To the Editor, The high demand for COVID19 vaccines, combined with a significant lack of supply, leaves smaller and developing countries behind in mass immunization. This prompts the question whether administering a single vaccine dose in SARSCoV2 seropositive individuals could be a method for rationing available vaccine doses. We report results from a prospective study on Macedonian healthcare workers who received two doses of the Pfizer/BioNTech BNT162b2 mRNA vaccine, comparing antibody titres and frequency of side effects after vaccine administration between individuals who were SARSCoV2 seropositive (SeroPOS group) and seronegative (SeroNEG group) prior to immunization. The study included 226 participants recruited through convenience sampling, of whom 41 were SeroPOS (73.17% female; mean age 43 years, SD: 10.571), and 185 were SeroNEG (68.11% female, mean age 46 years, SD: 10.523). Baseline patients’ characteristics are provided in the Supplementary Appendix (Table S1). Blood samples were collected 18– 21 days after the first vaccine dose and 25– 28 days after the second dose. Baseline antibody levels were obtained from patient records. All participants gave blood samples after the first dose and filed a questionnaire for side effects, and 189 participants (83.63%) returned for assessment four weeks after the second dose. Serological testing was performed using the commercially available quantitative CLIA antiSARSCoV2 RBD kit (Snibe, Shenzhen, China),1 which targets the S1 subunit of the viral spike protein. More details on methods are available in the Supplementary Appendix. AntiSARSCoV2 RBD IgG antibody levels after the first dose of BNT162b2 were on average 11.7times higher in SeroPOS individuals (mean: 923.40 AU/ml, SD: 948.119, range 15.04– 5034.70) compared to SeroNEG individuals (mean: 79.06 AU/ml, SD: 253.243, range 0.912– 1867.30; Wilcoxon ranksum test, p < 2e16, Figure 1A). After the second dose, antiSARSCoV2 RBD IgG antibody levels were still higher in SeroPOS individuals (mean: 602.59 AU/ml, SD: 511.545, range 25.41– 1986.00) compared to SeroNEG individuals (mean: 375.567 AU/ml, SD: 437.088, range 9.617– 3704.40; Wilcoxon ranksum test, p = 0.006, Figure 1B). SeroNEG individuals had on average a 5.35fold increase in antiSARSCoV2 RBD IgG antibody levels after the second dose (Wilcoxon signedrank test, p < 2e16, Figure 1B), whereas SeroPOS individuals had no benefit of increased antibody levels after the second dose (Wilcoxon signedrank test, p = 0.529, Figure 1B). SeroPOS individuals had higher antibody levels after the first dose than SeroNEG individuals after the second dose (Wilcoxon ranksum test, p = 0.0039, Figure 1B). Exploratory analysis of the influence of sex and age on antibody response showed that older age had a reducing effect on antibody levels after the first and second vaccine dose (Supplementary Appendix, Table S2). The vast majority of the study participants reported at least one side effect after the first dose (91.15%, Figure 1C), mostly minor local pain (69.47%). A higher proportion of study participants reported at least one side effect after the second dose (97.35%, Figure 1D), again mostly minor local pain (53.97%). Our findings are in line with previous reports of higher antibody levels in SeroPOS individuals after a single dose of BNT162b2 compared to SeroNEG individuals25 and support the hypothesis that a single dose of BNT162b2 in SARSCoV2 seropositive individuals might provide sufficient humoral immunity towards SARSCoV2. These findings should be validated in a clinical trial setting as soon as possible, due to direct implications for public health policy in developing countries with limited access to vaccines. Future investigations should incorporate analyses of the cellular immunity and take into consideration the duration of the immune response, which have not been evaluated in this study. The more rational use of vaccines could accelerate the attainment of collective immunity at reduced costs.

Internet enabled the emergence of social media platforms, opened new markets and changed the way businesses and organizations operate, communicate with their target audience, promote their activities and achievements. Non-profit organizations worldwide use social media platforms and strive to increase user engagement and the number of reactions to achieve their goals. Although in Bosnia and Herzegovina, non-profit organizations use social media platforms, there is no empirical evidence of their success in increasing user engagement and the number of reactions. This study investigates which social media factors have an influence on user engagement, particularly on the number of reactions on Facebook posts. The real data from the period of 15 months were collected from a Facebook business profile of a non-profit organization from Bosnia and Herzegovina. SPSS and Excel were used to analyze the data. According to the research results, the period of publishing and the number of photos have an effect on the number of reactions on Facebook posts, while the amount of text and the day of publishing do not have an effect on the number of reactions.

Stereo 3D video from surgical procedures can be highly valuable for medical education and improve clinical communication. But access to the operating room and the surgical field is restricted. It is a sterile environment, and the physical space is crowded with surgical staff and technical equipment. In this setting, unobscured capture and realistic reproduction of the surgical procedures are difficult. This paper presents a method for rapid and reliable data collection of stereoscopic 3D videos at different camera baseline distances and distances of convergence. To collect test data with minimum interference during surgery, with high precision and repeatability, the cameras were attached to each hand of a dual-arm robot. The robot was ceiling-mounted in the operating room. It was programmed to perform a timed sequence of synchronized camera movements stepping through a range of test positions with baseline distance between 50-240 mm at incremental steps of 10 mm, and at two convergence distances of 1100 mm and 1400 mm. Surgery was paused to allow 40 consecutive 5-s video samples. A total of 10 surgical scenarios were recorded.

Conjugated polymers need to be doped to increase charge carrier density and reach the electrical conductivity necessary for electronic and energy applications. While doping increases carrier density, Coulomb interactions between the dopant molecules and the localized carriers are poorly screened, causing broadening and a heavy tail in the electronic density‐of‐states (DOS). The authors examine the effects of dopant‐induced disorder on two complimentary charge transport properties of semiconducting polymers, the Seebeck coefficient and electrical conductivity, and demonstrate a way to mitigate them. Their simulations, based on a modified Gaussian disorder model with Miller‐Abrahams hopping rates, show that dopant‐induced broadening of the DOS negatively impacts the Seebeck coefficient versus electrical conductivity trade‐off curve. Increasing the dielectric permittivity of the polymer mitigates dopant‐carrier Coulomb interactions and improves charge transport, evidenced by simultaneous increases in conductivity and the Seebeck coefficient. They verified this increase experimentally in iodine‐doped P3HT and P3HT blended with barium titanate (BaTiO3) nanoparticles. The addition of 2% w/w BaTiO3 nanoparticles increased conductivity and Seebeck across a broad range of doping, resulting in a fourfold increase in power factor. Thus, these results show a promising path forward to reduce the dopant‐charge carrier Coulomb interactions and mitigate their adverse impact on charge transport.