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Background: Scarce information is available on the duration of the protective effect of COVID-19 vaccination against the risk of SARS-CoV-2 infection and its severe clinical consequences. We investigated the effect of time since vaccine completion on the SARS-CoV-2 infection and its severe forms. Methods: In this retrospective observational analysis using the vaccination campaign integrated platform of the Italian region of Lombardy, 5 351 085 individuals aged 12 years or older who received complete vaccination from Jan 17 to July 31, 2021, were followed up from 14 days after vaccine completion until Oct 20, 2021. Changes over time in outcome rates (ie, SARS-CoV-2 infection and severe illness among vaccinated individuals) were analysed with age-period-cohort models. Trends in vaccine effectiveness (ie, outcomes comparison in vaccinated and unvaccinated individuals) were also measured. Findings: Overall, 14 140 infections and 2450 severe illnesses were documented, corresponding to incidence rates of 6·7 (95% CI 6·6–6·8) and 1·2 (1·1–1·2) cases per 10 000 person-months, respectively. From the first to the ninth month since vaccine completion, rates increased from 4·6 to 10·2 infections, and from 1·0 to 1·7 severe illnesses every 10 000 person-months. These figures correspond to relative reduction of vaccine effectiveness of 54·9% (95% CI 48·3–60·6) for infection and of 40·0% (16·2–57·0) for severe illness. The increasing infection rate was greater for individuals aged 60 years or older who received adenovirus-vectored vaccines (from 4·0 to 23·5 cases every 10 000 person-months). The increasing severe illness rates were similar for individuals receiving mRNA-based vaccines (from 1·1 to 1·5 every 10 000 person-months) and adenovirus-vectored vaccines (from 0·5 to 0·9 every 10 000 person-months). Interpretation: Although the risk of infection after vaccination, and even more of severe illness, remains low, the gradual increase in clinical outcomes related to SARS-CoV-2 infection suggests that the booster campaign should be accelerated and that social and individual protection measures against COVID-19 spread should not be abandoned. Funding: None.

Background: Using the knowledge gained during the first eleven months of the vaccine campaign in Lombardy, Italy, we provide an overview of the benefits of using reliable, complete, and rapidly available observational data to monitor the progress of the vaccine strategy. Methods: A population-based platform was implemented by linking four registries reporting individual data on: (i) date, type, and dose of vaccine dispensed; (ii) SARS-CoV-2 infections and hospital admissions and deaths due to COVID-19; (iii) inpatient diagnoses and outpatient services supplied by the Regional Health Services (RHS); and the (iv) health registry reporting and updating data on patient status. Background, methods, findings, and implications of eight COVID-19 relevant questions are reported. Results: Before starting the vaccine campaign, we identified high-risk individuals who need to be prioritized. During the vaccine campaign, we: (i) monitored the trend in the speed of the vaccine campaign progression and the number of prevented clinical outcomes; (ii) verified that available vaccines work in real-life, assessed their effectiveness-harm profile, and measured their reduced effectiveness against the delta variant. Finally, we studied the reduced effectiveness of the vaccine over time and identified risk factors of post-vaccine infection and severe illness. Conclusions: The correct use of rapidly available observational data of good quality and completeness generates reliable evidence to promptly inform patients and policymakers.