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PurposeIn a country-wide seroprevalence study of COVID-19 in Estonia we aimed to determine the seroprevalence and the dynamics of IgG against SARS-CoV-2 after vaccination or positive PCR-test. MethodsLeftover blood samples were selected between February 8 to March 25, 2021, by SYNLAB Estonia from all counties and age groups (0-9, 10-19, 20-59, 60-69, 70-79, 80-100 years) proportionally to the whole Estonian population and tested for IgG against SARS-CoV-2 spike protein receptor-binding domain (anti-S-RBD IgG) using Abbott SARS-CoV-2 IgG II Quant assay. Antibody levels after positive PCR-test or vaccination were described by nonlinear model. ResultsA total of 2517 samples were tested. Overall seroprevalence (95% CI) was 20.1% (18.5-21.7%), similar in all age groups. If all individuals vaccinated with the first dose at least 14 days before antibody measurement were assumed to be seronegative, the overall seroprevalence was 15.8% (14.4-17.3%), 4-fold larger than the proportion of confirmed COVID-19 cases. According to nonlinear models, age increased anti-S-RBD IgG production after positive PCR-test but decreased after vaccination. The peak of anti-S-RBD IgG in a 52-year-old (median age of PCR-positive and/or vaccinated individuals) was significantly higher after vaccination compared with positive PCR-test (22082 (12897...26875) vs 6732 (2321...8243) AU/mL), but half-life was similar (26.5 (6.9...46.1) vs 38.3 (8.2...68.5) days). ConclusionOne year after the start of COVID-19 pandemic the actual prevalence of infection is still underestimated compared with confirmed COVID-19 cases, underlining the importance of seroepidemiological studies. Older individuals have lower anti-S-RBD IgG level after vaccination, but similar decline rate to younger.
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BackgroundIn Estonia, during the first wave of COVID-19 total number of cases confirmed by PCR was 13.3/10,000, similar in most regions, including capital Tallinn, but in the hotspot of Estonian epidemic, an island Saaremaa, the cumulative incidence was 166.1/10,000. AimWe aimed to determine the prevalence of SARS-CoV-2 IgG antibodies in these two regions, symptoms associated with infection and factors associated with antibody concentrations. MethodsParticipants were selected using stratified (formed by age decades) random sampling and recruited by general practitioners. IgG were determined from sera by four assays. Symptoms of acute respiratory illness associated with seropositivity were analyzed by multiple correspondence analysis, antibody concentrations by multiple linear regression. ResultsTotal of 3608 individual were invited and 1960 recruited From May 8 to July 31, 2020. Seroprevalence was 1.5% (95% confidence interval (CI) 0.9-2.5) and 6.3% (95% CI 5.0-7.9), infection fatality rate 0.1% (95% CI 0.0-0.2) and 1.3% (95% CI 0.4-2.1) in Tallinn and Saaremaa, respectively. Of seropositive subjects 19.2% (14/73) had acute respiratory illness. Fever, diarrhea and the absence of cough and runny nose were associated with seropositivity in individuals aged 50 or more years. IgG concentrations were higher if fever, difficulty breathing, shortness of breath, chest pain or diarrhea was present, or hospitalization required. ConclusionSimilarly to other European countries the seroprevalence of SARS-CoV-2 in Estonia was low even in the hotspot region Saaremaa suggesting that majority of population is still susceptible to SARS-CoV-2. Focusing only on respiratory symptoms may delay accurate diagnosis of SARS-CoV-2 infection.
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BackgroundHigh number of SARS-CoV-2 antibody tests are available in different formats, detect different types of antibodies, and use different target proteins. Sensitivity of these tests varies and could be also related to clinical symptoms and testing time. MethodsSerum samples from 97 COVID-19 patients and 100 controls were tested with 9 antibody tests (SNIBE, Epitope, Euroimmun, Roche, Abbott, DiaSorin, Biosensor, LIPS N, and LIPS S-RBD). The results were analyzed in context of clinical data. FindingsPositivity rate was of tests was following: N-LIPS test (91.8% cases), Epitope (85.6%), Abbott and in-house LIPS S-RBD (both 84.5%), Roche (83.5%), Euroimmun (82.5%), DiaSorin (81.4%), SNIBE (70.1%), and Biosensor (64.9%). Agreement between tests varied (71-95%). Correlation between patient symptoms score and antibody value was test-dependent: varied from strongest in LIPS N ({rho}=0.41; p<0.001) to nonsignificant (LIPS S-RBD). Testing time from symptoms influenced sensitivity in some tests more than anothers. InterpretationSensitivity of tests varied highly and combination of different tests may improve it. Relation of results to symptoms and testing time was test-dependent. Thus, some antibody tests seems to be more sensitive to detect antibodies early and in asymptomatic patients than others. FundingStudy was funded by SYNLAB Estonia and Estonian Research Council grant PRG377. Research in contextO_ST_ABSEvidence before this studyC_ST_ABSHigh variation in COVID-19 antibody tests sensitivity has been described. Relation between antibody response and clinical cause has been found in some studies but not in others. Its known that antibody response is time dependent, however seroconversion medians varied in different studies. Added value of this studyWe confirmed that SARS-CoV-2 antibody response depends on clinical symptoms and time of testing, but we also found that this relation is dependent on test setup and viral antigens used in the tests. This may explain contradictory results of previous studies. Implications of all the available evidenceOur study has practical implications showing that not all antibody tests work uniformly well in symptomatic and asymptomatic cases and in different time periods from disease onset. This should be taken into consideration in clinical practice for diagnosing COVID-19 and in epidemiological studies evaluating the seroprevalence especially in asymptomatic population.
