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Background: Risk communication is necessary to improve the booster vaccination rate, but Vietnam does not have a system to collect and disclose such information. Therefore, the purpose of this study was to clarify adverse reactions and their frequency in the early period after booster vaccination, and to obtain primary data for improving the booster vaccination rate. Methods: A cross-sectional survey was conducted among adults aged ≥18 years. Clinical data were collected 14 days after booster vaccination by using a standard questionnaire. Results: A total of 1322 participants were included with median age = 23 and sex ratio (Male/Female) = 0.53. AstraZeneca was the most commonly used vaccine for the first and second doses, while Pfizer was the most commonly used vaccine for booster shots. Injection site pain, fatigue, and myalgia were the most common side effect reported (71.9%, 28.1%, and 21.8%, respectively). Compared to previous COVID-19 vaccine injections, 81.9% of participants reported that their symptoms were similar or milder after receiving the booster dose. They were more likely to present injection site pain (OR = 1.43, p < 0.0001) and lymphadenopathy (OR = 4.76, p < 0.0001) after receiving the booster shot. Fever (OR = 0.33, p < 0.0001) and fatigue (OR = 0.77, p = 0.002) were less often reported after booster shots compared to the first and second injections. The severity of symptoms occurring after booster dose versus first and second doses increased significantly with each additional year of age and among participants receiving the Pfizer and Moderna vaccines. Conclusion: Adverse reactions to booster vaccination are minor and their incidence is the same as for the first or the second vaccination. Multicenter studies with larger sample sizes on the side effects and safety of COVID-19 vaccine booster shots need to be conducted to make the population less worried, in order to increase the vaccination rate, to protect individuals’ and communities’ health.

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Using the density functional method, we investigated the crystal and electronic structures and the electrochemical properties of NaxVOPO4 (x = 0, 1) and explored the diffusion mechanism of Na ions in these materials. The van der Waals interaction was also taken into account to include the non-local electron correlation in the calculation of structural parameters and voltage. The diffusion of Na ions is treated as a process of the Na vacancy-positive small polaron complex in NaVOPO4 and the Na ion-negative small polaron complex in VOPO4, respectively. During the charging (discharging) process, the removal (insertion) of a Na ion would result in the formation of a positive (negative) small polaron at one of the two first nearest vanadium sites to the Na vacancy. Three elementary diffusion processes, including the single, crossing and parallel diffusion processes, are explored. It is found that the [010] direction is preferable for Na ion diffusion in both the charging and discharging processes. The influence of small polaron migration on Na ion diffusion in the charging state is negligible, whereas such effect is considerably strong in the discharging process. Moreover, while three elementary diffusion processes in NaVOPO4 require the same energy, the parallel diffusion process in VOPO4 is not preferred. The diffusion of Na vacancy accompanied by a positive polaron in the full charging process requires an activation energy of 395 meV, while the diffusion of Na ion accompanied by a negative polaron in the discharging state, VOPO4, has a higher activation energy of 627 meV. With a voltage and activation barrier similar to that of the olivine phosphate LiFePO4, these sodium-based materials are expected to be promising cathode materials for sodium ion batteries.