Robust and persistent B-cell responses following SARS-CoV-2 vaccine determine protection from SARS-CoV-2 infection.

Introduction: A clear immune correlate of protection from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has not been defined. We explored antibody, B-cell, and T-cell responses to the third-dose vaccine and relationship to incident SARS-CoV-2 infection. Methods: Adults in a prospective cohort provided blood samples at day 0, day 14, and 10 months after the third-dose SARS-CoV-2 vaccine. Participants self-reported incident SARS-CoV-2 infection. Plasma anti-SARS-CoV-2 receptor-binding domain (RBD) and spike-subunit-1 and spike-subunit-2 antibodies were measured. A sub-study assessed SARS-CoV-2-specific plasma and memory B-cell and memory T-cell responses in peripheral blood mononuclear cells by enzyme-linked immunospot. Comparative analysis between participants who developed incident infection and uninfected participants utilised non-parametric t-tests, Kaplan-Meier survival analysis, and Cox proportional hazard ratios. Results: Of the 132 participants, 47 (36%) reported incident SARS-CoV-2 infection at a median 16.5 (16.25-21) weeks after the third-dose vaccination. RBD titres and B-cell responses, but not T-cell responses, increased after the third-dose vaccine. Whereas no significant difference in day 14 antibody titres or T-cell responses was observed between participants with and without incident SARS-CoV-2 infection, RBD memory B-cell frequencies were significantly higher in those who did not develop infection [10.0% (4.5%-16.0%) versus 4.9% (1.6%-9.3%), p = 0.01]. RBD titres and memory B-cell frequencies remained significantly higher at 10 months than day 0 levels (p < 0.01). Discussion: Robust antibody and B-cell responses persisted at 10 months following the third-dose vaccination. Higher memory B-cell frequencies, rather than antibody titres or T-cell responses, predicted protection from subsequent infection, identifying memory B cells as a correlate of protection.

[Refusal and hesitation regarding vaccination against COVID-19 in Douala, Cameroon]. / Refus et hésitation vis-à-vis de la vaccination anti-COVID-19 à Douala, Cameroun.

Introduction: vaccination is one of the strategies the World Health Organization recommends to reduce the burden of COVID-19. However, many African countries like Cameroon have low COVID-19 vaccination coverage. The purpose of this study was to investigate the reasons for the refusal of the population of the city of Douala to be vaccinated against COVID-19. Methods: we conducted a cross-sectional and analytical study in Douala from February 10 to May 31, 2022. Participants, aged at least 21 years and residing in the city of Douala, were interviewed. Associations between the variables of interest were measured using Chi-square and Fisher tests, with a 95% confidence interval. Results: a total of 1555 people were included in the study. Only 168 (11%) had been vaccinated. The proportion of vaccine refusal was high, with 711 (45.7%) refusing, 640 participants (41.1%) hesitating, and 204 people (13.2%) being in favor of COVID-19 vaccination. The main reasons for refusing anti-COVID-19 vaccination were fear of adverse effects (406; 44.8%), lack of information about vaccines (331; 36.5%), and lack of confidence (302; 33.3%). Factors associated with vaccine refusal were religion (p=0.026) and level of education (p=0.002). Conclusion: this study revealed low COVID-19 vaccination coverage in Douala, with a significant proportion of refusal and hesitation towards vaccination. Communication strategies should take into account the reasons and factors associated with refusal.

Health Care Delivery Site- and Patient-Level Factors Associated With COVID-19 Primary Vaccine Series Completion in a National Network of Community Health Centers.

Objectives. To assess multilevel factors associated with variation in COVID-19 vaccination rates in a US network of community health centers. Methods. Using multilevel logistic regression with electronic health record data from ADVANCE (Accelerating Data Value Across a National Community Health Center Network; January 1, 2022-December 31, 2022), we assessed associations between health care delivery site-level (n = 1219) and patient-level (n = 1 864 007) characteristics and COVID-19 primary vaccine series uptake. Results. A total of 1 337 440 patients completed the COVID-19 primary vaccine series. Health care delivery site characteristics were significantly associated with lower series completion rates, including being located in non-Medicaid expansion states and isolated or rural communities and serving fewer patients. Patient characteristics associated with significantly lower likelihood of completing the vaccine series included being Black/African American or American Indian/Alaska Native (vs White), younger age, lower income, being uninsured or publicly insured (vs using private insurance), and having fewer visits. Conclusions. Both health care delivery site- and patient-level factors were significantly associated with lower COVID-19 vaccine uptake. Community health centers have been a critical resource for vaccination during the pandemic. (Am J Public Health. 2024;114(11):1242-1251. https://doi.org/10.2105/AJPH.2024.307773).

Reach and Engagement With Population Health Management Interventions to Address COVID-19 Among Safety-Net Health Care Systems.

Interventions designed to address COVID-19 needed to be rapidly scaled up to the population level, and to address health equity by reaching historically marginalized populations most affected by the pandemic (e.g., racial/ethnic minorities and rural and low socioeconomic status populations). From February 2021 to June 2022, SCALE-UP Utah used text messaging interventions to reach 107 846 patients from 28 clinics within seven safety-net health care systems. Interventions provided informational and motivational messaging regarding COVID-19 testing and vaccination, and were developed using extensive community partner input. (Am J Public Health. 2024;114(11):1207-1211. https://doi.org/10.2105/AJPH.2024.307770).

[Management of HIT/anti-PF4 disorders].

Heparin-induced thrombocytopenia (HIT) was widely known as a disease characterized by development of thrombosis with thrombocytopenia after heparin exposure. In addition, vaccine-induced immune thrombotic thrombocytopenia (VITT) has been described as a fatal disease involving simultaneous bleeding and thrombosis after COVID-19 adenovirus vector vaccination. These were caused by HIT antibodies and anti-PF4 antibodies, respectively, but both were autoantibodies that recognized PF4, and were found to have the same pathology with different severities. In recent years, many pathologies in which anti-PF4 antibodies are produced have been reported, and a new concept of anti-PF4 disorder has been proposed. Anti-PF4 disorders are often difficult to identify due to their diverse range of causes, and the prognosis varies greatly depending on whether anti-PF4 antibodies can be measured and early treatment performed after observation of thrombocytopenia of unknown cause or thrombosis at an unusual site. To avoid overlooking anti-PF4 disorders, clinicians should become familiar with the classification of these disorders and accurately select the necessary tests.

Tracking temporal variations of fatality and symptomology correlated with COVID-19 dominant variants and vaccine effectiveness in the United States.

Introduction: We described how COVID-19 fatality and symptoms varied by dominant variant and vaccination in the US. Methods: Using the Restricted Access Dataset from the US CDC (1/1/2020-10/20/2022), we conducted a cross-sectional study assessing differences in COVID-19 deaths, severity indicators (hospitalization, ICU, pneumonia, abnormal X-ray, acute respiratory distress syndrome, mechanical ventilation) and 12 mild symptoms by dominant variant/vaccination periods using logistic regression after controlling for confounders. Results: We found the highest fatality during the dominant periods of Wild (4.6%) and Delta (3.4%). Most severe symptoms appeared when Delta was dominant (Rate range: 2.0-9.4%). Omicron was associated with higher mild symptoms than other variants. Vaccination showed consistent protection against death and severe symptoms for most variants (Risk Ratio range: 0.41-0.93). Boosters, especially the second, provided additional protection, reducing severe symptoms by over 50%. Discussion: This dataset may serve as a useful tool to monitor temporospatial changes of fatality and symptom for case management and surveillance.

Time to COVID-19 Vaccination by Language and Country of Origin.

Importance: Disparities in COVID-19 vaccination rates by race and ethnicity are well documented. Less is known about primary language and COVID-19 vaccine uptake. Objective: To describe the time to COVID-19 primary series vaccination and booster doses by primary language and country of origin. Design, Setting, and Participants: This retrospective cohort study included patients aged 6 months or older with at least 1 health encounter from July 1, 2019, to June 30, 2023, at a single health care system serving patients across Minnesota and western Wisconsin. Exposure: Primary language and country of origin documented in the electronic health record. Main Outcomes and Measures: Three COVID-19 vaccine coverage outcomes were evaluated: (1) primary series (1 Ad26.COV.S vaccine or 2 mRNA COVID-19 vaccines), (2) first-generation booster (primary series Ad26.COV.S vaccine plus 1 Ad26.COV.S or mRNA COVID-19 vaccine at least 2 months after the second dose or primary series mRNA vaccine plus 1 mRNA vaccine at least 5 months after the second dose), and (3) bivalent booster. Vaccine coverage was described by patient characteristics. Associations of primary language, race and ethnicity, and other patient characteristics with COVID-19 vaccine uptake were evaluated using time-to-event analysis in multivariable Cox proportional hazards regression models, and adjusted hazard ratios (AHRs) with 95% CIs were reported. Results: There were 1 001 235 patients included (53.7% female). Most patients reported English as a primary language (94.1%) and were born in the US (91.8%). Primary series coverage was 63.7%; first-generation booster coverage, 64.4%; and bivalent booster coverage, 39.5%. Coverage for all outcomes was lower among those with a non-English primary language compared with English as the primary language (56.9% vs 64.1% for primary series; 47.5% vs 65.3% for first-generation booster; 26.2% vs 40.3% for bivalent booster). Those with a non-English primary language had lower COVID-19 vaccine uptake for the primary series (AHR, 0.85; 95% CI, 0.84-0.86), first-generation booster (AHR, 0.74; 95% CI, 0.73-0.75), and bivalent booster (AHR, 0.65; 95% CI, 0.64-0.67) compared with patients with English as their primary language. Non-US-born patients had higher primary series uptake compared with US-born patients (AHR, 1.19; 95% CI, 1.18-1.20) but similar first-generation booster (AHR, 1.01; 95% CI, 0.99-1.02) and bivalent booster (AHR, 1.00; 95% CI, 0.98-1.02) uptake. Conclusions and Relevance: In this retrospective cohort study, patients with a non-English primary language had both lower coverage and delays in receiving COVID-19 vaccines compared with those with English as their primary language. Reporting on language may identify health disparities that can be addressed with language-specific interventions.

Strategies for developing self-assembled nanoparticle vaccines against SARS-CoV-2 infection.

In the recent history of the SARS-CoV-2 outbreak, vaccines have been a crucial public health tool, playing a significant role in effectively preventing infections. However, improving the efficacy while minimizing side effects remains a major challenge. In recent years, there has been growing interest in nanoparticle-based delivery systems aimed at improving antigen delivery efficiency and immunogenicity. Among these, self-assembled nanoparticles with varying sizes, shapes, and surface properties have garnered considerable attention. This paper reviews the latest advancements in the design and development of SARS-CoV-2 vaccines utilizing self-assembled materials, highlighting their advantages in delivering viral immunogens. In addition, we briefly discuss strategies for designing a broad-spectrum universal vaccine, which provides insights and ideas for dealing with possible future infectious sarbecoviruses.

Co-designing and pilot testing a digital game to improve vaccine attitudes and misinformation resistance in Ghana.

Misinformation related to vaccines has been shown to potentially negatively impact public perceptions and intentions to vaccinate in many contexts including COVID-19 vaccination in Ghana. Psychological inoculation - where recipients are warned about the misleading techniques used in misinformation - is a potential intervention which could preemptively boost public resistance against misinformation. Cranky Uncle Vaccine is an interactive, digital game that applies inoculation, offering a scalable tool building public resilience against vaccine misinformation and promoting positive health-related behaviors. In this study, we document the process of developing and testing a West African version of Cranky Uncle Vaccine, with co-design workshops and a pilot test conducted in urban and peri-urban areas of the Greater Accra region of Ghana with 829 young people who had access to mobile and computer devices. The average age was 21.8 and participants were highly educated (median education level "Some/all university") with slightly more females (51.2%) than males (48.4%). Pilot participants filled out surveys before and after playing the game, measuring vaccine attitudes (pre-game M = 3.4, post-game M = 3.6), intent to get vaccinated (pre-game M = 3.5, post-game M = 3.6), and discernment between vaccine facts and fallacies (pre-game AUC = 0.72, post-game AUC = 0.75). We observed a significant improvement in attitudes toward vaccines, with players demonstrating increased likelihood to get vaccinated after completing the game. Among players who indicated that they were unlikely to get vaccinated in the pre-game survey (N = 52, or 6.3% of participants), just over half of these participants (53%) switched to likely to get vaccinated after playing the game. Perceived reliability of vaccine facts remained the same, while perceived reliability of vaccine fallacies significantly decreased, indicating improved ability to spot misleading arguments about vaccines. These results demonstrate the effectiveness of a digital game in building public resilience against vaccine misinformation as well as improving vaccine attitudes and intent to get vaccinated.

SARS-CoV-2, Tdap, and influenza vaccination during pregnancy from 2019 to 2022 in Ontario, Canada: a population-based retrospective cohort study.

BACKGROUND: Hesitancy about vaccination during pregnancy posed challenges to SARS-CoV-2 vaccination efforts. We aimed to examine rates of SARS-CoV-2 vaccination among Ontario residents who gave birth in early 2022, and to compare rates of SARS-CoV-2 vaccine uptake with rates of tetanus, diphtheria, and pertussis (Tdap) and influenza vaccination during pregnancy in 2019, 2021, and 2022. METHODS: We conducted a population-based retrospective cohort study to describe vaccination rates among pregnant and comparable nonpregnant populations in Ontario using linked administrative data. Provincially insured females who had a live, in-hospital birth from Jan. 1 to Mar. 31 in 2019, 2021, or 2022 were our primary cohort. Using log-binomial regression, we tested associations between SARS-CoV-2 (2022) and Tdap and influenza (2019, 2021, 2022) vaccination status, with birth group and covariates. We compared SARS-CoV-2 vaccination status with the status of a matched cohort of nonpregnant females and conducted subgroup analyses by age and prenatal clinician type. RESULTS: Among birthing people, 78.7% received their first SARS-CoV-2 vaccine dose and 74.2% received a second dose. The rate was significantly higher among nonpregnant comparators (dose 1: relative risk [RR] 0.94, 95% confidence interval [CI] 0.93-0.94; dose 2: RR 0.91, 95% CI 0.90-0.91). However, the rate of SARS-CoV-2 vaccination uptake among birthing people was higher than uptake of Tdap or influenza vaccination. Tetanus, diphtheria, and pertussis vaccination increased over time from 22.2% in 2019 to 32.6% in 2022, and influenza vaccination rose to 35.3% in 2021 but returned to prepandemic levels in 2022 (27.7%). Vaccination rates were lower among pregnant people who were young, multiparous, or residents of rural or economically deprived areas for all 3 vaccines. INTERPRETATION: Rates of SARS-CoV-2 vaccination were lower among pregnant people than among nonpregnant comparators but were higher than rates of routinely recommended Tdap and influenza vaccinations. Pandemic urgency may have overcome a great deal of hesitancy about vaccinating against SARS-CoV-2 during pregnancy in 2022, but uptake of routinely recommended vaccines in pregnancy remains a challenge. TRIAL REGISTRATION: Clinicaltrials.gov, no. NCT05663762.

COVID-19 vaccine hesitancy among unvaccinated individuals in a primary care setting, Pretoria.

BACKGROUND:  South Africa faced challenges while implementing coronavirus disease 2019 (COVID-19) measures such as mass vaccination. Some people rejected or were hesitant to receive government-recommended vaccines. This study explored COVID-19 vaccination hesitancy among unvaccinated individuals in a primary care setting in Pretoria, South Africa. METHODS:  This was an exploratory phenomenological study that included one-on-one interviews with 12 individuals at Temba Community Health Centre in Pretoria, South Africa. RESULTS:  The research revealed five themes: perceptions of COVID-19 disease, perceptions of COVID-19 vaccine, factors related to non-vaccination, information sources about the COVID-19 vaccine, and long-term vaccination decisions. There were seven linked sub-themes. CONCLUSION:  Overall, participants had a good understanding of COVID-19 disease, but limited knowledge about the vaccine, causing hesitancy to get vaccinated. Reasons for not getting vaccinated included health-related concerns, safety concerns, personal experiences, and social and political factors. Safety and health-related concerns were prevalent, with adverse vaccine outcomes being the most common concern. Most participants had experienced a historic encounter with a vaccine-related death or illness.Contribution: Vaccine hesitancy should be viewed as a powerful concern from the community, and a key source of worry for the health authorities over any vaccine-related doubt.

Effectiveness of the Bivalent mRNA COVID-19 Vaccine for Preventing Critical Infection From the SARS-CoV-2 Omicron Variant in the Republic of Korea.

BACKGROUND: This retrospective observational matched cohort study assessed the differences in critical infections caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) during the omicron-predominant period of the coronavirus disease 2019 (COVID-19) pandemic. We evaluated the vaccine effectiveness of bivalent mRNA vaccine compared to unvaccinated individuals. METHODS: We collected COVID-19 case data from the Korean COVID-19 vaccine effectiveness cohort. We calculated the probability of critical COVID-19 cases by comparing the vaccinated and unvaccinated groups. RESULTS: The risk of being critically infected due to SAR-CoV-2 infection was 5.96 times higher (95% confidence interval, 5.63-6.38) among older individuals who were unvaccinated compared to those who received the bivalent COVID-19 vaccine. CONCLUSION: Our findings indicate that the bivalent vaccine reduces the disease burden of the SARS-CoV-2 omicron variant, particularly among the older population. Further studies are warranted to determine the effectiveness of booster doses of vaccines for SARS-CoV-2 infection.

Hesitation towards COVID-19 booster vaccination among dialysis patients: a cross-sectional study in Taizhou, China.

PURPOSE: This research aimed to explore hesitation towards the COVID-19 booster vaccine among dialysis patients and study the association between COVID-19 pandemic-induced health behavior and vaccination hesitancy. METHODS: A self-administered online questionnaire evaluating dialysis patients' hesitation to take COVID-19 booster vaccination was conducted between March 24 and 22 April 2022 in Taizhou, China. The logistic regression method was applied to identify factors associated with vaccination hesitancy, and all data were analyzed using R software. RESULTS: Of the 365 study participants, 272 (74.5%) individuals hesitated to take the booster dose. Health behavior was found to be a significant factor for hesitation to take COVID-19 vaccines, with OR (95% CI) of 1.09 (1.02-1.17). Influenza vaccination history was also significantly associated with the hesitation (OR (95% CI) = 0.39 (0.21-0.74)). In addition, participants with higher education levels exhibited lower vaccine hesitancy compared to those with junior secondary or below, with ORs (95% CIs) of 0.49 (0.27-0.91) for senior secondary and 0.35 (0.14-0.89) for junior college or above, respectively. CONCLUSION: The proportion of hesitancy for taking the booster vaccination of the COVID-19 vaccine was high among dialysis patients. Health behaviors, influenza vaccination history, and education levels were risk factors in their vaccination hesitancy. These findings may aid efforts to help vaccinate people with underlying diseases against future pandemics.

Common measures of vaccination intention generate substantially different estimates that can reduce predictive validity.

Surveys often estimate vaccination intentions using dichotomous ("Yes"/"No") or trichotomous ("Yes," "Unsure," "No") response options presented in different orders. Do survey results depend on these variations? This controlled experiment randomized participants to dichotomous or trichotomous measures of vaccine intentions (with "Yes" and "No" options presented in different orders). Intentions were measured separately for COVID-19, its booster, and influenza vaccines. Among a sample of U.S. adults (N = 4,764), estimates of vaccine intention varied as much as 37.5 ± 17.4 percentage points as a function of the dichotomous or trichotomous response set. Among participants who had not received the COVID-19 vaccine, the "Unsure" option was more likely to reduce the share of "No" (versus "Yes") responses, whereas among participants who had received the COVID-19 vaccine, the "Unsure" option was more likely to reduce the share of "Yes" (versus "No") responses. The "Unsure" category may increase doubt and decrease reliance on past vaccination behavior when forming intentions. The order of "Yes" and "No" responses had no significant effect. Future research is needed to further evaluate why the effects of including the "Unsure" option vary in direction and magnitude.

Assessing vaccinated persons' intention to take the COVID-19 boosters using a combined theoretical framework: an online survey in Egypt.

Vaccines, like the Corona Virus Disease-2019 (COVID-19) vaccines, can control diseases, but vaccine hesitancy reduces their use. It is important to assess the intention to use COVID-19 vaccines boosters and the determinants of this intention to help in developing programs to promote the uptake of boosters. An online survey collected data from adults in Egypt between March and June 2022 using a questionnaire that assessed demographic characteristics, and constructs of the Theory of Planned Behaviour (TPB) and the Health Belief Model (HBM). The survey was uploaded to SurveyMonkey and the links were posted on social media platforms. Binary regression analysis was used and the dependent variable was intention to use boosters of COVID-19 vaccines. The independent variables were indicators of the HBM including perceived susceptibility to COVID-19 infection (medical history) and possibility of disease prevention (awareness of the availability of types of COVID-19 vaccines); and indicators of the TPB including attitude toward COVID-19 vaccines (that they are harmful, that they may lead to death and confidence in locally and foreign manufactured vaccines), perceived norms (the percentage of vaccinated persons in one's circle) and perceived control over booster uptake (presence of government mandates for COVID-19 vaccination). The confounders were sociodemographic factors (age, sex, education, and place of residence). Complete responses were available from 1113 out of 1401 participants (79.4%), with mean (SD) age = 25 (9.5) years, of whom, 66.7% (n = 742) were females and 68.6% (n = 764) were university students. About 39.4% and 31.2% indicated that they would get or would definitely get the booster dose of the COVID-19 vaccines. In multiple regression, intention to use a booster dose was significantly related to not agreeing (AOR = 4.87, P < 0.001) or not agreeing at all (AOR = 8.46, P = 0.001) that vaccines are harmful and to having no confidence (AOR = 0.21, P < 0.001) or no confidence at all (AOR = 0.14, P < 0.001) in foreign-manufactured vaccines. Most university-educated Egyptians in the study intended to take the COVID-19 vaccine booster dose and this intention was associated with attitude toward the harm of the vaccine and confidence in foreign-manufactured vaccines. Awareness campaigns are needed to counteract misinformation and promote booster dose uptake.

An integrated blockchain-enabled multi-channel vaccine supply chain network under hybrid uncertainties.

The recent pandemic caused by COVID-19 is considered an unparalleled disaster in history. Developing a vaccine distribution network can provide valuable support to supply chain managers. Prioritizing the assigned available vaccines is crucial due to the limited supply at the final stage of the vaccine supply chain. In addition, parameter uncertainty is a common occurrence in a real supply chain, and it is essential to address this uncertainty in planning models. On the other hand, blockchain technology, being at the forefront of technological advancements, has the potential to enhance transparency within supply chains. Hence, in this study, we develop a new mathematical model for designing a COVID-19 vaccine supply chain network. In this regard, a multi-channel network model is designed to minimize total cost and maximize transparency with blockchain technology consideration. This addresses the uncertainty in supply, and a scenario-based multi-stage stochastic programming method is presented to handle the inherent uncertainty in multi-period planning horizons. In addition, fuzzy programming is used to face the uncertain price and quality of vaccines. Vaccine assignment is based on two main policies including age and population-based priority. The proposed model and method are validated and tested using a real-world case study of Iran. The optimum design of the COVID-19 vaccine supply chain is determined, and some comprehensive sensitivity analyses are conducted on the proposed model. Generally, results demonstrate that the multi-stage stochastic programming model meaningfully reduces the objective function value compared to the competitor model. Also, the results show that one of the efficient factors in increasing satisfied demand and decreasing shortage is the price of each type of vaccine and its agreement.

Adverse event following immunisation of adsorbed-inactivated Coronavac (Sinovac) and ChAdOx1 nCOV-19 (Astra Zeneca) of COVID-19 vaccines.

INTRODUCTION: Countries around the world organised mass vaccinations using various types of vaccines against COVID-19, like inactivated viruses and mRNA. The study aimed to look at adverse events following immunisation (AEFI) of Coronavac® (SIN) and ChAdOx1 nCOV-19 ® (AZ) COVID-19 vaccines in Indonesia. MATERIALS AND METHODS: Subjects who received SIN or AZ vaccines were sent questionnaires twice: after they received the first and the second doses of vaccine, respectively. AEFI data on the first- and second-day post-vaccination were collected and analyzed descriptively. RESULTS: A total of 1547 people vaccinated with SIN vaccine, 529 (33.3%) responded to the first-dose and 239 (47%) to the second-dose questionnaires, whereas 936 people vaccinated with AZ vaccine, 483 (51.6%) answered the firstdose and 123 (25%) to the second-dose questionnaires. Some important AEFIs on the first- and second-day post receiving SIN vs. AZ vaccination were as follows: fever 4% vs 59%; pain at the injection site 27% vs 87%; redness and swelling at the injection site 4% vs 18%; nausea 5% vs 30%; diarrhea 1.8% vs 5.7%, respectively. CONCLUSION: SIN seemed to have fewer AEFIs than AZ. Apart from different vaccine materials and excipients, the gap in AEFIs between SIN and AZ could be caused by the distinct population where AZ recipients were more exposed to COVID-19.