR may be a valuable biomarker of disease severity in children with COVID-19 or MIS-C. This could facilitate the identification of children in need of intensive anti-inflammatory treatment, as it has been shown in adults with severe COVID-19.

Kinetics of SARS-CoV-2 Spike Antibodies after the Second and Third Dose of the BNT162b2 COVID-19 Vaccine and Association with Epidemiological Characteristics and Breakthrough Infection in a Cohort Study of Healthcare Workers.

To prospectively study the kinetics of immune responses after immunization with the BNT162b2 mRNA COVID-19 vaccine and their association with epidemiological parameters and breakthrough infection (BI), we measured total (TAbs-WT) and neutralizing antibodies against wild-type (NAbs-WT) and Omicron (NAbs-O) SARS-CoV-2 spike proteins in healthcare workers (HCWs) after the second (4 and 8 months) and third dose (1 and 8 months). Vaccinated HCWs (n = 486), with a median age (IQR) of 49 years (38-56), were included in this prospective cohort study. BI was observed 4 and 8 months after the second dose in 8/486 (1.6%) and 15/486 (3.1%) HCWs, respectively, and 1 and 8 months after the third dose in 17/486 (3.5%) and 152/486 (31.3%) HCWs, respectively. A comparison of immune responses 1 month after the third dose in vaccinated HCWs without a BI or with a BI in the next 7 months did not detect any statistically significant differences in the TAbs-WT (median (IQR): 16,611.0 (13,011.0) U/mL vs. 17,572.5 (14,501.0) U/mL, p = 0.529) and NAbs-WT (median (IQR): 96.5% (1.7) vs. 96.7% (1.9), p = 0.555). After infection, HCWs with a BI had significantly increased TAbs-WT levels at all time points compared to healthy HCWs. The findings of the present study indicate that antibody levels after three doses of the BNT162b2 vaccine are not directly associated with the possibility of a BI.

Epidemiological study of unusual rotavirus strains and molecular characterization of emerging P[14] strains isolated from children with acute gastroenteritis during a 15-year period.

Rotavirus group A (RVA) is characterized by molecular and epidemiological diversity. To date, 42 G and 58 P RVA genotypes have been identified, some of which, like P[14], have a zoonotic origin. In this study, we describe the epidemiology of unusual RVA genotypes and the molecular characteristics of P[14] strains. Fecal samples from children ≤ 16 years of age with acute gastroenteritis (AGE) who were hospitalized during 2007-2021 in Greece were tested for RVA by immunochromatography. Positive RVA samples were G and P genotyped, and part of the VP7 and VP4 genes were sequenced by the Sanger method. Epidemiological data were also recorded. Phylogenetic analysis of P[14] was performed using MEGA 11 software. Sixty-two (1.4%) out of 4427 children with RVA AGE were infected with an unusual G (G6/G8/G10) or P (P[6]/P[9]/P[10]/P[11]/P[14]) genotype. Their median (IQR) age was 18.7 (37.3) months, and 67.7% (42/62) were males. None of the children were vaccinated against RVA. P[9] (28/62; 45.2%) was the most common unusual genotype, followed by P[14] (12/62; 19.4%). In the last two years, during the period of the COVID-19 pandemic, an emergence of P[14] was observed (5/12, 41.6%) after an 8-year absence. The highest prevalence of P[14] infection was seen in the spring (91.7%). The combinations G8P[14] (41.7%), G6P[14] (41.7%), and G4P[14] (16.6%) were also detected. Phylogenetic analysis showed a potential evolutionary relationship of three human RVA P[14] strains to a fox strain from Croatia. These findings suggest a possible zoonotic origin of P[14] and interspecies transmission between nondomestic animals and humans, which may lead to new RVA genotypes with unknown severity.

Evaluation of T cell responses with the QuantiFERON SARS-CoV-2 assay in individuals with 3 doses of BNT162b2 vaccine, SARS-CoV-2 infection, or hybrid immunity.

Cellular immunity after SARS-CoV-2 infection or immunization may be important for long-lasting protection against severe COVID-19 disease. We investigated cellular immune responses after SARS-CoV-2 infection and/or vaccination with an interferon-γ release assay (QuantiFERON, QFN), in parallel, with humoral immunity assessment. We recruited 41 participants: unvaccinated convalescent children and adults and vaccinated uninfected or vaccinated convalescent adults. All vaccinated adults had received three doses of the BNT162b2 COVID-19 vaccine at 6.2 to 10.9 months prior to their inclusion to the study. All the unvaccinated participants were tested negative with QFN. Regarding the vaccinated population, 50% (8/16) of the vaccinated uninfected adults and 57.1% (8/14) of the vaccinated convalescent adults were tested positive. QFN did not detect T cell responses in unvaccinated individuals and in a significant number of vaccinated individuals. Further comparative studies with different immunoassays are required to elucidate whether this is the result of waning immunity or low sensitivity of the assay.

Comparison of a rapid fluorescence immunochromatographic test with an enzyme-linked immunosorbent assay for measurement of SARS-CoV-2 spike protein antibody neutralizing activity.

BACKGROUND: SARS-CoV-2 Spike protein Receptor Binding Domain neutralizing antibodies (NAbs-RBD) inhibit the viral binding to angiotensin-converting enzyme 2 (ACE2) receptors. We compared an ELISA and a fluorescence immunochromatography (FIC) method in NAbs-RBD detection after COVID-19 immunization. METHOD: Serum samples from healthcare workers (HCWs) vaccinated with BNT162b2 were collected one and four months after the second dose. NAbs-RBD (%) detection was performed using ELISA cPass™ (FDA approved) and FIC n-AbCOVID-19® assays. RESULTS: Samples from 200 HCWs [median age (IQR): 45(35-53)] were tested with both assays. There was a good qualitative agreement between the two methods [AUC: 0.92(95%C.I.: 0.89-0.94, P-value:0.007)]. NAbs-RBD (%), one and four months after immunization, were significantly lower with FIC compared to ELISA for all age groups (P-value<0.0001). The quantitative comparison between FIC and ELISA detected slight agreement one month after the second dose [(Lin's Concordance Correlation Coefficient (CCC): 0.21(95%CI: 0.15-0.27)] which improved four months after the second dose [CCC: 0.6(95%CI: 0.54-0.66)]. CONCLUSION: FIC had good qualitative agreement with ELISA in the detection of positive NAbs-RBD (%) and could be an alternative for rapid NAbs-RBD (%) testing.

Immunology of Multisystem Inflammatory Syndrome after COVID-19 in Children: A Review of the Current Evidence.

Immune responses following severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in children are still under investigation. Even though coronavirus disease 2019 (COVID-19) is usually mild in the pediatric population, some children exhibit severe clinical manifestations, require hospitalization, or develop the most severe condition: a multisystem inflammatory syndrome in children (MIS-C) associated with SARS-CoV-2 infection. The activated innate, humoral and T-cell-mediated immunological pathways that lead certain pediatric populations to present with MIS-C or remain asymptomatic after SARS-CoV-2 infection are yet to be established. This review focuses on the immunological aspects of MIS-C with respect to innate, humoral, and cellular immunity. In addition, presents the role of the SARS-CoV-2 Spike protein as a superantigen in the pathophysiological mechanisms, discusses the great heterogeneity among the immunological studies in the pediatric population, and highlights possible reasons why some children with a certain genetic background present with MIS-C.

SARS-CoV-2 seroepidemiology in paediatric population during Delta and Omicron predominance.

Limited prospective severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) data in children regarding the impact of Omicron variant in seropositivity have been reported. We investigated SARS-CoV-2 seropositivity in children between 1 September 2021 and 30 April 2022, representing Delta and Omicron predominance periods. Serum samples from children admitted to the major tertiary Greek paediatric hospital for any cause, except for COVID-19, were randomly collected and tested for SARS-CoV-2 natural infection antibodies against nucleocapsid antigen (Elecsys® Anti-SARS-CoV-2 reagent). A total of 506/1312 (38.6%) seropositive children (0-16 years) were detected (males: 261/506(51.6%); median age (IQR): 95.2 months(24-144)). Seropositivity rates (%) increased from Delta to Omicron period from 29.7% to 48.5% (P-value<0.0001). Seropositivity increased for all age groups, except for the age group of 0-1 year (P-value:0.914). The highest seropositivity rate was detected in April 2022 (52.6%) and reached 73.9% specifically for the age group 12-16 years. No significant differences were detected in seropositivity with respect to gender, origin, or hospitalisation status. Median (IQR) antibody titres were higher in the Omicron vs. Delta period in all age groups, especially in 12-16 years [32.2 COI (7-77.1) vs. 11.4 COI(2.8-50.2), P-value:0.009). During Omicron variant period increased SARS-CoV-2 seropositivity was detected in paediatric population, especially in adolescents, implicating either increased transmissibility or reinfection rates.

Post­COVID­19 syndrome in children (Review).

The persistence of symptoms for a long time after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is now familiar as post-COVID syndrome (PCS). To the best of our knowledge, the risk of long-term clinical outcomes in children after SARS-CoV-2 infection is still unclear. Unlike in adults, current evidence suggests a lower prevalence of persistent symptoms in children. However, since several studies are characterized by great heterogeneity, it is difficult to accurately estimate the exact incidence of PCS in children. The presence and course of recovery depend on risk factors that are more common in adults than children. Proposed pathophysiological mechanisms in PCS in children include age-dependent immune responses, angiotensin-converting enzyme 2 expression, blood-brain barrier development or social issues affecting children behavior, such as school closure and social isolation. However, further longitudinal studies are required for unanswered issues to be clarified. The aim of the present review is to describe the long-term symptoms per biological system in children, potential risk factors and the role of the immune system in the presence of PCS.

Whole Exome Sequencing Points towards a Multi-Gene Synergistic Action in the Pathogenesis of Congenital Combined Pituitary Hormone Deficiency.

Combined pituitary hormone deficiency (CPHD) is characterized by deficiency of growth hormone and at least one other pituitary hormone. Pathogenic variants in more than 30 genes expressed during the development of the head, hypothalamus, and/or pituitary have been identified so far to cause genetic forms of CPHD. However, the etiology of around 85% of the cases remains unknown. The aim of this study was to unveil the genetic etiology of CPHD due to congenital hypopituitarism employing whole exome sequencing (WES) in two newborn patients, initially tested and found to be negative for PROP1, LHX3, LHX4 and HESX1 pathogenic variants by Sanger sequencing and for copy number variations by MLPA. In this study, the application of WES in these CPHD newborns revealed the presence of three different heterozygous gene variants in each patient. Specifically in patient 1, the variants BMP4; p.Ala42Pro, GNRH1; p.Arg73Ter and SRA1; p.Gln32Glu, and in patient 2, the SOX9; p.Val95Ile, HS6ST1; p.Arg306Gln, and IL17RD; p.Pro566Ser were identified as candidate gene variants. These findings further support the hypothesis that CPHD constitutes an oligogenic rather than a monogenic disease and that there is a genetic overlap between CPHD and congenital hypogonadotropic hypogonadism.

Time of Day of BNT162b2 COVID-19 Immunization Affects Total SARS-CoV-2 Antibody Levels but Not Neutralizing Activity.

To examine whether immunization time affects the immune responses elicited by the BNT162b2 COVID-19 vaccine, we investigated the possible association between total SARS-CoV-2 spike protein receptor binding domain (TAbs-RBD) and neutralizing (NAbs-RBD) antibodies with vaccination time. A cohort of 468 healthcare workers (mean age [±SD]: 48 [±13] years), were included in the study. One month after the second dose, healthcare workers who were vaccinated between 1500-2200 h had higher TAbs-RBD compared to 0700-1100 h and 1100-1500 h (p = 0.006). One month after the third dose, healthcare workers who were vaccinated between 0700-1100 h and 1500-2200 h had significantly higher TAbs-RBD compared to 1100-1500 h (p = 0.034). However, no association of NAbs-RBD with vaccination time was detected after each of the 3 doses (p > 0.4). Despite the possible effect of BNT162b2 vaccination time in TAbs-RBD levels, possibly due to rhythmic expression of clock genes, neutralizing activity was not associated with vaccination time and, therefore, further investigation is required.

Immunogenicity of the COVID-19 BNT162b2 vaccine in adolescents and young adults with cystic fibrosis.

Data regarding immunogenicity of SARS-CoV-2 BNT162b2 vaccine in cystic fibrosis (CF) patients are limited. We prospectively measured total (TAbs-RBD; U/ml) and neutralizing (NAbs-RBD; %) antibodies of SARS-CoV-2 spike-receptor binding domain (RBD) protein in 33 CF patients and 66 healthy controls with median age (IQR): 19.6 (17.6-24.3) years and 31 (29-36) years, respectively and investigated possible associations with epidemiological and clinical parameters. Compared to healthy controls, CF patients had higher levels of TAbs-RBD and NAbs-RBD after both doses (P-value < 0.001). One month after the second dose, CF patients and controls had TAbs-RBD: median (IQR): 3396 (2443) and 1452 (1231) U/ml, respectively. Similarly, the NAbs-RBD (%) were: 97.30 (1.00) and 95.70 (3.71) %, respectively. CF patients also had fewer local and systemic adverse events (AEs) (P-value < 0.001). Among CF patients, no significant differences in immunogenicity were detected regarding the phenotype, genotype, medications, or severity of the disease. BNT162b2 vaccine was immunogenic with limited reactogenicity in CF patients regardless of the phenotype or severity of disease.

Seroepidemiology of SARS-CoV-2 in pediatric population during a 16-month period prior to vaccination.

Limited prospective serosurveillance data in children regarding severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection have been reported. We prospectively investigated SARS-CoV-2 seropositivity in children during a 16-month period of the coronavirus disease 2019 (COVID-19) pandemic, including the four waves of the pandemic, before SARS-CoV-2 adolescents' vaccination. Serum samples from children admitted to the major tertiary Greek pediatric hospital for any cause, except for COVID-19 infection, were randomly collected from 05/2020 to 08/2021. The study period was divided into four 4-month periods representing relevant epidemic waves. Total SARS-CoV-2 antibodies for nucleocapsid protein were determined using the Elecsys® Anti-SARS-CoV-2 reagent. A total of 3099 children (0-16 years) were included in the study. A total of 344 (11.1%) seropositive children were detected (males: 205 [59.5%]; median age [interquartile range [IQR]]: 3 years [0.6-10]). Seropositivity rates (%) increased during the four 4-month periods: 1.4%, 8.6%, 17.2%, and 17.6%, respectively. A correlation of seropositivity rates in children with new diagnosed SARS-CoV-2 cases in the community was detected. No significant differences were detected between males and females. Seropositivity was significantly higher in hospitalized than in nonhospitalized children and in non-Greek compared to Greek children (p < 0.001). The lowest seropositivity rate before school opening (9/2021) was detected in the age groups 6-12 years (14.4%) and 12-16 years (16.1%). However, compared with the other age groups, the lowest median antibody titers were observed in children 0-1 year (median [IQR]: 13.9 cut-off index: [4.5-53.9] [p < 0.001]). Although the seropositivity of children was related to the community epidemic waves, the exposure was limited. Low seropositivity rates in school-age children support the need for SARS-CoV-2 immunization.

SARS-CoV-2 seroepidemiological study in healthcare workers and discordant results using seven different diagnostic methods.

The aim of the study was to access the SARS-CoV-2 antibody seroprevalence in healthcare workers (HCWs) of a tertiary pediatric hospital after the first wave of the pandemic and to compare the results among seven commercially available antibody detection assays, including chemiluminescence (CMIA), electroluminescence (ECLIA), Εnzyme-Linked Immunosorbent Assay (ELISA), and rapid immunochromatography (RIC). SARS-CoV-2 antibody detection was performed in serum samples of 1216 HCWs, using a reference CMIA assay and 8/1216 (0.66%) were detected positive. Positive serum samples were further tested with other assays; however, only one sample was positive by all tests. The rest 7 cases were negative with ECLIA and ELISA and gave discordant results with RIC test. Six months later, new serum samples of seropositive HCWs were analyzed with the same 7 tests, with inconsistent results again. Identification of reliable SARS-CoV-2 antibody tests is important to determine the actual number of past infections, the duration of antibodies, and guide public health decisions.

Rotavirus epidemiology and genotype distribution in hospitalised children, Greece, 2008 to 2020: A prospective multicentre study.

BackgroundTwo rotavirus (RV) vaccines were licensed in Greece in late 2006 and included in the national immunisation programme in 2012.AimTo study the epidemiology and genotype distribution of RV in children during the post-vaccination period and assess the impact of increased vaccination coverage.MethodsIn a prospective multicentre hospital-based study, hospitalised children (≤ 16 years) with an RV-positive faecal sample were recruited. Epidemiological and genotyping analyses were performed; periods of low (2008-12) and moderate (2012-20) RV vaccination coverage were compared. Statistical analysis was performed with a chi-squared or Mann-Whitney U test and logistic regression.ResultsA total of 3,874 children (55.6% male; n = 2,153) with median age of 1.4 years (IQR: 0.5-3.3) were studied during 2008-20. Most RV-infected children were aged ≤ 3 years (72.2%) and hospitalised during December-May (69.1%). Common RV genotypes (G1P[8], G2P[4], G3P[8], G4P[8], G9P[8], G12P[8]) were detected in 92.2% of samples; G-P combinations with prevalence above 1% were G4P[8] (44.1%), G1P[8] (25.4%), G2P[4] (14.9%), G9P[8] (3.5%), G12P[8] (2.2%), G3P[8] (2.1%), other (4.3%) and mixed (3.5%). Of all samples, 97.6% were homotypic or partially heterotypic to vaccines' genotypes. With moderate vaccination coverage, the seasonal peak was detected earlier, children were older and partially or fully heterotypic genotypes were increased (p < 0.001).ConclusionsIn the era of moderate RV vaccination coverage in Greece, epidemiology of RV in hospitalised children seemed to change. However, most circulating genotypes remain homotypic or partially heterotypic to RV vaccines. Continuous epidemiological surveillance and genotyping are important to monitor possible changes arising from RV vaccines' implementation.

Association of clinical and epidemiological characteristics with COVID-19 BNT162b2 mRNA vaccine short-term adverse reactions in healthcare workers.

INTRODUCTION: The aim of the study was to investigate the prevalence and severity of adverse reactions (ARs) after immunization of healthcare workers (HCWs) with BNT162b2 vaccine and to associate them with clinical and epidemiological characteristics. METHODS: A form containing demographic and clinical data as well as ARs after both doses of the vaccine was completed, and statistical association analysis was performed. RESULTS: A total of 502 HCWs (females 78.3%) with mean age (±SD) 48.17 years (±12.97) participated. After the first dose, 404 (80.5%) HCWs reported at least one local AR (LAR) and 366 (72.9%) after the second dose (p-value=0.004). After the first dose, 121 (24.1%) HCWs reported at least one systemic AR (SAR) and 275 (54.8%) after the second dose (p-value<0.0001).In the logistic regression analysis, there was no association of gender or medical history of underlying disease with LARs. There was a negative association of age with the cumulative score (CS) of LARs (OR: 0.82, 95% CI: 0.69-0.96) after the first dose. Females had a positive association with CS of SARs following both doses (OR, 95% CI: 2.57, 1.39-4.73 and 2.71, 1.76-4.19, respectively). Age was negatively associated with CS of SARs (OR: 0.66, 95% CI: 0.57-0.76) after the second dose. Severe ARs included Bell's palsy (1) and tinnitus with temporary hearing loss (1). CONCLUSION: The administration of the BNT162b2 vaccine in our HCWs cohort had a good safety profile with the most common ARs being self-limited. An increasing rate of SARs following the second vaccine dose was noticed. Rare but severe possible ARs should be further investigated.

SARS-CoV-2 variants and effectiveness of vaccines: a review of current evidence.

The SARS-CoV-2 virus is rapidly evolving via mutagenesis, lengthening the pandemic, and threatening the public health. Until August 2021, 12 variants of SARS-CoV-2 named as variants of concern (VOC; Alpha to Delta) or variants of interest (VOI; Epsilon to Mu), with significant impact on transmissibility, morbidity, possible reinfection and mortality, have been identified. The VOC Delta (B.1.617.2) of Indian origin is now the dominant and the most contagious variant worldwide as it provokes a strong binding to the human ACE2 receptor, increases transmissibility and manifests considerable immune escape strategies after natural infection or vaccination. Although the development and administration of SARS-CoV-2 vaccines, based on different technologies (mRNA, adenovirus carrier, recombinant protein, etc.), are very promising for the control of the pandemic, their effectiveness and neutralizing activity against VOCs varies significantly. In this review, we describe the most significant circulating variants of SARS-CoV-2, and the known effectiveness of currently available vaccines against them.

Immune response to SARS-CoV-2 in children: A review of the current knowledge.

Host immune responses to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), especially in children, are still under investigation. Children with coronavirus disease 2019 (COVID-19) constitute a significant study group of immune responses as they rarely present with severe clinical manifestations, require hospitalization, or develop complications such as multisystem inflammatory syndrome in children (MIS-C) associated with SARS-CoV-2 infection. The deciphering of children's immune responses during COVID-19 infection will provide information about the protective mechanisms, while new potential targets for future therapies are likely to be revealed. Despite the limited immunological studies in children with COVID-19, this review compares data between adults and children in terms of innate and adaptive immunity to SARS-CoV-2, discusses the possible reasons why children are mostly asymptomatic, and highlights unanswered or unclear immunological issues. Current evidence suggests that the activity of innate immunity seems to be crucial to the early phases of SARS-CoV-2 infection and adaptive memory immunity is vital to prevent reinfection.