This was a retrospective cohort study, which aimed to investigate the factors associated with hesitancy to receive a third dose of a coronavirus disease 2019 (COVID-19) vaccine. A paper-based questionnaire survey was administered to all participants. This study included participants who provided answers in the questionnaire about whether they had an intent to receive a third dose of a vaccine. Data on sex, age, area of residence, adverse reactions after the second vaccination, whether the third vaccination was desired, and reasons to accept or hesitate over the booster vaccination were retrieved. Among the 2439 participants, with a mean (±SD) age of 52.6 ± 18.9 years, and a median IgG-S antibody titer of 324.9 (AU/mL), 97.9% of participants indicated their intent to accept a third vaccination dose. The logistic regression revealed that participants of a younger age (OR = 0.98; 95% CI: 0.96–1.00) and with a higher antibody level (OR = 2.52; 95% CI: 1.27–4.99) were positively associated with hesitancy over the third vaccine. The efficacy of the COVID-19 vaccine and concerns about adverse reactions had a significant impact on behavior regarding the third vaccination. A rapid increase in the booster dose rate is needed to control the pandemic, and specific approaches should be taken with these groups that are likely to hesitate over the third vaccine, subsequently increasing booster contact rate.

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Performing a cohort-based SARS-CoV-2 antibody assay is crucial for understanding infection status and future decision-making. The objective of this study was to examine consecutive antibody seroprevalence changes among hospital staff, a high-risk population. A two-time survey was performed in May and October 2020 for 545 hospital staff to investigate the changes in the results of the rapid kit test and chemiluminescence immunoassay (CLIA). The seroprevalence of each assay was summarized at both the survey periods. The proportion of seropositive individuals in the CLIA for each survey period and the number of confirmed COVID-19 cases in Central Fukushima were then compared. We chose 515 participants for the analysis. The proportion of IgM seroprevalence in CLIA increased from 0.19% in May to 0.39% in October, and IgG seroprevalence decreased from 0.97% in May to 0.39% in October. The proportion of IgM seroprevalence in the rapid kit test decreased from 7.96% in May to 3.50% in October, and IgG seroprevalence decreased from 7.77% in May to 2.14% in October. The IgG and IgM antibody seroprevalence among hospital staff in rural Central Fukushima decreased; the seroprevalence among hospital staff was consistent with the number of confirmed COVID-19 cases in the Central Fukushima area. Although it is difficult to interpret the results of the antibody assay in a population with a low prior probability, constant follow-up surveys of antibody titers among hospital staff had several merits in obtaining a set of criteria regarding the accuracy of measures against COVID-19 and estimating the COVID-19 infection status among hospital staff.

The purpose of this study was to identify factors associated with the increase in the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S1) protein and neutralizing antibody titer following SARS-CoV-2 vaccination. This observational study was conducted among healthcare workers working for a private hospital group in Fukushima Prefecture, Japan. Two blood samples were obtained from each participant. The first sample was obtained before the first dose of BNT162b2 (Pfizer-BioNTech) vaccine, and a second sample was obtained approximately 6 weeks later. Immunoglobulin G (IgG) antibody against the SARS-CoV-2 spike (S1) protein, immunoglobulin M (IgM) antibody against SARS-CoV-2 N-protein, and neutralizing activity were measured using the chemiluminescent immunoassay with iFlash 3000. A total of 231 healthcare workers who agreed to participate, and were negative for anti-SARS-CoV-2 IgM antibodies at enrollment, were included in the analysis. All participants had elevated IgG antibodies and neutralizing activity above the cutoff values. A total of 174 (75.3%) and 208 (90.0%) participants experienced adverse reactions after the first and second vaccine doses, respectively. Younger age, female sex, not taking immunosuppressive or antipyretic analgesic medication regularly, a lack of local adverse reactions after the first dose, and the presence of adverse reactions (fever, muscle, and joint pain) after the second dose were associated with higher IgG antibody titers and neutralizing activity. Intake of analgesic antipyretic for adverse reactions to vaccines was not significantly associated with antibody and neutralizing activity titer production. Immune responses after vaccination may differ among individuals, and continued countermeasures to prevent SARS-CoV-2 infection are vital.

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Recent studies have provided insights into the effect of vaccine boosters on recall immunity. Given the limited global supply of COVID-19 vaccines, identifying vulnerable populations with lower sustained vaccine-elicited antibody titers is important for targeting individuals for booster vaccinations. Here we investigated longitudinal data in a cohort of 2,526 people in Fukushima, Japan, from April 2021 to December 2021. Antibody titers following two doses of a COVID-19 vaccine were repeatedly monitored and information on lifestyle habits, comorbidities, adverse reactions, and medication use was collected. Using mathematical modeling and machine learning, we stratified the time-course patterns of antibody titers and identified vulnerable populations with low sustained antibody titers. Moreover, we showed that only 5.7% of the participants in our cohort were part of the "durable" population with high sustained antibody titers, which suggests that this durable population might be overlooked in small cohorts. We also found large variation in antibody waning within our cohort. There is a potential usefulness of our approach for identifying the neglected vulnerable population.