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Introduction: The rapid emergence of COVID-19 urged policy responses worldwide, focusing on vaccination and mobility restrictions. Chile represents a unique scenario for analyzing personal preventive measures amid intensive communication and vaccination campaigns. This study aims to explore changes in population adherence to non-pharmacological preventive measures during the pandemic and the factors that explain this adherence each year., Methods: 386 individuals who participated in two population-based studies (2021 and 2022) were considered. An interview was conducted to measure adherence to self-care practices, and case and contact tracing by the health authorities. The Wilcoxon signed-rank test was performed to measure change between 2021 and 2022, bivariate analysis, and a linear regression model for each year were performed., Results: Mask-wearing in public places was the most commonly used measure (95.9% in 2021, 89.9% in 2022). Follow-up of cases and cases contacts by the health authority had high coverage in 2021 (94.3% and 83% respectively). A greater decrease was observed in contact tracing in 2022 totaling 33.3%. An increase in the score of adherence to preventive practices was observed in 2022 (p < 0.00). The regression model showed in 2021 that women were more likely to adopt preventive behaviors (95% confidence interval: 0.27 to 1.13) and the overweight/obese had higher adherence compared to normal body mass index (95% confidence interval: 0.06 to 0.98). In 2022 being a young adult (30 to 49 years) predicted the adoption of behavioral precautions (95% confidence interval: 0.00 to 1.32)., Conclusions: Adherence to preventive measures increased even with high vaccination coverage, likely due to the epidemiological situation with the Omicron variant circulating in 2022., Competing Interests: The authors declare no conflict of interest. At the same time, the funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results., (This work is licensed under a Creative Commons Attribution 4.0 International License.)

Background: Chile has achieved the highest coverage for vaccines against the SARS-CoV-2 virus worldwide., Objective: To assess the progression of immunity (natural and acquired by vaccine) in a cohort from two Chilean cities., Methods: Individuals ( n = 386) who participated in three phases of population-based serial prevalence studies were included (2020-2021 and 2022). Presence of SARS-CoV-2 antibodies was measured in serum. Data including time of vaccination and type of vaccine received were analysed with descriptive statistics., Results: Seroprevalence was 3.6% in the first round and increased to 96.9% in the second and 98.7% in the third. In the third round, 75% of individuals who had received the basal full scheme were seropositive at 180 days or more since their last dose; 98% of individuals who received one booster dose were seropositive at 180 days or more, and 100% participants who received two boosters were seropositive, regardless of time since their last dose. Participants receiving mRNA vaccines had higher seroprevalence rates over time., Conclusions: The high vaccination coverage in Chile enabled the population to maintain high levels of antibodies. Vaccination boosters are essential to maintain immunity over time, which also depends on the type of vaccine administered.

Background: Seroprevalence studies provide an accurate measure of SARS-CoV-2 spread and the presence of asymptomatic cases. They also provide information on the uneven impact of the pandemic, pointing out vulnerable groups to prioritize which is particularly relevant in unequal societies. However, due to their high cost, they provide limited evidence of spatial spread of the pandemic specially in unequal societies. Our objective was to estimate the prevalence of SARS-CoV-2 antibodies in Chile and model its spatial risk distribution., Methods: During Oct-Nov 2020, we conducted a population-based serosurvey in Santiago, Talca, and Coquimbo-La Serena (2493 individuals). We explored the individual association between positive results and socio-economic and health-related variables by logistic regression for complex surveys. Then, using an Empirical Bayesian Kriging model, we estimated the infection risk spatial distribution using individual and census information, and compared these results with official records., Results: Seroprevalence was 10.4% (95% CI 7.8-13.7%), ranging from 2% (Talca) to 11% (Santiago), almost three times the number officially reported. Approximately 36% of these were asymptomatic, reaching 82% below 15 years old. Seroprevalence was associated with the city of residence, previous COVID-19 diagnosis, contact with confirmed cases (especially at household), and foreign nationality. The spatial model accurately interpolated the distribution of disease risk within the cities finding significant differences in the predicted probabilities of SARS-CoV-2 infection by census zone (IQR 2.5-15.0%), related to population density and education., Conclusions: Our results underscore the transmission heterogeneity of SARS-CoV-2 within and across three urban centers of Chile. Socio-economic factors and the outcomes of this seroprevalence study enable us to identify priority areas for intervention. Our methodological approach and results can help guide the design of interdisciplinary strategies for urban contexts, not only for SARS-CoV-2 but also for other communicable diseases., (© 2022. The Author(s).)

Chile is among the most successful nations worldwide in terms of its COVID-19 vaccine rollout. By 31 December 2021, 84.1% of the population was fully vaccinated, and 56.1% received booster doses using different COVID-19 vaccines. In this context, we aimed to estimate the prevalence of anti-SARS-CoV-2 antibodies following the infection and vaccination campaign. Using a three-stage stratified sampling, we performed a population-based cross-sectional serosurvey based on a representative sample of three Chilean cities. Selected participants were blood-sampled on-site and answered a short COVID-19 and vaccination history questionnaire using Wantai SARS-CoV-2 Ab ELISA to determine seroprevalence. We recruited 2198 individuals aged 7–93 between 5 October and 25 November 2021; 2132 individuals received COVID-19 vaccinations (97%), 67 (3.1%) received one dose, 2065 (93.9%) received two doses, and 936 received the booster jab (42.6%). Antibody seroprevalence reached 97.3%, ranging from 40.9% among those not vaccinated to 99.8% in those with booster doses (OR = 674.6, 154.8–2938.5). SARS-CoV-2 antibodies were associated with vaccination, previous COVID-19 diagnosis, age group, and city of residence. In contrast, we found no significant differences in the type of vaccine used, education, nationality, or type of health insurance. We found a seroprevalence close to 100%, primarily due to the successful vaccination program, which strongly emphasizes universal access. [ABSTRACT FROM AUTHOR]