Immunovirological and environmental screening reveals actionable risk factors for fatal COVID-19 during post-vaccination nursing home outbreaks.

Coronavirus Disease 2019 (COVID-19) vaccination has resulted in excellent protection against fatal disease, including in older adults. However, risk factors for post-vaccination fatal COVID-19 are largely unknown. We comprehensively studied three large nursing home outbreaks (20-35% fatal cases among residents) by combining severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) aerosol monitoring, whole-genome phylogenetic analysis and immunovirological profiling of nasal mucosa by digital nCounter transcriptomics. Phylogenetic investigations indicated that each outbreak stemmed from a single introduction event, although with different variants (Delta, Gamma and Mu). SARS-CoV-2 was detected in aerosol samples up to 52 d after the initial infection. Combining demographic, immune and viral parameters, the best predictive models for mortality comprised IFNB1 or age, viral ORF7a and ACE2 receptor transcripts. Comparison with published pre-vaccine fatal COVID-19 transcriptomic and genomic signatures uncovered a unique IRF3 low/IRF7 high immune signature in post-vaccine fatal COVID-19 outbreaks. A multi-layered strategy, including environmental sampling, immunomonitoring and early antiviral therapy, should be considered to prevent post-vaccination COVID-19 mortality in nursing homes.

Assessment of the clinical and analytical performance of the Aptima SARS-CoV-2 assay using the VALCOR protocol.

BACKGROUND: The COVID-19 pandemic highlighted the importance of diagnostic testing against curbing the spread of SARS-CoV-2. The urgent need and scale for diagnostic tools resulted in manufacturers of SARS-CoV-2 assays receiving emergency authorization that lacked robust analytical or clinical evaluation. As it is highly likely that testing for SARS-CoV-2 will continue to play a central role in public health, the performance characteristics of assays should be evaluated to ensure reliable diagnostic outcomes are achieved. METHODS: VALCOR or "VALidation of SARS-CORona Virus-2 assays" is a study protocol designed to set up a framework for test validation of SARS-CoV-2 virus assays. Using clinical samples collated from VALCOR, the performance of Aptima SARS-CoV-2 assay was assessed against a standard comparator assay. Diagnostic test parameters such as sensitivity, specificity and overall per cent agreement were calculated for the clinical performance of Aptima SARS-CoV-2 assay. RESULTS: A total of 180 clinical samples were tested with an addition of 40 diluted clinical specimens to determine the limit of detection. When compared to the standard comparator assay Aptima had a sensitivity of 100.0% [95% CI 95.9-100.0] and specificity of 96.7% [95% CI 90.8-99.3]. The overall percent agreement was 98.3% with an excellent Cohen's coefficient of κ = 0.967 [95% CI 0.929-1.000]. For the limit of detection, Aptima was able to detect all of the diluted clinical samples. CONCLUSION: In conclusion. validation of Aptima SARS-CoV-2 assay using clinical samples collated through the VALCOR protocol showed excellent test performance. Additionally, Aptima demonstrated high analytical sensitivity by detecting all diluted clinical samples corresponding to a low limit of detection.

Homologous and Heterologous Prime-Boost Vaccination: Impact on Clinical Severity of SARS-CoV-2 Omicron Infection among Hospitalized COVID-19 Patients in Belgium.

We investigated effectiveness of (1) mRNA booster vaccination versus primary vaccination only and (2) heterologous (viral vector-mRNA) versus homologous (mRNA-mRNA) prime-boost vaccination against severe outcomes of BA.1, BA.2, BA.4 or BA.5 Omicron infection (confirmed by whole genome sequencing) among hospitalized COVID-19 patients using observational data from national COVID-19 registries. In addition, it was investigated whether the difference between the heterologous and homologous prime-boost vaccination was homogenous across Omicron sub-lineages. Regression standardization (parametric g-formula) was used to estimate counterfactual risks for severe COVID-19 (combination of severity indicators), intensive care unit (ICU) admission, and in-hospital mortality under exposure to different vaccination schedules. The estimated risk for severe COVID-19 and in-hospital mortality was significantly lower with an mRNA booster vaccination as compared to only a primary vaccination schedule (RR = 0.59 [0.33; 0.85] and RR = 0.47 [0.15; 0.79], respectively). No significance difference was observed in the estimated risk for severe COVID-19, ICU admission and in-hospital mortality with a heterologous compared to a homologous prime-boost vaccination schedule, and this difference was not significantly modified by the Omicron sub-lineage. Our results support evidence that mRNA booster vaccination reduced the risk of severe COVID-19 disease during the Omicron-predominant period.

Longitudinal analysis of serum neutralization of SARS-CoV-2 Omicron BA.2, BA.4, and BA.5 in patients receiving monoclonal antibodies.

The emergence of Omicron sublineages impacts the therapeutic efficacy of anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) monoclonal antibodies (mAbs). Here, we evaluate neutralization and antibody-dependent cellular cytotoxicity (ADCC) activities of 6 therapeutic mAbs against Delta, BA.2, BA.4, and BA.5. The Omicron subvariants escape most antibodies but remain sensitive to bebtelovimab and cilgavimab. Consistent with their shared spike sequence, BA.4 and BA.5 display identical neutralization profiles. Sotrovimab is the most efficient at eliciting ADCC. We also analyze 121 sera from 40 immunocompromised individuals up to 6 months after infusion of Ronapreve (imdevimab + casirivimab) or Evusheld (cilgavimab + tixagevimab). Sera from Ronapreve-treated individuals do not neutralize Omicron subvariants. Evusheld-treated individuals neutralize BA.2 and BA.5, but titers are reduced. A longitudinal evaluation of sera from Evusheld-treated patients reveals a slow decay of mAb levels and neutralization, which is faster against BA.5. Our data shed light on antiviral activities of therapeutic mAbs and the duration of effectiveness of Evusheld pre-exposure prophylaxis.

Severity of COVID-19 among Hospitalized Patients: Omicron Remains a Severe Threat for Immunocompromised Hosts.

The Omicron variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged in the general population in the context of a relatively high immunity gained through the early waves of coronavirus disease 19 (COVID-19), and vaccination campaigns. Despite this context, a significant number of patients were hospitalized, and identifying the risk factors associated with severe disease in the Omicron era is critical for targeting further preventive, and curative interventions. We retrospectively analyzed the individual medical records of 1501 SARS-CoV-2 positive hospitalized patients between 13 December 2021, and 13 February 2022, in Belgium, of which 187 (12.5%) were infected with Delta, and 1036 (69.0%) with Omicron. Unvaccinated adults showed an increased risk of moderate/severe/critical/fatal COVID-19 (crude OR 1.54; 95% CI 1.09-2.16) compared to vaccinated patients, whether infected with Omicron or Delta. In adults infected with Omicron and moderate/severe/critical/fatal COVID-19 (n = 323), immunocompromised patients showed an increased risk of in-hospital mortality related to COVID-19 (adjusted OR 2.42; 95% CI 1.39-4.22), compared to non-immunocompromised patients. The upcoming impact of the pandemic will be defined by evolving viral variants, and the immune system status of the population. The observations support that, in the context of an intrinsically less virulent variant, vaccination and underlying patient immunity remain the main drivers of severe disease.

Two Years of Genomic Surveillance in Belgium during the SARS-CoV-2 Pandemic to Attain Country-Wide Coverage and Monitor the Introduction and Spread of Emerging Variants.

An adequate SARS-CoV-2 genomic surveillance strategy has proven to be essential for countries to obtain a thorough understanding of the variants and lineages being imported and successfully established within their borders. During 2020, genomic surveillance in Belgium was not structurally implemented but performed by individual research laboratories that had to acquire the necessary funds themselves to perform this important task. At the start of 2021, a nationwide genomic surveillance consortium was established in Belgium to markedly increase the country's genomic sequencing efforts (both in terms of intensity and representativeness), to perform quality control among participating laboratories, and to enable coordination and collaboration of research projects and publications. We here discuss the genomic surveillance efforts in Belgium before and after the establishment of its genomic sequencing consortium, provide an overview of the specifics of the consortium, and explore more details regarding the scientific studies that have been published as a result of the increased number of Belgian SARS-CoV-2 genomes that have become available.

The role of airborne transmission in a large single source outbreak of SARS-CoV-2 in a Belgian nursing home in 2020.

OBJECTIVES: To better understand the conditions which have led to one of the largest COVID-19 outbreaks in Belgian nursing homes in 2020. SETTING: A nursing home in Flanders, Belgium, which experienced a massive outbreak of COVID-19 after a cultural event. An external volunteer who dressed as a legendary figure visited consecutively the 4 living units on December, 4th and tested positive for SARS-CoV-2 the next day. Within days, residents started to display symptoms and the outbreak spread rapidly within the nursing home. METHODS: We interviewed key informants and collected standardized data from all residents retrospectively. A batch of 115 positive samples with a Ct value of < 37 by qRT-PCR were analyzed using whole-genome sequencing. Six months after the outbreak, ventilation assessment of gathering rooms in the nursing home was done using a tracer gas test with calibrated CO2 sensors. RESULTS: Timeline of diagnoses and symptom onsets clearly pointed to the cultural event as the start of the outbreak, with the volunteer as index case. The genotyping of positive samples depicted the presence of one large cluster, suggesting a single source outbreak. By the end of December, a total of 127 residents and 40 staff were diagnosed with SARS-CoV-2 since the beginning of the outbreak. The attack rate among residents was 77 % and significantly associated with presence at the event but not with close contact or mask wearing. The ventilation assessment showed a high background average CO2 level in four main rooms varying from 657 ppm to 846 ppm. CONCLUSIONS: Our investigation shows a rapid and widespread single source outbreak of SARS-CoV-2 in a nursing home, in which airborne transmission was the most plausible explanation for the massive intra-facility spread. Our results underscore the importance of ventilation and air quality for the prevention of future outbreaks in closed facilities.

Clinical Severity of SARS-CoV-2 Omicron Variant Compared with Delta among Hospitalized COVID-19 Patients in Belgium during Autumn and Winter Season 2021-2022.

This retrospective multi-center matched cohort study assessed the risk for severe COVID-19 (combination of severity indicators), intensive care unit (ICU) admission, and in-hospital mortality in hospitalized patients when infected with the Omicron variant compared to when infected with the Delta variant. The study is based on a causal framework using individually-linked data from national COVID-19 registries. The study population consisted of 954 COVID-19 patients (of which, 445 were infected with Omicron) above 18 years old admitted to a Belgian hospital during the autumn and winter season 2021-2022, and with available viral genomic data. Patients were matched based on the hospital, whereas other possible confounders (demographics, comorbidities, vaccination status, socio-economic status, and ICU occupancy) were adjusted for by using a multivariable logistic regression analysis. The estimated standardized risk for severe COVID-19 and ICU admission in hospitalized patients was significantly lower (RR = 0.63; 95% CI (0.30; 0.97) and RR = 0.56; 95% CI (0.14; 0.99), respectively) when infected with the Omicron variant, whereas in-hospital mortality was not significantly different according to the SARS-CoV-2 variant (RR = 0.78, 95% CI (0.28-1.29)). This study demonstrates the added value of integrated genomic and clinical surveillance to recognize the multifactorial nature of COVID-19 pathogenesis.

Nationwide Harmonization Effort for Semi-Quantitative Reporting of SARS-CoV-2 PCR Test Results in Belgium.

From early 2020, a high demand for SARS-CoV-2 tests was driven by several testing indications, including asymptomatic cases, resulting in the massive roll-out of PCR assays to combat the pandemic. Considering the dynamic of viral shedding during the course of infection, the demand to report cycle threshold (Ct) values rapidly emerged. As Ct values can be affected by a number of factors, we considered that harmonization of semi-quantitative PCR results across laboratories would avoid potential divergent interpretations, particularly in the absence of clinical or serological information. A proposal to harmonize reporting of test results was drafted by the National Reference Centre (NRC) UZ/KU Leuven, distinguishing four categories of positivity based on RNA copies/mL. Pre-quantified control material was shipped to 124 laboratories with instructions to setup a standard curve to define thresholds per assay. For each assay, the mean Ct value and corresponding standard deviation was calculated per target gene, for the three concentrations (107, 105 and 103 copies/mL) that determine the classification. The results of 17 assays are summarized. This harmonization effort allowed to ensure that all Belgian laboratories would report positive PCR results in the same semi-quantitative manner to clinicians and to the national database which feeds contact tracing interventions.

Early SARS-CoV-2 Reinfections within 60 Days and Implications for Retesting Policies.

Illustrated by a clinical case supplemented by epidemiologic data, early reinfections with SARS-CoV-2 Omicron BA.1 after infection with Delta variant, and reinfection with Omicron BA.2 after Omicron BA.1 infection, can occur within 60 days, especially in young, unvaccinated persons. The case definition of reinfection, which influences retesting policies, should be reconsidered.

Severity of infection with the SARS-CoV-2 B.1.1.7 lineage among hospitalized COVID-19 patients in Belgium.

INTRODUCTION: The pathogenesis of COVID-19 depends on the interplay between host characteristics, viral characteristics and contextual factors. Here, we compare COVID-19 disease severity between hospitalized patients in Belgium infected with the SARS-CoV-2 variant B.1.1.7 and those infected with previously circulating strains. METHODS: The study is conducted within a causal framework to study the severity of SARS-CoV-2 variants by merging surveillance registries in Belgium. Infection with SARS-CoV-2 B.1.1.7 ('exposed') was compared to infection with previously circulating strains ('unexposed') in terms of the manifestation of severe COVID-19, intensive care unit (ICU) admission, or in-hospital mortality. The exposed and unexposed group were matched based on the hospital and the mean ICU occupancy rate during the patient's hospital stay. Other variables identified as confounders in a Directed Acyclic Graph (DAG) were adjusted for using regression analysis. Sensitivity analyses were performed to assess the influence of selection bias, vaccination rollout, and unmeasured confounding. RESULTS: We observed no difference between the exposed and unexposed group in severe COVID-19 disease or in-hospital mortality (RR = 1.15, 95% CI [0.93-1.38] and RR = 0.92, 95% CI [0.62-1.23], respectively). The estimated standardized risk to be admitted in ICU was significantly higher (RR = 1.36, 95% CI [1.03-1.68]) when infected with the B.1.1.7 variant. An age-stratified analysis showed that among the younger age group (≤65 years), the SARS-CoV-2 variant B.1.1.7 was significantly associated with both severe COVID-19 progression and ICU admission. CONCLUSION: This matched observational cohort study did not find an overall increased risk of severe COVID-19 or death associated with B.1.1.7 infection among patients already hospitalized. There was a significant increased risk to be transferred to ICU when infected with the B.1.1.7 variant, especially among the younger age group. However, potential selection biases advocate for more systematic sequencing of samples from hospitalized COVID-19 patients.

Two Separate Clusters of SARS-CoV-2 Delta Variant Infections in a Group of 41 Students Travelling from India: An Illustration of the Need for Rigorous Testing and Quarantine.

We report two clusters of SARS-CoV-2 B.1.617.2 (Delta variant) infections in a group of 41 Indian nursing students who travelled from New Delhi, India, to Belgium via Paris, France. All students tested negative before departure and had a second negative antigen test upon arrival in Paris. Upon arrival in Belgium, the students were quarantined in eight different houses. Four houses remained COVID-free during the 24 days of follow-up, while all 27 residents of the other four houses developed an infection during quarantine, including the four residents who were fully vaccinated and the two residents who were partially vaccinated. Genome sequencing revealed two distinct clusters affecting one and three houses, respectively. In this group of students, vaccination status did not seem to prevent infection nor decrease the viral load. No severe symptoms were reported. Extensive contact tracing and 3 months of nationwide genomic surveillance confirmed that these outbreaks were successfully contained and did not contribute to secondary community transmission in Belgium. These clusters highlight the importance of repeated testing and quarantine measures among travelers coming from countries experiencing a surge of infections, as all infections were detected 6 days or more after arrival.

Genomic evidence of co-identification with Omicron and Delta SARS-CoV-2 variants: a report of two cases.

On November 24, 2021, a new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant assigned to the lineage B.1.1.529 (Omicron) was first reported to the World Health Organization from South Africa. Despite the co-circulation of several SARS-CoV-2 variants, co-infection by different variants is not commonly identified. Here, we report two cases of SARS-CoV-2 co-identifications with the Omicron and Delta variants.

VALCOR: a protocol for the validation of SARS-corona virus-2 assays.

BACKGROUND: Testing for SARS-CoV-2, together with vaccination, is one of the most vital strategies in curbing the current COVID-19 pandemic. The pandemic has led to an unprecedented need for diagnostic testing and the rapid emergence of an abundance of commercial assays on the market. Due to the nature of the pandemic and in the interest of health protection, many of these assays received provisional authorisation for emergency use without thorough validation. To limit false negative and false positive results, it is key to define common criteria that SARS-CoV-2 assays need to fulfil. VALCOR or "VALidation of SARS-CORona Virus-2 assays" is a protocol designed to set up a framework for test validation of SARS-CoV-2 virus assays. OBJECTIVES: VALCOR is a study protocol for the validation of assays used for confirmation of the presence of SARS-CoV-2 in patients with COVID-19 disease or the screening of carriers of SARS-CoV-2 virus by the identification of viral RNA in oropharyngeal and/or nasopharyngeal specimens or other specimens from the human respiratory tract. METHODS: The VALCOR panel of samples will contain clinical human specimens and standardised artificial specimens. The collection of clinical specimens will include nasopharyngeal or oropharyngeal specimens or other specimens from the respiratory tract obtained from COVID-19 patients and healthy carriers of SARS-CoV-2 as well as specimens from subjects not carrying SARS-CoV-2. Artificial specimens include calibrated amounts of viral RNA of SARS-CoV-2 sequences provided by established competent agencies that produce reference materials for the assessment of the limit of detection of each assay. The panel of samples are sent from a central reference laboratory (having access to biobanks of clinical specimens tested already for SARS-CoV-2 with a reference comparator assay) to participating laboratories for testing with a SARS-CoV-2 index assay that requires evaluation. DISCUSSION: VALCOR provides a harmonised and standard framework to benchmark the testing performance of SARS-CoV-2 assays that are rapidly evolving. As the pandemic incited an urgent need for testing capacity, there is a gap in the comprehensive validation of SARS-CoV-2 assays. This study will generate comprehensive validation data for assays used for the diagnosis of SARS-CoV-2 and may serve as a basis for other validation protocols.

Serum neutralization of SARS-CoV-2 Omicron sublineages BA.1 and BA.2 in patients receiving monoclonal antibodies.

The severe acute respiratory syndrome coronavirus 2 Omicron BA.1 sublineage has been supplanted in many countries by the BA.2 sublineage. BA.2 differs from BA.1 by about 21 mutations in its spike. In this study, we first compared the sensitivity of BA.1 and BA.2 to neutralization by nine therapeutic monoclonal antibodies (mAbs). In contrast to BA.1, BA.2 was sensitive to cilgavimab, partly inhibited by imdevimab and resistant to adintrevimab and sotrovimab. We then analyzed sera from 29 immunocompromised individuals up to 1 month after administration of Ronapreve (casirivimab and imdevimab) and/or Evusheld (cilgavimab and tixagevimab) antibody cocktails. All treated individuals displayed elevated antibody levels in their sera, which efficiently neutralized the Delta variant. Sera from Ronapreve recipients did not neutralize BA.1 and weakly inhibited BA.2. Neutralization of BA.1 and BA.2 was detected in 19 and 29 out of 29 Evusheld recipients, respectively. As compared to the Delta variant, neutralizing titers were more markedly decreased against BA.1 (344-fold) than BA.2 (nine-fold). We further report four breakthrough Omicron infections among the 29 individuals, indicating that antibody treatment did not fully prevent infection. Collectively, BA.1 and BA.2 exhibit noticeable differences in their sensitivity to therapeutic mAbs. Anti-Omicron neutralizing activity of Ronapreve and, to a lesser extent, that of Evusheld is reduced in patients' sera.

Evaluation of the SARS-CoV-2 positivity ratio and upper respiratory tract viral load among asymptomatic individuals screened before hospitalization or surgery in Flanders, Belgium.

INTRODUCTION: The incidence of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infections in the Belgian community is mainly estimated based on test results of patients with coronavirus disease (COVID-19)-like symptoms. The aim of this study was to investigate the evolution of the SARS-CoV-2 reverse transcriptase polymerase chain reaction (RT-PCR) positivity ratio and distribution of viral loads within a cohort of asymptomatic patients screened prior hospitalization or surgery, stratified by age category. MATERIALS/METHODS: We retrospectively studied data on SARS-CoV-2 real-time RT-PCR detection in respiratory tract samples of asymptomatic patients screened pre-hospitalization or pre-surgery in nine Belgian hospitals located in Flanders over a 12-month period (1 April 2020-31 March 2021). RESULTS: In total, 255925 SARS-CoV-2 RT-PCR test results and 2421 positive results for which a viral load was reported, were included in this study. An unweighted overall SARS-CoV-2 real-time RT-PCR positivity ratio of 1.27% was observed with strong spatiotemporal differences. SARS-CoV-2 circulated predominantly in 80+ year old individuals across all time periods except between the first and second COVID-19 wave and in 20-30 year old individuals before the second COVID-19 wave. In contrast to the first wave, a significantly higher positivity ratio was observed for the 20-40 age group in addition to the 80+ age group compared to the other age groups during the second wave. The median viral load follows a similar temporal evolution as the positivity rate with an increase ahead of the second wave and highest viral loads observed for 80+ year old individuals. CONCLUSION: There was a high SARS-CoV-2 circulation among asymptomatic patients with a predominance and highest viral loads observed in the elderly. Moreover, ahead of the second COVID-19 wave an increase in median viral load was noted with the highest overall positivity ratio observed in 20-30 year old individuals, indicating they could have been the hidden drivers of this wave.

Infant Pneumococcal Carriage in Belgium Not Affected by COVID-19 Containment Measures.

Streptococcus pneumoniae is an important and frequently carried respiratory pathogen that has the potential to cause serious invasive diseases, such as pneumonia, meningitis, and sepsis. Young children and older adults are among the most vulnerable to developing serious disease. With the arrival of the COVID-19 pandemic and the concomitant restrictive measures, invasive disease cases caused by respiratory bacterial species, including pneumococci, decreased substantially. Notably, the stringency of the containment measures as well as the visible reduction in the movement of people appeared to coincide with the drop in invasive disease cases. One could argue that wearing protective masks and adhering to social distancing guidelines to halt the spread of the SARS-CoV-2 virus, also led to a reduction in the person-to-person transmission of respiratory bacterial species. Although plausible, this conjecture is challenged by novel data obtained from our nasopharyngeal carriage study which is performed yearly in healthy daycare center attending children. A sustained and high pneumococcal carriage rate was observed amid periods of stringent restrictive measures. This finding prompts us to revisit the connection between nasopharyngeal colonization and invasion and invites us to look closer at the nasopharyngeal microbiome as a whole.

Identification of the First SARS-CoV-2 Lineage B.1.1.529 Virus Detected in Europe.

We report the complete genome sequence of a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant (lineage B.1.1.529) from a Belgian patient with a history of recent travel to Egypt. At the time of writing, this genome constituted the first confirmed case of an infection with the Omicron variant in Europe.

Considerable escape of SARS-CoV-2 Omicron to antibody neutralization.

The SARS-CoV-2 Omicron variant was first identified in November 2021 in Botswana and South Africa1-3. It has since spread to many countries and is expected to rapidly become dominant worldwide. The lineage is characterized by the presence of around 32 mutations in spike-located mostly in the N-terminal domain and the receptor-binding domain-that may enhance viral fitness and enable antibody evasion. Here we isolated an infectious Omicron virus in Belgium from a traveller returning from Egypt. We examined its sensitivity to nine monoclonal antibodies that have been clinically approved or are in development4, and to antibodies present in 115 serum samples from COVID-19 vaccine recipients or individuals who have recovered from COVID-19. Omicron was completely or partially resistant to neutralization by all monoclonal antibodies tested. Sera from recipients of the Pfizer or AstraZeneca vaccine, sampled five months after complete vaccination, barely inhibited Omicron. Sera from COVID-19-convalescent patients collected 6 or 12 months after symptoms displayed low or no neutralizing activity against Omicron. Administration of a booster Pfizer dose as well as vaccination of previously infected individuals generated an anti-Omicron neutralizing response, with titres 6-fold to 23-fold lower against Omicron compared with those against Delta. Thus, Omicron escapes most therapeutic monoclonal antibodies and, to a large extent, vaccine-elicited antibodies. However, Omicron is neutralized by antibodies generated by a booster vaccine dose.

One year of laboratory-based COVID-19 surveillance system in Belgium: main indicators and performance of the laboratories (March 2020-21).

BACKGROUND: With the spread of coronavirus disease 2019 (COVID-19), an existing national laboratory-based surveillance system was adapted to daily monitor the epidemiological situation of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the Belgium by following the number of confirmed SARS-CoV-2 infections, the number of performed tests and the positivity ratio. We present these main indicators of the surveillance over a one-year period as well as the impact of the performance of the laboratories, regarding speed of processing the samples and reporting results, for surveillance. METHODS: We describe the evolution of test capacity, testing strategy and the data collection methods during the first year of the epidemic in Belgium. RESULTS: Between the 1st of March 2020 and the 28th of February 2021, 9,487,470 tests and 773,078 COVID-19 laboratory confirmed cases were reported. Two epidemic waves occurred, with a peak in April and October 2020. The capacity and performance of the laboratories improved continuously during 2020 resulting in a high level performance. Since the end of November 2020 90 to 95% of the test results are reported at the latest the day after sampling was performed. CONCLUSIONS: Thanks to the effort of all laboratories a performant exhaustive national laboratory-based surveillance system to monitor the epidemiological situation of SARS-CoV-2 was set up in Belgium in 2020. On top of expanding the number of laboratories performing diagnostics and significantly increasing the test capacity in Belgium, turnaround times between sampling and testing as well as reporting were optimized over the first year of this pandemic.