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Background : Despite adverse events following immunization (AEFI) being well described in vaccine trials, there is a need to produce more real-world data on events supposedly attributed to vaccination against COVID-19. This study aims to estimate the prevalence of AEFI in the first dose of COVID-19 vaccines in the state of Brazil and to verify whether such events differ among the types of vaccines offered in this country. Methods : A population-based study using linked administrative data on vaccine registry and adverse events following immunization in 2021 and 2022. The study included 10,169,378 individuals aged 18 or over who lived in Bahia and received the first dose of COVID-19 vaccines. We calculated AEFI prevalence and verified differences among vaccines by logistic regression to estimate crude and adjusted by sex and age group prevalence ratio (PR). Results : The prevalence of AEFI was 74.3 per 100,000 doses applied, with a higher rate of nonserious events, mainly following the ChAdOx1-S. More than two-thirds of these adverse effects occurred in women, and almost half were between 30 and 49 years old. The individuals who received ChAdOx1-S had a 125% higher prevalence than those who received CoronaVac. Those who received BNT162b2 and Ad26.COV2.S had a 71% and 58%, respectively, lower prevalence of AEFI than those who received CoronaVac. Conclusions : The use of vaccines against COVID-19 has proven to be positive and effective in combating SARS-CoV-2, significantly reducing morbidity and mortality from the disease. We cannot deny the presence of adverse events in the context of vaccination. However, the vaccines have proven to be safe and reliable. The results of this study offer relevant data that can contribute to the qualification of AEFI pharmacovigilance in Brazil and worldwide.

Objective: To analyse vaccination coverage and factors associated with incomplete polio vaccination in a cohort of children born in 2017-2018, in state capitals and interior region municipalities of Northeast Brazil., Methods: Household survey of children aged ≤24 months conducted between 2020 and 2022. Vaccination coverage and dropout rates were estimated, as well as factors associated with incomplete vaccination, analyzed by calculating odds ratios (OR) and 95% confidence intervals (95%CI)., Results: : Among 12,137 children, vaccination coverage (4 doses) was 80.9% (95%CI 78.4;83.1); 8.4% were not vaccinated. Not having a vaccination card (OR=18.06; 95%CI 10.01;32.61) and use of private services (OR=1.46; 95%CI 1.23;1.74) were associated with incomplete vaccination. Higher dropout rates were found for the booster dose, especially in the highest stratum., Conclusion: Low vaccination coverage, poor dose follow-up and high dropout rates were found for polio vaccines in the areas studied.

Objective: To analyze vaccination coverage and factors associated with incomplete vaccination in inland municipalities of Northeastern Brazil., Methods: This was a household survey using cluster sampling conducted in Vitória da Conquista, Bahia state, Caruaru, Pernambuco state, Sobral, Ceará state and Imperatriz, Maranhão state between 2020 and 2022. Vaccination coverage by valid doses and vaccine hesitancy were analyzed, with the odds ratio (OR) estimated and adjusted using logistic regression., Results: Among 1,847 children, complete vaccination coverage was 49.2% (95%CI 43.9;54.5). Factors associated with incomplete vaccination included: higher income (OR 1.53; 95%CI 1.02;2.31), residence in Sobral (OR 4.35; 95%CI 3.04; 6.21) and >1 child (OR 1.20; 95%CI 1.11;1.32). Parental decision not to vaccinate and difficulties in traveling to vaccination centers contributed to vaccine hesitancy., Conclusion: Low vaccination coverage and incomplete vaccination were associated with social issues in the socioeconomic strata analyzed.

Objective: To analyze measles, mumps, and rubella vaccination coverage among children up to 24 months old and factors associated with non-vaccination in a 2017-2018 live birth cohort, in state capitals and large interior region cities in Northeast Brazil., Methods: Population-based survey analyzing vaccination coverage and sociodemographic factors through logistic regression., Results: For 12,137 children, vaccination coverage was 79.3% (95%CI 76.5;81.8), and the dropout rate was 10.6%. Association with non-vaccination: socioeconomic stratum A (OR-a 1.29; 95%CI 1.10;1.50), living in the interior region (OR-a 1.22; 95%CI 1.07;1.39), no access to the Bolsa Família Program (OR-a 1.19, 95%CI 1.05;1.34), family income ≤BRL 1,000 (OR-a 1.17, 95%CI 1.03;1.31), mother not working (OR-a 1.28, 95%CI 1.15;1.42), >1 child per mother (OR-a 1.12, 95%CI 1.08;1.17), and no vaccination card (OR-a 10.69, 95%CI 6.27;18.20)., Conclusion: Low vaccination coverage and a high dropout rate in state capitals and municipalities in the interior region of Northeast Brazil., Main Results: 79.3% vaccination coverage with incomplete vaccination associated with living in higher socioeconomic strata and in interior region municipalities, and children without vaccination records. High dropout rates in the state capitals Natal and João Pessoa., Implications for Services: Recognition of low coverage and factors associated with non-vaccination provides health services with relevant information with a view to planning strategic actions to increase measles, mumps and rubella vaccination coverage., Perspectives: Low coverage and identification of non-vaccination indicate the importance of targeting actions in the Brazilian National Health System to achieve the targets recommended by the National Immunization Program, for populations in Northeast Brazil.

Objective: To estimate vaccination coverage and analyze sociodemographic factors associated with non-vaccination in children born in 2017 and 2018 in the state capitals of Northeast Brazil., Methods: A household survey using cluster sampling was conducted from 2020-2022 to estimate vaccination coverage and hesitancy. Factors associated with non-vaccination were analyzed using logistic regression to calculate Odds Ratios (OR) and their Confidence Intervals (95%CI)., Results: Natal was the capital with the lowest vaccination coverage, below 75.0% for most immunizers. Teresina had rates equal to or greater than 90.0% for all vaccines. Among those interviewed, 99.1% (95%CI 98.9;99.3) believe that vaccines are important for health; 95.4% (95%CI 95.0;95.8) trust immunobiologicals and 79.6% (95%CI% 78.8;80.3) are not afraid of reactions. Belonging to the highest socioeconomic stratum (adjusted OR: 1.34 - 95%CI 1.20;1.50) was as a factor associated with non-vaccination., Conclusion: Low coverage highlights the need for a better understanding of regional specificities and social inequalities.

The evaluation of vaccine effectiveness is conducted with real-world data. They are essential to monitor the performance of vaccination programmes over time, and in the context of the emergence of new variants. Until now, the effectiveness of COVID-19 vaccines has been assessed based on classic methods, such as cohort and test-negative case-control studies, which may often not allow for adequate control of inherent biases in the assignment of vaccination campaigns. The aim of this review was to discuss the study designs available to evaluate vaccine effectiveness, highlighting quasi-experimental studies, which seek to mimic randomized trials, by introducing an exogenous component to allocate to treatment, in addition to the advantages, limitations, and applicability in the context of Brazilian data. The use of quasi-experimental approaches, such as interrupted time series, difference-in-differences, propensity scores, instrumental variables, and regression discontinuity design, are relevant due to the possibility of providing more accurate estimates of COVID-19 vaccine effectiveness. This is especially important in scenarios such as the Brazilian, which characterized by the use of various vaccines, with the respective numbers and intervals between doses, applied to different age groups, and introduced at different times during the pandemic.