This is a SARS-CoV-2 seroepidemiological study in pediatric population(0-16 years) during BA.5 Omicron predominance period in Athens metropolitan area. Serum samples were tested for SARS-CoV-2 nucleocapsid antibodies(Abs-N), representing natural infection during three sub-periods of BA.5 predominance: 01/05/2022-31/08/2022(period A), 01/09/2022-31/12/2022(period B) and July 2023(period C) and epidemiological data were collected. Additionally in period C Abs-N seronegative samples were tested for SARS-CoV-2 spike antibodies(Abs-S). A total of 878 children were tested (males:51.3%) with median age(IQR): 108(36-156) months and seropositivity during the 3 subperiods was: A:292/417(70%), B:288/356(80.9%), C:89/105(84.8%), P-value<0.001. SARS-CoV-2 seropositivity increased for from period A to C for 0-1-year (P-value: 0.044), 1-4 years (P-value: 0.028) and 6-12 years children (P-value: 0.003). Children 6-12y had the highest seroposi-tivity rates in all subperiods (A: 77.3%, B: 91.4% and C: 95.8%). A significant correlation of monthly SARS-CoV-2 median antibody titers with monthly seropositivity rates was detected (rs:0.812, P-value:0.008). During period C, an additional 12/105 (11.4%) Abs-S seropositive and Abs-N ser-onegative samples were detected and total seropositivity was estimated at 96.2% (101/105). The increased SARS-CoV-2 seropositivity detected in current seroepidemiology study illustrates a high exposure rate during BA.5 predominance period. These data could guide public health decisions regarding immunization strategies and protection measures.

Background/Objectives: Multisystem Inflammatory Syndrome in children (MIS-C) is a rare but severe post-infectious complication of COVID-19, that often requires admission to the Pediatric Intensive Care Unit (PICU). The present study aimed to compare the demographic, clinical and laboratory characteristics between children diagnosed with MIS-C who were admitted to the PICU or did not require PICU admission. Methods: Children diagnosed with MIS-C from September 2020 to April 2023 were included in this case-control study. Patient’s demographic, clinical and laboratory data, were collected from medical records. Results: Fifty children with MIS-C were included in the study [median (IQR) age: 7.5 (4.3, 11.4) years, 28/50 (56%) males]. Twenty-two (22/50, 44%) children required admission to the PICU. In the multivariate regression analysis, hepatic (OR:12.89, 95%CI: 1.35-123.41, p-value=0.03) and cardiological involvement (OR:34.55, 95%CI: 2.2-541.91, p-value=0.01) were significantly associated with hospitalization at the PICU. Regarding the laboratory and imaging parameters during the first 48 hours from admission, D-dimer levels higher than 4μg/mL and decreased Left Ventricular Ejection Fraction were associated with an increased risk of PICU admission (OR:7.95, 95%CI:1.48-42.78, p-value=0.02 and OR=1.28, 95%CI:1.07-1.53, p-value=0.01). Children who were admitted to the PICU were more likely to develop complications during their hospitalization (10/22, 45.5% vs. 3/28, 10.7%, p-value=0.005) and were hospitalized for more days, than children in the pediatric wards (median length of stay (IQR): 20 (15,28) days vs. 8.5 (6, 14) days, p-value&lt;0.001). Conclusions: In the present study, cardiovascular, hepatic involvement and increased D-dimer levels in children with MIS-C were associated with admission to the PICU.