Kinetics of SARS-CoV-2 antibody titers after booster vaccinations during an Omicron surge in Japan.

BACKGROUND: Despite the emergence of SARS-CoV-2 variants and waning immunity after initial vaccination, data on antibody kinetics following booster doses, particularly those adapted to Omicron subvariants like XBB.1.5, remain limited. This study assesses the kinetics of anti-spike protein receptor-binding domain (S-RBD) IgG antibody titers post-booster vaccination in a Japanese population during the Omicron variant epidemic. METHODS: A prospective cohort study was conducted in Bizen City, Japan, from November 2023 to January 2024. Participants included residents and workers aged ≥18 years, with at least three COVID-19 vaccinations. Antibody levels were measured from venous blood samples. The study analyzed 424 participants and 821 antibody measurements, adjusting for variables such as age, sex, underlying conditions, and prior infection status. Mixed-effects models were employed to describe the kinetics of log-transformed S-RBD antibody titers. RESULTS: The study found that S-RBD antibody titers declined over time but increased with the number of booster vaccinations, particularly those adapted to Omicron and its subvariant XBB.1.5 (Pfizer-BioNTech Omicron-compatible: 0.156, 95%CI -0.032 to 0.344; Pfizer-BioNTech XBB-compatible: 0.226; 95%CI -0.051 to 0.504; Moderna Omicron-compatible: 0.279, 95%CI 0.012 to 0.546; and Moderna XBB-compatible: 0.338, 95%CI -0.052 to 0.728). Previously infected individuals maintained higher antibody titers, which declined more gradually compared to uninfected individuals (coefficient for interaction with time 0.006; 95%CI 0.001 to 0.011). Sensitivity analyses using Generalized Estimating Equations and interval-censored random intercept model confirmed the robustness of these findings. CONCLUSIONS: The study provides specific data on antibody kinetics post-booster vaccination, including the XBB.1.5-adapted vaccine, in a highly vaccinated Japanese population. The results highlight the importance of considering individual demographics and prior infection history in optimizing vaccination strategies.

Survey of pharyngeal foreign bodies in Japan: An ecological study using the nationwide claims data.

OBJECTIVE: Pharyngeal foreign bodies (PFBs) are a prevalent disease affected by food culture and dietary habits, with fish bones as the leading cause. Most studies were limited to specific regions, and a nationwide survey was not conducted in Japan. In this ecological study, we aimed to conduct a nationwide analysis of outpatient PFB cases in Japan over three years, focusing on seasonal trends, sex- and age-stratified cases, and regional differences. METHODS: We used the National Database of Health Insurance Claims and Specific Health Checkups of Japan open data from April 2019 to March 2022. The case data were analyzed by month, age, sex, and prefecture. Additionally, we calculated the standardized claim ratios (SCRs) for each prefecture and investigated the association between dietary habits, food culture, and SCR of PFBs using a two-level linear regression model. RESULTS: We analyzed a total of 164,337 outpatient PFB cases in Japan, revealing an average incidence rate of 45.6 per 100,000 persons. The seasonal trend revealed a peak in July each year from 2019 to 2021, confirming seasonality in PFB incidents. Children reported a higher incidence rate. Living west of Japan and expenditure on fish and shellfish had a strongly positive association with the SCR of PFBs. CONCLUSION: Our nationwide survey reveals that, even within Japan, there were regional variations influenced by food culture and dietary habits. The data showed that PFB incidence was higher among children, highlighting the need for preventive education. LEVEL OF EVIDENCE: Level 3.

Association between mean airway pressure during high-frequency oscillatory ventilation and pulmonary air leak in extremely preterm infants during the first week of life.

Background: While positive pressure ventilation has been considered an important contributing factor associated with pulmonary air leaks, studies examining the association between specific ventilatory settings during acute-phase high-frequency oscillatory ventilation (HFOV) and pulmonary air leaks among extremely preterm infants are limited. Methods: This was a single-center retrospective cohort study conducted at an institution that primarily used HFOV after intubation in extremely preterm infants. We analyzed data from extremely preterm infants born between 2010 and 2021. The primary outcome was pulmonary air leakage during the first 7 days of life. The exposure variable was the maximum mean airway pressure (MAP) on HFOV during the first 7 days of life or before the onset of pulmonary air leaks. Maximum MAP was categorized into three groups: low (7-10 cmH2O), moderate (11-12 cmH2O), and high (13-15 cmH2O) MAP categories. We conducted robust Poisson regression analyses after adjustment for perinatal confounders, using the low MAP category as the reference. Results: The cohort included 171 infants (low MAP, 123; moderate MAP, 27; and high MAP, 21). The median (interquartile range) gestational age and birth weight were 25.7 (24.3-26.7), 25.7 (24.9-26.9), and 25.3 (24.3-26.6) weeks and 760 (612-878), 756 (648-962), and 734 (578-922) g for infants in the low, moderate, and high MAP categories, respectively. Compared to infants in the low MAP category, those in the high MAP category had a higher incidence of pulmonary air leaks (4.1% vs. 33.3%; adjusted risk ratio, 5.4; 95% confidence interval, 1.6-18.5). In contrast, there was no clear difference in the risk of pulmonary air leaks between the moderate and low MAP categories (3.7% vs. 4.1%; adjusted risk ratio, 0.9; 95% confidence interval, 0.1-6.1). Conclusion: Extremely preterm infants requiring high MAP (≥13 cmH2O) in acute-phase HFOV had a higher risk of pulmonary air leak during the first 7 days of life.

Pediatric otitis media in Japan: A nationwide longitudinal study of the pre- and post-pneumococcal conjugate vaccine eras born in 2001 and 2010.

BACKGROUND: Otitis media (OM) is a prevalent respiratory disease in children and poses significant public health challenges due to its impact on child health and economic burdens. However, there have no nationwide epidemiological studies conducted in Japan. This study investigates the epidemiological trends of OM in Japan, taking into account the impact of the 7-valent pneumococcal conjugate vaccine (PCV7) introduction. METHOD: This study was retrospective cohort study using secondary data on the nationwide longitudinal birth cohort. This survey followed two cohorts born in 2001 (pre-PCV era) and 2010 (post-PCV era) until the age of 9. Every year, parents were surveyed about their children's health status, including occurrences of OM. The annual period prevalence and cumulative incidence of OM were assessed in this study, and the two cohorts were compared using a modified Poisson regression model adjusted environmental factors with the 2001 cohort as reference. RESULT: The study included 47,015 children from the 2001 cohort and 38,554 from the 2010 cohort. Peak annual period prevalence of OM varied by era. Cumulative incidence was 13.8 % for the 2001 cohort and 18.5 % for the 2010 cohort by 1.5 years of age and 28.9 % and 33.3 %, respectively, by 3.5 years of age. In particular, from the fourth survey onward, covering ages 2.5-3.5 years, a shift was observed from an increased risk to a decreased risk of OM. CONCLUSION: This nationwide longitudinal study emphasizes variations in OM epidemiology across Japan over time, with changes potentially influenced by the introduction of PCV7. In this study, due to the absence of individual PCV7 vaccination data, the effect of PCV7 was estimated based on the vaccination rate at the population level. The results suggest a notable decrease in the incidence of OM in later years, aligning with the increased uptake of PCV7.

Antibody titers and the risk of infection during the SARS-CoV-2 Omicron phase in Bizen City, Japan.

BACKGROUND: Understanding the association between the immune response and the risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has implications for forthcoming prevention strategies. We evaluated the association between antibody titers and the risk of infection for the general population during the Omicron-dominant phase. METHODS: This was a prospective cohort study of residents or people affiliated with institutions in Bizen City, which included 1,899 participants. We measured the titers of antibodies against SARS-CoV-2 repeatedly every 2 months from June 2022 to March 2023. Infection status was obtained from self-reported questionnaires and the official registry. We estimated risk ratios (RRs) for infection within 2 months of the date of each antibody measurement with 95% confidence intervals (CIs) based on antibody titer categories and spline functions. RESULTS: Compared with the <2,500 arbitrary unit (AU)/mL category, the 2,500-5,000, 5,000-10,000, and ≥10,000 AU/mL categories had adjusted RRs (95% CI) of 0.81 (0.61-1.08), 0.51 (0.36-0.72), and 0.41 (0.31-0.54), respectively. The spline function showed a non-linear relationship between antibody titer and risk. CONCLUSIONS: Higher antibody titers were associated with a lower risk. We demonstrate the usefulness of measuring an antibody titers to determine the appropriate timing for future vaccination.

Treatment interruption in hypertensive patients during the COVID-19 pandemic: An interrupted time series analysis using prescription data in Okayama, Japan.

Background: The COVID-19 pandemic has impacted healthcare behaviors, leading to fewer pediatric visits in Japan and potentially fewer visits by adult patients. However, existing Japanese studies on treatment interruptions have generally relied on questionnaire-based methods. In this study, we assessed the impact of the pandemic on antihypertensive treatment interruption using real-world prescription data. Methods: We conducted an interrupted time series analysis using the National Health Insurance Database in Okayama Prefecture, Japan. Participants included individuals aged 40-69 years with at least one antihypertensive prescription between 2018 and 2020. Treatment interruption was defined as a 3-month or longer gap in prescriptions after medication depletion. We used segmented Poisson regression with models unadjusted and adjusted for seasonality and over-dispersion to assess monthly treatment interruptions before and after Japan's April 2020 emergency. Results: During the study period, 23.0% of 55,431 participants experienced treatment interruptions. Cyclical fluctuations in interruptions were observed. The crude analysis indicated a 1.2-fold increase in treatment interruptions following the pandemic; however, the adjusted models showed no significant changes. Even among higher-risk groups, such as women, younger adults, and those with shorter prescriptions, no significant alterations were observed. Conclusion: We found no significant impact of the COVID-19 pandemic on antihypertensive treatment interruption in Okayama Prefecture. The less severe outbreak in the area or increased use of telemedicine and extended prescriptions may have contributed to treatment continuity. Further research is needed using a more stable and comprehensive database, broader regional data, and detailed prescription records to validate and extend our findings.

Longitudinal antibody dynamics after COVID-19 vaccine boosters based on prior infection status and booster doses.

Global concern over COVID-19 vaccine distribution disparities highlights the need for strategic booster shots. We explored longitudinal antibody responses post-booster during the Omicron wave in a Japanese cohort, emphasizing prior infection and booster doses. This prospective cohort study included 1763 participants aged 18 years and older with at least three vaccine doses (7376 datapoints). Antibody levels were measured every 2 months. We modeled temporal declines in antibody levels after COVID-19 vaccine boosters according to prior infection status and booster doses using a Bayesian linear mixed-effects interval-censored model, considering age, sex, underlying conditions, and lifestyle. Prior infection enhanced post-booster immunity (posterior median 0.346, 95% credible interval [CrI] 0.335-0.355), maintaining antibody levels (posterior median 0.021; 95% CrI 0.019-0.023) over 1 year, in contrast to uninfected individuals whose levels had waned by 8 months post-vaccination. Each additional booster was correlated with higher baseline antibody levels and slower declines, comparing after the third dose. Female sex, older age, immunosuppressive status, and smoking history were associated with lower baseline post-vaccination antibodies, but not associated with decline rates except for older age in the main model. Prior infection status and tailored, efficient, personalized booster strategies are crucial, considering sex, age, health conditions, and lifestyle.

Respiratory Severity Score and Neurodevelopmental Outcomes at Age 3 Years in Extremely Preterm Infants.

OBJECTIVE: We aimed to examine the association between respiratory severity score (RSS; mean airway pressure × fraction of inspired oxygen) and neurodevelopmental outcomes in extremely preterm infants. STUDY DESIGN: This was a single-center, retrospective cohort study. We analyzed data from extremely preterm infants who were admitted to the neonatal intensive care unit at Okayama Medical Center between 2010 and 2019. Infants without invasive respiratory management during the first day of life were excluded. The exposure variable was the highest RSS during the first day of life. RSS was categorized into two groups: low (<3.5) and high (≥3.5) RSS. The primary outcome was death or neurodevelopmental impairment at age 3 years, defined as cognitive impairment (developmental quotient <70) or the presence of cerebral palsy. Secondary outcomes were the components of the primary outcome. We conducted robust Poisson regression analyses to investigate the association between RSS category and primary and secondary outcomes, adjusting for perinatal confounders. RESULTS: The cohort included 97 infants with neurodevelopmental data, of whom 34 and 63 infants were in the low- and high-RSS categories, respectively. The median (interquartile range) gestational age and birth weight were 26.0 (24.7-26.9) and 25.7 (24.6-26.7) weeks and 761 (584-866) and 806 (618-898) g for infants in the low- and high-RSS categories, respectively. Compared with infants in the low-RSS category, those in the high-RSS category had a greater risk of death or neurodevelopmental impairment at age 3 years (26.3 vs. 42.3%; adjusted risk ratio [RR], 2.0; 95% confidence interval [CI], 1.1-3.5) and neurodevelopmental impairment at age 3 years (17.6 vs. 28.6%; adjusted RR, 2.7; 95% CI, 1.3-5.9). CONCLUSION: High RSS (≥3.5) during the first day of life was associated with an increased risk of neurodevelopmental impairment at age 3 years in extremely preterm infants. KEY POINTS: · RSS is a valuable tool for assessing respiratory failure.. · RSS = Mean airway pressure × fraction of inspired oxygen.. · RSS at age 1 day was associated with neurodevelopment..

Longitudinal changes in antibody titers in a nursing home in which COVID-19 clusters occurred.

BACKGROUND: Many outbreaks of coronavirus disease 2019 have occurred in Japanese nursing homes in which residents and staff are in close daily contact. This study evaluated longitudinal changes in antibody titers in nursing homes in which clusters occurred and examined the association between antibody titer and the severe acute respiratory syndrome coronavirus 2 infection or severity. METHODS: This cohort study included 171 participants who had provided at least one antibody titer test between June 2022 and March 2023. A descriptive analysis estimated the association between the risk of infection and antibody titer level. RESULTS: The facility experienced 2 clusters during the study period that involved facility staff as the initial source of infection. Noninfected participants had less variation in antibody titer levels and a higher level of preinfection antibodies than infected participants. The risk of infection and severity was lower in participants with higher antibody titers than in those with lower titers. CONCLUSIONS: We showed the changes in antibody titers over time and the association between antibody titer and severe acute respiratory syndrome coronavirus 2 infection or severity. Vaccination schedules may need to be tailored to the dynamics of decreasing antibody titers over time and the occurrence of infectious diseases in facilities.

Role of the Cytosolic Domain of the a3 Subunit of V-ATPase in the Interaction with Rab7 and Secretory Lysosome Trafficking in Osteoclasts.

We previously reported that the a3 subunit of proton-pumping vacuolar-type ATPase (V-ATPase) interacts with Rab7 and its guanine nucleotide exchange factor, Mon1a-Ccz1, and recruits them to secretory lysosomes in osteoclasts, which is essential for anterograde trafficking of secretory lysosomes. The a3 subunit interacts with Mon1a-Ccz1 through its cytosolic N-terminal domain. Here, we examined the roles of this domain in the interaction with Rab7 and trafficking of secretory lysosomes. Immunoprecipitation experiments showed that a3 interacted with Rab7 through its cytosolic domain, similar to the interaction with Mon1a-Ccz1. We connected this domain with a lysosome localization signal and expressed it in a3-knockout (a3KO) osteoclasts. Although the signal connected to the cytosolic domain was mainly detected in lysosomes, impaired lysosome trafficking in a3KO osteoclasts was not rescued. These results indicate that the cytosolic domain of a3 can interact with trafficking regulators, but is insufficient to induce secretory lysosome trafficking. The C-terminal domain of a3 and other subunits of V-ATPase are likely required to form a fully functional complex for secretory lysosome trafficking.

Delivery room intubation and neurodevelopment among extremely preterm infants.

BACKGROUND: The impact of delivery room intubation (DRI) on neurodevelopment in extremely preterm infants remains unclear. METHODS: We retrospectively analyzed data for infants born at 24-27 gestational weeks between 2003 and 2018. The primary outcome was neurodevelopmental impairment (NDI), defined as cerebral palsy or hearing, visual, or cognitive impairment at age 3 years. Secondary outcomes were NDI components and death before and after discharge from the neonatal intensive care unit. We conducted robust Poisson regression analyses, adjusting for perinatal confounders. RESULTS: The full cohort included 4397 infants with NDI data, of whom 3703 were intubated in the delivery room and 694 were not intubated in the delivery room. The mean gestational age and birth weight were 26.0 ± 1.1 weeks and 778 ± 184 g for infants with DRI and 26.6 ± 1.0 weeks and 873 ± 184 g for infants without DRI. Compared with infants without DRI, those with DRI had a higher risk for NDI (32.4% vs. 23.3%; adjusted risk ratio 1.18, 95% confidence interval: 1.01-1.37). There were no differences in secondary outcomes between infants with and without DRI. CONCLUSIONS: DRI was associated with an increased risk for NDI at age 3 years among extremely preterm infants. IMPACT: Few studies have examined the impact of delivery room intubation on neurodevelopment in infants born extremely preterm, and the results have been inconsistent. A total of 4397 infants born at 24-27 gestational weeks who had neurodevelopmental data at age 3 years were included in the present study. The present study found that delivery room intubation was associated with an increased risk for neurodevelopmental impairment at age 3 years among extremely preterm infants.

A nationwide birth cohort in Japan showed increased risk of early childhood hospitalisation in infants born small for gestational age.

AIM: To examine associations between children being born small for gestational age and childhood hospitalisation following term and preterm births. METHODS: This study included 34 564 children from a nationwide population-based longitudinal survey starting in 2010, comprising 32 603 term births and 1961 preterm births. Children's hospitalisation history was examined during two observational periods, 6-18 and 6-66 months of age. Logistic regression analysis was conducted, adjusting for child and parental confounders, with children born appropriate for gestational age as reference. RESULTS: Children born small for gestational age were more likely to be hospitalised during early childhood than those born appropriate for gestational age. The odds ratio (95% confidence interval) for hospitalisation from 6 to 66 months of age was 1.19 (1.05-1.34) in term children born small for gestational age and 1.47 (1.05-2.06) for preterm children born small for gestational age, compared with those born appropriate for gestational age. The risk of hospitalisation from 6 to 66 months of age in children born small for gestational age was observed for bronchitis/pneumonia. CONCLUSION: We observed the adverse effects of small for gestational age on hospitalisation during early childhood in both term and preterm births, particularly for bronchitis and pneumonia.

COVID-19 pandemic and language development in children at 18 months: a repeated cross-sectional study over a 6-year period in Japan.

OBJECTIVE: To evaluate the impact of the COVID-19 pandemic experience on language development among children, we compared language development at 18 months of age, before and during the pandemic in Japan, where strict control measures continued over a long period. METHODS: This was a repeated cross-sectional study and we included children who attended the 18-month health check-up provided by the Okayama City Public Health Center between January 2017 and December 2022 (n=33 484). We compared indicators of language development before (from January 2017 to February 2020) and during (from March 2020 to December 2022) the pandemic. Our primary outcome was the proportion of children who required follow-up for language development by the Public Health Center. The secondary outcome was the proportion of children who could not say three or more meaningful words. We estimated risk ratios (RRs) and their 95% CIs, adjusted for potential confounders. RESULTS: The prevalence of the primary outcome was 33.5% before the pandemic and 36% during the pandemic. Compared with before the pandemic, increased RRs for the primary and secondary outcomes were observed during the pandemic, with RRs (95% CIs) of 1.09 (1.06-1.13) for the primary outcome and 1.11 (1.05-1.17) for the secondary outcome. Although the statistical interactions were not significant, the RRs were higher for children cared for at home than those in nursery schools and with ≤3 family members than those with ≥4 family members. CONCLUSIONS: The COVID-19 pandemic was associated with an increased risk of impaired language development in children at 18 months. More extensive support is needed for higher risk families, as well as follow-up of long-term language development in children affected by the COVID-19 pandemic.

Delayed diagnostic interval and survival outcomes in pediatric leukemia: A single-center, retrospective study.

OBJECTIVE: This study primarily focused on the diagnostic interval (DI), defined as the duration from the onset of leukemic symptoms to diagnosis. We investigated whether a prolonged DI is associated with the outcomes of pediatric leukemia. METHODS: We retrospectively collected data of children with newly diagnosed pediatric leukemia at Okayama University Hospital from January 2007 to December 2022. Survival analyses were conducted using Kaplan-Meier methods, and an unadjusted analysis to compare differences in survival was performed using the log-rank test. RESULTS: In total, 103 children with leukemia were included in the analysis. The median DI was 20 days (interquartile range, 9.5-33.5 days). A prolonged DI (≥30 days) demonstrated no association with either 5-year event-free survival (70.1% for <30 days and 68.3% for ≥30 days, p = .99, log-rank test) or overall survival (84.7% for <30 days and 89.4% for ≥30 days, p = .85, log-rank test). CONCLUSIONS: A prolonged DI was not associated with the survival of children with leukemia. If a precise classification of leukemia biology is provided for pediatric patients, a prolonged DI may have little impact on the prognosis of these patients.

Association Between Clinical Remission of Infantile-Onset Allergic Rhinitis During the School-Age Period and the Type of Housing: A Longitudinal Population-Based Japanese Study.

This study investigated the association between housing type and clinical remission of infantile-onset allergic rhinitis (AR) in 53,575 children born in 2001 in Japan. Infantile-onset AR was defined as the presence of AR symptoms reported between ages 1.5 and 4.5 years, and remission was assessed between ages 10 and 12. The type of housing was categorized into detached houses and multi-unit residential buildings with 1-2, 3-5, or ≥6 floors. Among the 4,352 infantile-onset AR, 42.9% experienced remission. Notably, living in multi-unit residential buildings, particularly those with 1-2 and ≥6 floors, was positively associated with AR remission.

Review of a Series of Surveys on Adverse Reactions to the COVID-19 mRNA-1273 Vaccine at Okayama University.

This paper presents the results of a series of surveys conducted from July 2021 to March 2023 to investigate the post-vaccination adverse reactions to the mRNA-1273 (Moderna) vaccine among faculty, staff, and students at Okayama University. These studies complement the official surveys conducted by the Ministry of Health, Labour and Welfare (MHLW) and provide a more representative picture of adverse reactions in the general population including large numbers of healthy young people. Pain, swelling, redness at the injection site, fever, headache, and malaise were the main adverse reactions reported. The proportion of adverse reactions was generally higher after the second vaccination and decreased with each additional vaccination. No statistically significant differences in the adverse reactions were found for males and females and those with/without a history of allergy, but a lower proportion of fever was observed in older participants and those with underlying medical conditions. We also evaluated the association between adverse reactions and antibody titers after the third vaccination and found no significant differences in antibody levels one month after vaccination. This series of studies highlights the importance of conducting surveys in diverse populations to provide a more representative picture of post-vaccination adverse reactions during a pandemic.

Effectiveness of the Original COVID-19 Vaccine against COVID-19 Exacerbations during the Omicron Wave: A Population-based Study in Okayama, Japan.

Introduction: In Japan, approximately 97 million individuals have received their primary two doses of coronavirus disease 2019 (COVID-19) vaccine at the end of 2022. In this study, we aim to examine the effectiveness of the primary vaccines and compare its efficacy to booster vaccine shots in terms of preventing COVID-19 exacerbations during the Omicron-predominant period in Japan. Methods: For this analysis, we have collected all the confirmed COVID-19-positive cases from different medical institutions in Okayama City and have also utilized the information from the public Vaccination Record System. Taking the number of vaccinations into consideration, we then conducted a population-based study to assess the effectiveness of the two primary vaccine doses in preventing COVID-19 exacerbations during the Omicron waves. Our primary and secondary outcomes were COVID-19 exacerbations with respiratory failure (i.e., oxygen saturation on room air ≤ 93%, requiring supplemental oxygen), intensive care unit admission and/or mechanical ventilator requirement, or death, in accordance with the Japanese COVID-19 guidelines, and pneumonia during the course of COVID-19 infection, respectively. Results: In total, 95,329 COVID-19-positive individuals, aged 5 years and above, were included in this analysis (study period from January 1 to September 10, 2022). As per our findings, the effectiveness of the primary two doses against COVID-19 exacerbations compared with those who had never been vaccinated was 55.5% (95% confidential interval [CI]: 32.6-71.7), whereas it was higher after the third dose (76.9%; 95% CI: 66.7-84.0) and the fourth dose (75.7%; 95% CI: 58.8-85.7). Effectiveness was sustained for ≥ 5 months after the third vaccination, and preventive effectiveness was observed in individuals aged ≥ 65 years. Conclusions: As per the results of this study, we can conclude that the efficacy of the primary two doses of SARS-CoV-2 vaccine can be further strengthened in terms of preventing COVID-19 exacerbations by administering third and fourth booster vaccine shots. The additional bivalent vaccine is anticipated to further increase its efficacy against the Omicron strain, suggesting that individuals who have not received their booster shots yet should consider getting them to prevent COVID-19 exacerbations.

Adverse reactions in young children receiving the coronavirus disease 2019 vaccine.

BACKGROUND: We sought to investigate the occurrence of adverse reactions in Japanese children aged 6 months to 4 years who received the BNT162b2 coronavirus disease 2019 (COVID-19) vaccine, to examine parental considerations, and to evaluate potential risk factors associated with post-vaccination fever. METHODS: This cross-sectional survey study targeted 1617 children aged 6 months to 4 years who received their primary doses of BNT162b2 from November 10, 2022, to April 30, 2023, in Okayama Prefecture. We surveyed the occurrence of local and systemic reactions within 1 week after vaccination, and described the incidence proportions of adverse reactions for 515 participants overall and by age group. The study also examined the impact of previous COVID-19 infection and co-administration of the seasonal influenza vaccine on post-vaccination fever. A survey also assessed parents' reasons for vaccinating their children and the sources of information they used. RESULTS: Adverse reactions were infrequent (5.2%, with fever ≥37.5°C; no cases exceeded 39°C) and did not increase with vaccine doses administered. The risk of post-vaccination fever was not statistically associated with a history of COVID-19-the adjusted risk ratio (aRR) was 0.99, and the 95% confidence interval (CI) was 0.41-2.39-but was associated with co-administration of the seasonal influenza vaccine (aRR 3.24, 95% CI 1.14-9.18). Parental decisions regarding vaccination were influenced by official government guidelines and primary care physicians' opinion. CONCLUSION: This study provides valuable insight into the safety profile of the BNT162b2 vaccine in Japanese children aged 6 months to 4 years. Further research involving larger cohorts and appropriate control groups is needed.