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Background: Understanding how the efficacy of COVID-19 vaccines translates from clinical trials to real-world settings is critical to inform evolving vaccination policies. The objective of this study was to assess COVID-19 vaccine effectiveness (VE) against severe COVID-19-related outcomes in children aged 5-11 years, including COVID-19-related hospital admissions and multisystem inflammatory syndrome in children (MIS-C)., Methods: We conducted a retrospective, population-based cohort study using linked health administrative data in the first year following the emergence of the Omicron variant (January 2 to December 31, 2022) in Ontario, Canada. Baseline differences between subgroups of interest were compared using standardized differences. We used multivariable Cox proportional hazard regression models to estimate VE by time since last vaccine dose by treating vaccination as a time-varying exposure, compared to unvaccinated children., Results: We included a total of 1,058,740 children, of which 583,867 (55.1 %) had received at least one vaccine dose by the end of the study period. In total, there were 185 COVID-19-related hospital admissions and 39 cases of MIS-C. The rate of COVID-19-related admission was substantially higher in children with an underlying comorbid condition compared to children who were previously healthy (adjusted hazard ratio [aHR] = 4.77, 95 %CI, 3.56-6.38). VE against COVID-19-related admission ranged from 93 % (95 %CI, 52-99 %) 7-29 days after a second dose to 63 % (95 %CI; 41-77 %) ≥120 days after a second dose. There was no statistically significant difference in the rate of MIS-C in children who received at least one dose of the vaccine compared to unvaccinated children (aHR = 0.71; 95 %CI, 0.38-1.34)., Conclusions: We found that, for children aged 5-11 years, VE against COVID-19-related hospitalization was high in the first four months after a second dose. Children with comorbid conditions were found to be at much higher risk of COVID-19-related severe outcomes and thus may benefit most from COVID-19 vaccination., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Jeffrey C. Kwong reports financial support was provided by Canadian Institutes of Health Research. Jeffrey C. Kwong reports financial support was provided by Public Health Agency of Canada. Kumanan Wilson reports a relationship with CANImmunize that includes: board membership. Shaun K. Morris reports a relationship with GlaxoSmithKline that includes: consulting or advisory and speaking and lecture fees. Shaun K. Morris reports a relationship with Sanofi Pasteur that includes: consulting or advisory and speaking and lecture fees. Shaun K. Morris reports a relationship with Pfizer that includes: consulting or advisory and speaking and lecture fees. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier Ltd.)

The COVID-19 vaccination program implementation in Ontario, Canada has spanned multiple years and is ongoing. To meet the challenges of the program, Ontario developed and implemented a new electronic COVID-19 immunization registry, COVaxON, which captures individual-level data on all doses administered in the province enabling comprehensive coverage assessment. However, the need for ongoing COVID-19 vaccine coverage assessments over a multi-year vaccination program posed challenges necessitating methodological changes. This paper describes Ontario's COVID-19 immunization registry, the methods implemented over time to allow for the ongoing assessment of vaccine coverage by age, and the impact of those methodological changes. Throughout the course of the vaccination program, four different methodological approaches were used to calculate age-specific coverage estimates using vaccination data (numerator) obtained from COVaxON. Age-specific numerators were initially calculated using age at time of first dose (method A), but were updated to the age at coverage assessment (method B). Database enhancements allowed for the exclusion of deceased individuals from the numerator (method C). Population data (denominator) was updated to 2022 projections from the 2021 national census following their availability (method D). The impact was most evident in older age groups where vaccine uptake was high. For example, coverage estimates for individuals aged 70-79 years of age for at least one dose decreased from 104.9 % (method B) to 95.0 % (method D). Thus, methodological changes improved estimates such that none exceeded 100 %. Ontario's COVID-19 immunization registry has been transformational for vaccine program surveillance. The implementation of a single registry for COVID-19 vaccines was essential for comprehensive near real-time coverage assessment, and enabled new uses of the data to support additional components of vaccine program surveillance. The province is well positioned to build on what has been achieved as a result of the COVID-19 pandemic and expand the registry to other routine vaccination programs., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Crown Copyright © 2023. Published by Elsevier Ltd. All rights reserved.)

Background: Population-based COVID-19 vaccine coverage estimates among pregnant individuals are limited. We assessed temporal patterns in vaccine coverage (≥1 dose before or during pregnancy) and evaluated factors associated with vaccine series initiation (receiving dose 1 during pregnancy) in Ontario, Canada., Methods: We linked the provincial birth registry with COVID-19 vaccination records from December 14, 2020 to December 31, 2021 and assessed coverage rates among all pregnant individuals by month, age, and neighborhood sociodemographic characteristics. Among individuals who gave birth since April 2021-when pregnant people were prioritized for vaccination-we assessed associations between sociodemographic, behavioral, and pregnancy-related factors with vaccine series initiation using multivariable regression to estimate adjusted risk ratios (aRR) and risk differences (aRD) with 95% confidence intervals (CI)., Results: Among 221,190 pregnant individuals, vaccine coverage increased to 71.2% by December 2021. Gaps in coverage across categories of age and sociodemographic characteristics decreased over time, but did not disappear. Lower vaccine series initiation was associated with lower age (<25 vs. 30-34 years: aRR 0.53, 95%CI 0.51-0.56), smoking (vs. non-smoking: 0.64, 0.61-0.67), no first trimester prenatal care visit (vs. visit: 0.80, 0.77-0.84), and residing in neighborhoods with the lowest income (vs. highest: 0.69, 0.67-0.71). Vaccine series initiation was marginally higher among individuals with pre-existing medical conditions (vs. no conditions: 1.07, 1.04-1.10)., Conclusions: COVID-19 vaccine coverage among pregnant individuals remained lower than in the general population, and there was lower vaccine initiation by multiple characteristics., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: K. Wilson is CEO of CANImmunize Inc, which hosts a national digital immunization record, and is a member of the independent data safety board for the Medicago COVID-19 vaccine trial; there were no other relationships or activities that could appear to have influenced the submitted work., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Background: Background incidence rates are critical in pharmacovigilance to facilitate identification of vaccine safety signals. We estimated background incidence rates of 11 adverse events of special interest related to COVID-19 vaccines in Ontario, Canada., Methods: We conducted a population-based retrospective observational study using linked health administrative databases for hospitalizations and emergency department visits among Ontario residents. We estimated incidence rates of Bell's palsy, idiopathic thrombocytopenia, febrile convulsions, acute disseminated encephalomyelitis, myocarditis, pericarditis, Kawasaki disease, Guillain-Barré syndrome, transverse myelitis, acute myocardial infarction, and anaphylaxis during five pre-pandemic years (2015-2019) and 2020., Results: The average annual population was 14 million across all age groups with 51% female. The pre-pandemic mean annual rates per 100,000 population during 2015-2019 were 191 for acute myocardial infarction, 43.9 for idiopathic thrombocytopenia, 28.8 for anaphylaxis, 27.8 for Bell's palsy, 25.0 for febrile convulsions, 22.8 for acute disseminated encephalomyelitis, 11.3 for myocarditis/pericarditis, 8.7 for pericarditis, 2.9 for myocarditis, 2.0 for Kawasaki disease, 1.9 for Guillain-Barré syndrome, and 1.7 for transverse myelitis. Females had higher rates of acute disseminated encephalomyelitis, transverse myelitis and anaphylaxis while males had higher rates of myocarditis, pericarditis, and Guillain-Barré syndrome. Bell's palsy, acute disseminated encephalomyelitis, and Guillain-Barré syndrome increased with age. The mean rates of myocarditis and/or pericarditis increased with age up to 79 years; males had higher rates than females: from 12 to 59 years for myocarditis and ≥12 years for pericarditis. Febrile convulsions and Kawasaki disease were predominantly childhood diseases and generally decreased with age., Conclusions: Our estimated background rates will permit estimating numbers of expected events for these conditions and facilitate detection of potential safety signals following COVID-19 vaccination., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Crown Copyright © 2022. Published by Elsevier Ltd. All rights reserved.)

Background: Many countries recommend influenza vaccination during pregnancy. Despite this recommendation, influenza vaccine among pregnant individuals remains under-utilized and uptake varies by country. Factors associated with influenza vaccine uptake during pregnancy may also vary across countries., Methods: As members of the Pregnancy Influenza Vaccine Effectiveness Network (PREVENT), five sites from four countries (Australia, Canada, Israel, and the United States) retrospectively identified cohorts of individuals aged 18-50 years who were pregnant during pre-defined influenza seasons. Influenza vaccine coverage estimates were calculated for the 2010-11 through 2015-16 northern hemisphere and the 2012 through 2015 southern hemisphere influenza seasons, by site. Sites used electronic health records, administrative data, and immunization registries to collect information on pregnancy, health history, demographics, and vaccination status. Each season, vaccination coverage was calculated as the percentage of individuals who received influenza vaccine among the individuals in the cohort that season. Characteristics were compared between those vaccinated and unvaccinated, by site., Results: More than two million pregnancies were identified over the study period. Influenza vaccination coverage ranged from 5% to 58% across sites and seasons. Coverage increased consistently over the study period at three of the five sites (Western Australia, Alberta, and Israel), and was highest in all seasons at the United States study site (39-58%). Associations with vaccination varied by country and across seasons; where available, parity >0, presence of a high-risk medical condition, and urban residence were consistently associated with increased likelihood of vaccination., Conclusions: Though increasing, uptake of influenza vaccine among pregnant individuals remains lower than recommended. Coverage varied substantially by country, suggesting an ongoing need for targeted strategies to improve influenza vaccine uptake in this population., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Dr. Naleway and Dr. Katz each report unrelated research support from Pfizer. Dr. Klein reports unrelated research support from Sanofi Pasteur, Protein Science (now Sanofi Pasteur), GlaxoSmithKline, Merck & Co, and Pfizer. All additional authors have no conflicts to disclose., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Background: Nursing home (NH) residents are prioritized for COVID-19 vaccination. We report monthly mortality, hospitalizations, and emergency department (ED) visit incidence rates (IRs) during 2010-2020 to provide context for COVID-19 vaccine safety assessments., Methods: We observed outcomes among all NH residents in Ontario using administrative databases. IRs were calculated by month, sex, and age group. Comparisons between months were assessed using one-sample t-tests; comparisons by age and sex were assessed using chi-squared tests., Results: From 2010 to 2019, there were 83,453 (SD: 652.4) NH residents per month, with an average of 2.3 (SD: 0.28) deaths, 3.1 (SD: 0.16) hospitalizations, and 3.6 (SD: 0.17) ED visits per 100 residents per month. From March to December 2020, mortality IRs were increased, but hospitalization and ED visit IRs were reduced (p < 0.05)., Conclusion: We identified consistent monthly mortality, hospitalization, and ED visit IRs during 2010-2019. Marked differences in these rates were observed during 2020, coinciding with the COVID-19 pandemic., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Introduction: In Ontario, Canada, parents have the responsibility to report their child's routine infant and childhood vaccines to the provincial immunization registry (the Digital Health Immunization Repository; DHIR) without healthcare provider validation. Despite its use in routine immunization coverage monitoring, no study has previously examined the completeness of immunization data within the DHIR., Methods: We assessed the completeness of DHIR immunizations, as compared to immunizations within the Electronic Medical Records-Primary Care (EMRPC) database, also known as EMRALD, a network of family physician electronic medical records (EMRs). We linked client records from the DHIR and EMRPC to a centralized population file. To create the study cohort, we examined children born during 2005-2008 and further defined the cohort based on those rostered to an EMRPC physician, visit criteria to ensure ongoing care by an EMRPC provider, and school attendance in Ontario at age 7. We calculated up-to-date (UTD) immunization coverage at age 7 for individual vaccines and overall using data from the DHIR and EMRPC separately, and compared the estimates., Results: The analytic cohort to assess DHIR data completeness included 2,657 children. Overall UTD coverage (all vaccines assessed) was 82.0% in the DHIR and 67.6% in EMRPC. UTD coverage was higher in the DHIR for all vaccines assessed individually, with the exception of meningococcal C conjugate vaccine (difference = 0.3%). After excluding two EMRPC sites with irregularities in immunization data, the difference in overall UTD coverage between systems decreased from 14.4% to 6.6% INTERPRETATION: These results validate the use of DHIR for coverage assessment but also suggest that bidirectional exchange of immunization information has the potential to increase immunization data completeness in both systems., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Crown Copyright © 2020. Published by Elsevier Ltd. All rights reserved.)

Background: Linking data on laboratory specimens collected during clinical practice with health administrative data permits highly powered vaccine effectiveness (VE) studies to be conducted at relatively low cost, but bias from using convenience samples is a concern. We evaluated the validity of using such data for estimating VE., Methods: We created the Flu and Other Respiratory Viruses Research (FOREVER) Cohort by linking individual-level data on respiratory virus laboratory tests, hospitalizations, emergency department visits, and physician services. For community-dwelling adults aged > 65 years, we assessed the presence and magnitude of information and selection biases, generated VE estimates under various conditions, and compared our VE estimates with those from other studies., Results: We included 65,648 unique testing episodes obtained from 54,434 individuals during the 2010-11 to 2015-16 influenza seasons. To examine information bias, we found the proportion testing positive for influenza for patients with unknown interval from illness onset to specimen collection was more similar to patients for whom illness onset date was ≤ 7 days before specimen collection than to patients for whom illness onset was > 7 days before specimen collection. To assess the presence of selection bias, we found the likelihood of influenza testing was comparable between vaccinated and unvaccinated individuals, although the adjusted odds ratios were significantly greater than 1 for some healthcare settings and during some influenza seasons. Over 6 seasons, VE estimates ranged between 36% (95%CI, 27-44%) in 2010-11 and 5% (95%CI, -2, 11%) in 2014-15. VE estimates were similar under a range of conditions, but were consistently higher when accounting for misclassification of vaccination status through a quantitative sensitivity analysis. VE estimates from the FOREVER Cohort were comparable to those from other studies., Conclusions: Routinely collected laboratory and health administrative data contained in the FOREVER Cohort can be used to estimate influenza VE in community-dwelling older adults., (Copyright © 2019 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Background: Our objectives were: (1) to quantify and describe un-immunized students in Ontario, Canada and assess the extent to which these students have exemptions; and (2) to quantify and describe students with non-medical exemptions (NMEs), including what proportion have up-to-date immunizations., Methods: We examined Ontario students 7 to 17 years-of-age in the 2016-2017 school year using information within a centralized immunization repository. We identified and described students with different immunization/exemption classifications by age, sex, school type, geography and area-level material deprivation using descriptive and multivariable logistic regression analyses. Finally, we assessed the immunization status of students with NMEs, by antigen., Results: We found that students could be recorded as un-immunized with or without an NME, or be immunized with an NME. From a cohort of 1.65 million students, 2.9% of students had zero vaccine doses recorded, and of these 68% had no exemption of any kind. A total of 2.4% of students had an NME. Of these, 39% were un-immunized and 61% had received ≥1 vaccine. Among all students with NMEs, 19-48% had up-to-date immunizations, varying by antigen. Factors associated with increased odds of having a NME and being un-immunized included: attendance at private and 'other' schools, rural residence, and geography. Older age and greater area-level deprivation were associated with a reduced odds., Conclusions: Our assessment revealed that Ontario students with NMEs cannot be assumed to be un-immunized and at risk for all vaccine-preventable diseases. Conversely, not all un-immunized students had NMEs suggesting that future studies of un-immunized children in Ontario must consider additional factors beyond NME status alone. Other jurisdictions that use NME data to inform research and surveillance of vaccine hesitancy and risks for VPD outbreaks may wish to undertake a similar assessment to determine how well student NMEs correlate with student immunization status., (Crown Copyright © 2019. Published by Elsevier Ltd. All rights reserved.)