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In 2021, Qatar experienced considerable incidence of SARS-CoV-2 infection that was dominated sequentially by the Alpha, Beta, and Delta variants. Using the cycle threshold (Ct) value of an RT-qPCR-positive test to proxy the inverse of infectiousness, we investigated infectiousness of SARS-CoV-2 infections by variant, age, sex, vaccination status, prior infection status, and reason for testing in a random sample of 18,355 RT-qPCR-genotyped infections. Regression analyses were conducted to estimate associations with the Ct value of RT-qPCR-positive tests. Compared to Beta infections, Alpha and Delta infections demonstrated 2.56 higher Ct cycles (95% CI: 2.35-2.78), and 4.92 fewer cycles (95% CI: 4.67-5.16), respectively. The Ct value declined gradually with age and was especially high for children <10 years of age, signifying lower infectiousness of small children. Children <10 years of age had 2.18 higher Ct cycles (95% CI: 1.88-2.48) than those 10-19 years of age. Compared to unvaccinated individuals, the Ct value was higher among individuals who had received one or two vaccine doses, but the Ct value decreased gradually with time since the second-dose vaccination. Ct value was 2.07 cycles higher (95% CI: 1.42-2.72) for those with a prior infection than those without prior infection. The Ct value was lowest among individuals tested because of symptoms and was highest among individuals tested as a travel requirement. Delta was substantially more infectious than Beta. Prior immunity, whether due to vaccination or prior infection, is associated with lower infectiousness of breakthrough infections, but infectiousness increases gradually with time since the second-dose vaccination.
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ObjectiveTo investigate the performance of a rapid point-of-care antibody test, the BioMedomics COVID-19 IgM/IgG Rapid Test, in comparison with a high-quality, validated, laboratory-based platform, the Roche Elecsys Anti-SARS-CoV-2 assay. MethodsSerological testing was conducted on 708 individuals. Concordance metrics were estimated. Logistic regression was used to assess associations with seropositivity. ResultsSARS-CoV-2 seroprevalence was 63.4% (449/708; 95% CI 59.8%-66.9%) using the BioMedomics assay and 71.9% (509/708; 95% CI 68.5%-75.1%) using the Elecsys assay. There were 62 discordant results between the two assays. One specimen was seropositive in the BioMedomics assay, but seronegative in the Elecsys assay, while 61 specimens were seropositive in the Elecsys assay, but seronegative in the BioMedomics assay. Positive, negative, and overall percent agreements between the two assays were 88.0% (95% CI 84.9%-90.6%), 99.5% (95% CI 97.2%-99.9%), and 91.2% (95% CI 88.9%-93.1%), respectively, with a Cohens kappa of 0.80 (95% CI 0.77-0.83), indicating excellent agreement. Excluding specimens with lower antibody titers, the agreement improved with positive, negative, and overall percent concordance of 91.2% (95% CI 88.2%-93.6%), 99.5% (95% CI 97.2%-99.9%), and 93.9% (95% CI 91.7%-95.5%), respectively, and a Cohens kappa of 0.87 (95% CI 0.84-0.89). Logistic regression confirmed better agreement with higher antibody titers. ConclusionThe BioMedomics COVID-19 IgM/IgG Rapid Test demonstrated excellent performance in measuring detectable antibodies against SARS-CoV-2, supporting the utility of such rapid point-of-care serological testing to guide the public health responses and possible vaccine prioritization.
RESUMEN
BackgroundQatar experienced a large severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) epidemic that disproportionately affected the craft and manual workers (CMWs) who constitute 60% of the population. This study aimed to investigate level of immunity in communities within this population as well as infection exposure required to achieve herd immunity. MethodsAnti-SARS-CoV-2 seropositivity was assessed in ten CMW communities between June 21 and September 9, 2020. PCR positivity, infection positivity (antibody and/or PCR positive), and infection severity rate were also estimated. Associations with anti-SARS-CoV-2 positivity were investigated using regression analyses. ResultsStudy included 4,970 CMWs who were mostly men (95.0%) and <40 years of age (71.5%). Seropositivity ranged from 54.9% (95% CI: 50.2-59.4%) to 83.8% (95% CI: 79.1-87.7%) in the different CMW communities. Pooled mean seropositivity across all communities was 66.1% (95% CI: 61.5-70.6%). PCR positivity ranged from 0.0% to 10.5% (95% CI: 7.4-14.8%) in the different CMW communities. Pooled mean PCR positivity was 3.9% (95% CI: 1.6-6.9%). Median cycle threshold (Ct) value was 34.0 (range: 15.8-37.4)--majority (79.5%) of PCR-positive individuals had Ct value >30 indicative of earlier rather than recent infection. Infection positivity (antibody and/or PCR positive) ranged from 62.5% (95% CI: 58.3-66.7%) to 83.8% (95% CI: 79.1-87.7%) in the different CMW communities. Pooled mean infection positivity was 69.5% (95% CI: 62.8-75.9%). Only five infections were ever severe and one was ever critical--an infection severity rate of 0.2% (95% CI: 0.1-0.4%). ConclusionsBased on an extended range of epidemiological measures, active infection is rare in these communities with limited if any sustainable infection transmission for clusters to occur. At least some CMW communities in Qatar have reached or nearly reached herd immunity for SARS-CoV-2 infection at a proportion of ever infection of 65-70%.
