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medrxiv; 2022.
Preprint in English | medRxiv | ID: ppzbmed-10.1101.2022.05.09.22274623


The COVID-19 vaccination campaign in Belgium aimed to reduce disease spread and severity. We quantified the observed vaccine effectiveness (VE) against symptomatic infection (VEi) and hospitalization (VEh). Exhaustive data on testing and vaccination was combined with a clinical hospital survey. We estimated VEi using a test negative design and VEh using a proportional hazard analysis. We controlled for prior infection, age, sex, province of residence and calendar week of sampling. Variant of concern specific VE-estimates were obtained by time since vaccination from July 2021 to April 2022. We included 1,433,135 persons. VEi against Delta waned from an initial estimate of 81% (95%CI 80-82) to 56% (95%CI 56-57) 100-150 days after primary-vaccination. Booster-vaccination increased initial VEi to 84% (95%CI 83-85). Against Omicron, an initial VEi of 37% (95%CI 34-40) waned to 18% (95%CI 17-20) 100-150 days after primary-vaccination. Booster-vaccination increased VEi to 52% (95%CI 51-53) and waned to 25% (95%CI 24-27) 100-150 days after vaccination. Hybrid immunity conferred by prior infection and booster-vaccination outperformed booster-vaccination only even if the infection was over one year ago, 67% (95%CI 66-68). Initial VEh for booster-vaccination decreased from 93% (95%CI 93-94) against Delta to 87% (95%CI 85-89) against Omicron. VEh for Omicron waned to 66% (95%CI 63-70) 100-150 days after booster-vaccination. In conclusion, we report significant immune-escape by Omicron. VEh was less affected than VEi and immune-escape was attenuated by booster-vaccination. Waning further reduced VEi- and VEh-estimates. Infection-acquired immunity offered additional protection against symptomatic infection in vaccinated persons which lasted at least one year.

COVID-19 , Infections
researchsquare; 2022.


Background: Contact tracing is one of the main public health tools in the control of coronavirus disease 2019 (COVID-19). A centralized contact tracing system was developed in Belgium in 2020. We aim to evaluate the performance and describe the results, between January 01, 2021, and September 30, 2021. The characteristics of COVID-19 cases and the impact of COVID-19 vaccination on testing and tracing are also described.Methods: We combined laboratory diagnostic test data (molecular and antigen test), vaccination data, and contact tracing data. A descriptive analysis was done to evaluate the performance of contact tracing and describe insights into the epidemiology of COVID-19 by contact tracing. Results: Between January and September 2021, 555.181 COVID-19 cases were reported to the central contact center and 91% were contacted. The average delay between symptom onset and contact tracing initiation was around 5 days, of which 4 days corresponded to pre-testing delay. High-Risk Contacts (HRC) were reported by 49% of the contacted index cases. The mean number of reported HRC was 2.7. In total, 666.869 HRC were reported of which 91% were successfully contacted and 89% of these were tested at least once following the interview. The estimated average secondary attack rate (SAR) among the contacts of the COVID-19 cases who reported at least one contact, was 27% and was significantly higher among household HRC. The proportion of COVID-19 cases who were previously identified as HRC within the central system was 24%.Conclusions:The contact-tracing system contacted more than 90% of the reported COVID-19 cases and their HRC. This proportion remained stable between January 1 2021 and September 30 2021 despite an increase in cases in March-April 2021. We report high SAR, indicating that through contact tracing a large number of infections were prospectively detected. The system can be further improved by (1) reducing the delay between onset of illness and medical consultation (2) having more exhaustive reporting of HRC by the COVID-19 case. 

medrxiv; 2022.
Preprint in English | medRxiv | ID: ppzbmed-10.1101.2022.01.28.22269756


The SARS-CoV-2 Omicron BA.1 variant is rapidly spreading worldwide, possibly outcompeting the Delta strain. We investigated the empirical serial interval for both variants using contact tracing data. Overall, we observed a shorter serial interval for Omicron compared to Delta, suggesting faster transmission. Furthermore, results indicate a relation between the empirical serial interval and the vaccination status for both the Omicron and the Delta variant. Consequently, with the progression of the vaccination campaign, the reasons for and extent of dominance of Omicron over Delta may need further assessment.