Background Data regarding the association of antibody levels elicited after immunization with the BNT162b2 mRNA Covid-19 vaccine with epidemiological and clinical parameters are limited. Methods We prospectively measured the total (TAbs-RBD) and the neutralizing antibodies (NAbs-RBD) against the receptor binding domain (RBD) of SARS-CoV-2 spike protein in a cohort of 268 Healthcare workers before immunization, 20 days after the 1 st dose and 30 days after the 2 nd dose of the BNT162b2 vaccine. A statistical analysis for possible association of antibodies’ levels with epidemiological and clinical parameters was performed. Results The mean age (±SD) of the participants was 45.45 years (± 11.93) (range: 24-70 years) and 211 (79.9%) were females. Statistically significant differences were detected regarding both TAbs-RBD and NAbs-RBD between the first and second doses of the vaccine (P<0.001). The median (IQR) percentage (%) of NAbs-RBD after the 1 st dose was 51.07% (31.60%) and after the 2 nd dose 95.31% (3.70%) (P<0.001).The correlation between the TAbs-RBD and NAbs-RBD was after the 1 st dose, Spearman’s, rho: 0.861 ( P <0.001) and after the 2 nd dose rho: 0.989 ( P <0.001). Twenty days after the 1 st dose, 56/264 (21.2%) of the participants did not have detectable NAbs-RBD, while one month after the 2 nd dose all of them had detectable NAbs-RBD. After the 2 nd vaccine dose, a statistically significant negative association of TAbs-RBD was detected for age (P<0.001), smoking ( P =0.011), and immunosuppressive medications (P<0.001), while a positive association was detected for BMI (P=0.004) and systemic adverse events after immunization (P=0.001). Conclusion A significant correlation of TAbs-RBD and NAbs-RBD was detected after both vaccine doses. Older age, smoking, and immunosuppressive medications negatively affected the final antibody level after SARS-CoV-2 immunization. Our findings emphasize the significance of the 2 nd vaccine dose especially in the older age groups.

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.

Limited prospective 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 pediatric 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 hospitalization status. Median (IQR) antibody titers 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), Pvalue:0.009]. During Omicron variant period increased SARS-CoV-2 seropositivity was detected in pediatric population, especially in adolescents, implicating either increased transmissibility or reinfection rates.

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