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We investigate differences in protection from previous infection and/or vaccination against infection with Omicron BA.4/5 or BA.2. We observed a higher percentage of registered previous SARS-CoV-2 infections among 19836 persons infected with Omicron BA.4/5 compared to 7052 persons infected with BA.2 (31.3% vs. 20.0%) between 2 May and 24 July 2022 (adjusted odds ratio (aOR) for testing week, age group and sex: 1.4 (95%CI: 1.3-1.5)). No difference was observed in the distribution of vaccination status between BA.2 and BA.4/5 cases (aOR: 1.1 for primary and booster vaccination). Among reinfections, those newly infected with BA4/5 had a shorter interval between infections and the previous infection was more often caused by BA.1, compared to those newly infected with BA.2 (aOR: 1.9 (1.5-2.6). This suggests immunity induced by BA.1 is less effective against a BA.4/5 infection than against a BA.2 infection.
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BackgroundWe aimed to estimate vaccine effectiveness (VE) against COVID-19 mortality, and to explore whether an increased risk in non-COVID-19 mortality exists in the weeks following a COVID- 19 vaccine dose. MethodsNational registries of causes of death, COVID-19 vaccination and long-term care reimbursements were linked by a unique identifier using data from 1 January 2021 to 31 January 2022. We used Cox regression with calendar time as underlying time scale to, firstly, estimate VE against COVID-19 mortality after primary and first booster vaccination, per month since vaccination and, secondly, estimate risk of non-COVID-19 mortality in the 8 weeks following a first, second or booster dose, adjusting for birth year, sex and country of origin. ResultsVE against COVID-19 mortality was >90% for all age groups two months after completion of the primary series. VE gradually decreased thereafter, to around 80% at 7-8 months post-primary series for most groups, and around 60% for elderly receiving a high level of long-term care and for people aged 90+ years. The risk of non-COVID-19 mortality was lower or similar in the 8 weeks following a first booster dose compared to no vaccination, first or second dose, respectively, for all age and long-term care groups. ConclusionCOVID-19 vaccination greatly reduced the risk of COVID-19 mortality and no increased risk of death from other causes was seen at the population level.
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ObjectivesTo assess the performances of three commonly used rapid antigen diagnostic tests (Ag-RDTs) used as self-tests in asymptomatic individuals in the Omicron period. DesignCross-sectional diagnostic test accuracy study. SettingThree public health service COVID-19 test sites in the Netherlands. Participants3,600 asymptomatic individuals aged [≥]16 years presenting for SARS-CoV-2 testing for any reason except confirmatory testing after a positive self-test. InterventionsParticipants were sampled for RT-PCR (reference test) and received one self-test (either Acon Flowflex (Flowflex), MP Biomedicals (MPBio), or Siemens-Healthineers Clinitest (Clinitest)) to perform unsupervised at home within three hours and blinded to the RT-PCR result. Main Outcome(s) and Measures(s)Diagnostic accuracies (sensitivity, specificity, positive and negative predictive values) of each self-test compared to RT-PCR. ResultsOverall sensitivities of the three self-tests were 27.5% (95% CI: 21.3-34.3%) for Flowflex, 20.9% (13.9-29.4%) for MPBio, and 25.6% (19.1-33.1%) for Clinitest. After applying a viral load cut-off ([≥]5.2 log10 SARS-CoV-2 E-gene copies/mL), sensitivities increased to 48.3% (95% CI: 37.6-59.2%), 37.8% (22.5-55.2%), and 40.0% (29.5-51.2%), respectively. No consistent differences were found in sensitivities by COVID-19 vaccination status, having had a prior SARS-CoV-2 infection, gender or age across the three self-tests. Specificities were >99% for all tests in most analyses. ConclusionsThe sensitivities of three commonly used SARS-CoV-2 Ag-RDTs when used as self-tests in asymptomatic individuals in the Omicron period, were very low. Our findings indicate that Ag-RDT self-testing in asymptomatic individuals may only detect the minority of infections at that point in time and may not be sufficient to prevent the spreading of the virus to other (vulnerable) persons. Repeated self-testing in case of a negative self-test is advocated to improve the diagnostic yield of the self-tests, and individuals should certainly be advised to re-test when symptoms develop. Summary boxO_ST_ABSWhat is already known on this topicC_ST_ABSO_LIIf sufficiently reliable, SARS-CoV-2 self-testing by asymptomatic persons prior to admission in places where groups gather could have a huge public health impact by lowering the reproduction number or keep it below one for longer periods. C_LIO_LICurrent evidence suggests that SARS-CoV-2 rapid antigen diagnostic tests (Ag-RDTs) when used as self-tests by asymptomatic individuals perform suboptimal, but sample sizes of the previous studies were too small to draw robust conclusions, and also empirical data on the accuracy of Ag-RDT self-tests in asymptomatic individuals during the Omicron period are scarce. C_LI What this study addsO_LICompared to RT-PCR testing, overall sensitivities of three commercially available SARS-CoV-2 Ag-RDTs when used as self-tests by asymptomatic individuals (primary analysis population of non-confirmatory testers; n= 3600, 87% of full analysis population) in the Omicron period, were very low: 27.5% (95% CI: 21.3-34.3%) for the Acon Flowflex test, 20.9% (13.9-29.4%) for the MP Biomedicals test, and 25.6% (19.1-33.1%) for the Siemens Healthineers Clinitest Ag-RDT, which increased to 48.3% (95% CI: 37.6-59.2%), 37.8% (22.5-55.2%), and 40.0% (29.5-51.2%), respectively, when applying a viral load cut-off ([≥]5.2 log10 SARS-CoV-2 E-gene copies/mL). C_LIO_LIOur findings indicate that Ag-RDT self-testing in asymptomatic individuals may only detect the minority of infections at that point in time and may not be sufficient to prevent the spreading of the virus to other (vulnerable) persons. Repeated self-testing in case of a negative self-test is advocated to improve the diagnostic yield of the self-tests, and individuals should certainly be advised to re-test when symptoms develop. C_LI
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BackgroundPerformances of rapid antigen diagnostic tests (Ag-RDTs) with nasal self-sampling, and oropharyngeal plus nasal (OP-N) self-sampling, in the Omicron period are unknown. MethodsProspective diagnostic accuracy study among 6,497 symptomatic individuals aged >16 years presenting for SARS-CoV-2 testing at three test-sites. Participants were sampled for RT-PCR (reference test) and received one Ag-RDT to perform unsupervised with either nasal self-sampling (during the emergence of Omicron, and after Omicron share was >90%, phase-1) or with OP-N self-sampling (in a subsequent phase-2; Omicron share >99%). The evaluated tests were Acon Flowflex (Flowflex; phase-1 only), MP Biomedicals (MPBio), and Siemens-Healthineers Clinitest (Clinitest). FindingsDuring phase-1, 45% of Flowflex, 29% of MPBio, and 35% of Clinitest participants were confirmatory testers (previously tested positive by a self-test at own initiative). Overall sensitivities with nasal self-sampling were 79.0% (95% CI: 74.7-82.8%) for Flowflex, 69.9% (65.1-74.4%) for MPBio, and 70.2% (65.6-74.5%) for Clinitest. Sensitivities were substantially higher in confirmatory testers (93.6%, 83.6%, and 85.7%, respectively) than in those who tested for other reasons (52.4%, 51.5%, and 49.5%, respectively). Sensitivities decreased by 6.1 (p=0.16 by Chi-square test), 7.0 (p=0.60), and 12.8 (p=0.025) percentage points, respectively, when transitioning from 29% to >95% Omicron. During phase-2, 53% of MPBio, and 44% of Clinitest participants were confirmatory testers. Overall sensitivities with OP-N self-sampling were 83.0% (78.8%-86.7%) for MPBio and 77.3% (72.9%-81.2%) for Clinitest. Comparing OP-N to nasal sampling, sensitivities were slightly higher in confirmatory testers (87.4% and 86.1%, respectively), and substantially higher in those testing for other reasons (69.3% and 59.9%, respectively). InterpretatioSensitivities of three Ag-RDTs with nasal self-sampling decreased during Omicron emergence but was only statistically significant for Clinitest. Sensitivities were substantially influenced by the proportion of confirmatory testers. Addition of oropharyngeal to nasal self-sampling improved sensitivities of MPBio and Clinitest. FundingDutch Ministry of Health, Welfare, and Sport. Research into contextO_ST_ABSEvidence before this studyC_ST_ABSSARS-CoV-2 rapid antigen diagnostic tests (Ag-RDTs) require no or minimal equipment, provide a result within 15-30 minutes, and can be used in a range of settings including for self-testing at home. Self-testing may potentially lower the threshold to testing and allows individuals to obtain a test result quickly and at their own convenience, which could support the early detection of infectious cases and reduce community transmission. Real world evidence on the performance of unsupervised nasal and oropharyngeal plus nasal (OP-N) self-sampling in the Omicron variant period is needed to accurately inform end-users and policymakers. Therefore, we conducted a large prospective diagnostic accuracy study of three commercially available Ag-RDTs with self-sampling (the Acon Flowflex test, the MP Biomedicals test, and the Siemens-Healthineers Clinitest) during and after the emergence of Omicron using RT-PCR as the reference standard. Our aims were to evaluate whether the accuracies of Ag-RDTs with nasal self-sampling changed over time with the emergence of Omicron; and to determine whether addition of oropharyngeal to nasal self-sampling with the same swab yielded higher diagnostic accuracies. What this study addsThe large comprehensive study was conducted in almost 6,500 participants with symptoms when presenting for routine SARS-CoV-2 testing at three public health service COVID-19 test-sites in the Netherlands. During the study, conducted between 21 December 2021 and 10 February 2022, the percentage of the Omicron variant in samples from the national SARS-CoV-2 pathogen surveillance increased from 29% in the first week to 99% in the last week of the study. The period during which the Omicron variant was dominant was divided into a nasal sampling phase (phase-1; Omicron present in >90% of surveillance samples) and an OP-N sampling phase (phase-2; Omicron share was >99%). In phase-1, 45% of Flowflex, 29% of MPBio, and 35% of Clinitest participants visited the test-site because of a positive self-test (confirmatory testers). Overall sensitivities with nasal self-sampling were 79.0% (95% CI: 74.7-82.8%) for the Flowflex, 69.9% (65.1-74.4%) for the MPBio, and 70.2% (65.6-74.5%) for the Clinitest Ag-RDT. Sensitivities were 94%, 84%, and 86%, respectively, for confirmatory testers, and 52%, 52%, and 50%, respectively, for those who had other reasons for getting tested. Sensitivities were 87.0% (79.7-92.4%), 83.1% (72.9-90.7%), and 80.0% (51.9-95.7%), respectively, in the first week, and decreased by 6.1 (p=0.16 by Chi-square test), 7.0 (p=0.60), and 12.8 (p=0.025) percentage points in the final week of the study. In Phase-2, 53% of MPBio and 44% of Clinitest participants were confirmatory testers. Overall sensitivities with OP-N self-sampling were 83.0% (78.8%-86.7%) for MPBio and 77.3% (72.9%-81.2%) for Clinitest. When comparing OP-N to nasal sampling, sensitivities were slightly higher in confirmatory testers (87.4% and 86.1%, respectively), and substantially higher in those testing for other reasons (69.3% and 59.9%). Implications of all the available evidenceThe sensitivities of three commercially available Ag-RDTs performed with nasal self-sampling decreased during the emergence of Omicron, but this trend was only statistically significant for Clinitest. Addition of oropharyngeal to nasal self-sampling improved the sensitivity of the MPBio and Clinitest, most notably in individuals who visited the test-site for other reasons than to confirm a positive self-test. Based on these findings, the manufacturers of MPBio and Clinitest may consider extending their instructions for use to include combined oropharyngeal and nasal sampling, and other manufacturers may consider evaluating this as well.
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Given the emergence of the SARS-CoV-2 Omicron BA.1 and BA.2 variants and the roll-out of booster COVID-19 vaccination, evidence is needed on protection conferred by primary vaccination, booster vaccination and previous SARS-CoV-2 infection by variant. We employed a test-negative design and used multinomial logistic regression on data from community PCR testing in the Netherlands. S-gene target failure (SGTF) was used as proxy to discern Delta, Omicron BA.1 and Omicron BA.2 infections. Two cohorts were defined to assess protection from vaccination and previous infection by variant: Delta-Omicron BA.1 cohort including data from 22 November 2021 to 7 January 2022 (n = 354,653) and Omicron BA.1-BA.2 cohort including data from 26 January to 31 March 2022 (n = 317,110). In the Delta-Omicron BA.1 cohort, including 39,889 Delta and 13,915 Omicron BA.1 infections, previous infection, primary vaccination or both protected well against Delta infection (76%, 71%, 92%, respectively, at 7+ months after infection or vaccination). Protection against Omicron BA.1 was much lower compared to Delta infections, but BA.1 estimates were imprecise. In the Omicron BA.1-BA.2 cohort, including 67,887 BA.1 and 41,670 BA.2 infections, protection was similar against Omicron BA.1 compared to BA.2 infection for previous infection (34 and 38% at 7+ months post-infection), primary (39 and 32% at 7+ months post-vaccination) and booster vaccination (68 and 63% at 1 month post-vaccination). Higher protection was observed against all variants in individuals with both vaccination and previous infection compared with either one. Protection against all variants by either vaccination or infection decreased over time since last vaccination or infection. Primary vaccination with current COVID-19 vaccines and previous SARS-CoV-2 infections offer low protection against Omicron BA.1 and BA.2 infection. Booster vaccination considerably increases protection against Omicron infection, but decreases rapidly after vaccination.
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The SARS-CoV-2 Omicron variant has a growth advantage over the Delta variant, due to higher transmissibility, immune evasion, or a shorter serial interval. Using S-gene target failure (SGTF) as indication for Omicron BA.1, we identify 908 SGTF and 1621 non-SGTF serial intervals in the same period. Within households, we find that the mean serial interval for SGTF cases is 0.2-0.6 days shorter than for non-SGTF cases. This suggests that the growth advantage of Omicron is partly due to a shorter serial interval.
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Infections by the Omicron SARS-CoV-2 variant are rapidly increasing worldwide. Among 70,983 infected individuals (age [≥] 12 years), we observed an increased risk of S-gene target failure, predictive of the Omicron variant, in fully vaccinated (odds ratio: 5.0; 95% confidence interval: 4.0-6.1) and previously infected individuals (OR: 4.9: 95% CI: 3.1-7.7) compared with infected naive individuals. This suggests a substantial decrease in protection from vaccine- or infection-induced immunity against SARS-CoV-2 infections caused by the Omicron variant compared with the Delta variant.
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BackgroundSARS-CoV-2 self-tests may lower the threshold of testing and produce a result quickly. This could support the early detection of infectious cases and reduce further community transmission. However, the diagnostic accuracy of (unsupervised) self-testing with rapid antigen diagnostic tests (Ag-RDTs) is mostly unknown. We therefore conducted a large-scale head-to-head comparison of the diagnostic accuracy of a self-performed SARS-CoV-2 saliva and nasal Ag-RDT, each compared to a molecular reference test, in the general population in the Netherlands. MethodsIn this cross-sectional study we consecutively included individuals aged 16 years and older presenting for SARS-CoV-2 testing at three Dutch public health service test sites irrespective of their indication for testing, vaccination status, and symptomatology. Participants were sampled for molecular testing at the test site and received two self-tests (the Hangzhou AllTest saliva self-test and the SD Biosensor nasal self-test by Roche Diagnostics) to perform at home within a few hours without knowledge of their molecular test result. Information on presence and type of symptoms, user experiences, and results of both self-tests were collected via an online questionnaire. For each self-test, sensitivity, specificity, positive and negative predictive values were determined with molecular testing as reference standard. FindingsThe SARS-CoV-2 molecular reference test positivity rate was 6.5% in the 2,819 participants. Overall sensitivities with 95% confidence intervals were 46.7% (85/182; 39.3%-54.2%) for the saliva Ag-RDT, and 68.9% (124/180; 61.6%-75.6%) for the nasal Ag-RDT. With a viral load cut-off ([≥]5.2 log10 SARS-CoV-2 E-gene copies/mL) as a proxy of infectiousness, sensitivities increased to 54.9% (78/142; 46.4%-63.3%) for the saliva Ag-RDT and 83.9% (120/143; 76.9%-89.5%) for the nasal Ag-RDT. For the nasal Ag-RDT, sensitivities were 78.5% [71.1%-84.8%] and 22.6% [9.6%-41.1%] in those with and without symptoms at the time of sampling, which increased to 90.4% (113/125; 83.8%-94.9%) and 38.9% (7/18; 17.3%-64.3%) after applying the viral load cut-off. In those with and without prior confirmed SARS-CoV-2, sensitivities were 36.8% [19/372; 16.3%-61.6%] and 72.7% [161/2437; 65.1%-79.4%] for the nasal Ag-RDT, which increased to 100% (7/7; 59.0%-100%) and 83.1% (113/126; 75.7%-89.0%) after applying the viral load cut-off. The diagnostic accuracy of the nasal Ag-RDT did not differ by COVID-19 vaccination status, sex, and age. Specificities were >99%, positive predictive values >70% and negative predictive values >95%, for the saliva Ag-RDT, and >99%, >90%, and >95% for the nasal Ag-RDT, respectively, in most analyses. Interpreting the results was considered (very) easy for both self-tests. InterpretationThe Hangzhou AllTest self-performed saliva Ag-RDT is not reliable for SARS-CoV-2 infection detection overall nor in the studied subgroups. The SD Biosensor self-performed nasal Ag-RDT had high sensitivity in individuals with symptoms and in those without a prior SARS-CoV-2 infection. The overall accuracy in individuals with symptoms was comparable to that found in previous studies with professional sampling for this Ag-RDT. The extremely low sensitivity of the nasal Ag-RDT in asymptomatic individuals and in individuals who had had a prior SARS-CoV-2 infection is an important finding and warrants further investigation. FundingDutch Ministry of Health, Welfare, and Sport.
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ObjectiveTo assess the diagnostic accuracy of three rapid antigen tests (Ag-RDTs) for detecting SARS-CoV-2 infection in the general population. DesignCross-sectional study with follow-up using pseudonymised record linkage. SettingThree Dutch public health service COVID-19 test sites. ParticipantsConsecutively included individuals aged 16 years and older presenting for SARS-CoV-2 testing. Main outcome measuresSensitivity, specificity, positive and negative predictive values of BD-Veritortm System (Becton Dickinson), PanBio (Abbott), and SD-Biosensor (Roche Diagnostics), applying routinely used sampling methods (combined oropharyngeal and nasal [OP-N] or nasopharyngeal [NP] swab), with molecular testing as reference standard. For SD-Biosensor, the diagnostic accuracy with OP-N sampling was also assessed. A viral load cut-off ([≥]5.2 log10 SARS-CoV-2 E-gene copies/mL) served as a proxy of infectiousness. ResultsSARS-CoV-2 prevalence and overall sensitivities with 95% confidence intervals were 188/1441 (13.0%) and 129/188 (68.6% [61.5%-75.2%]) for BD-Veritor, 173/2056 (8.4%) and 119/173 (68.8% [61.3%-75.6%]) for PanBio, and 215/1769 (12.2%) and 160/215 (74.4% [68.0%-80.1%]) for SD-Biosensor with routine sampling, and 164/1689 (9.7%) and 123/164 (75.0% [67.7%-81.4%]) for SD-Biosensor with OP-N sampling. In those symptomatic or asymptomatic at sampling, sensitivities were 72.2%-83.4% and 54.0%-55.9%, respectively. With a viral load cut-off, sensitivities were 125/146 (85.6% [78.9%-90.9%]) for BD-Veritor, 108/121 (89.3% [82.3%-94.2%]) for PanBio, 160/182 (87.9% [82.3%-92.3%]) for SD-Biosensor with routine sampling, and 118/141 (83.7% [76.5%-89.4%]) with OP-N sampling. Specificities were >99%, and positive and negative predictive values >95%, for all tests in most analyses. 61.3% of false negative Ag-RDT participants returned for testing within 14 days (median of 3 days, interquartile range 3) of whom 90.3% tested positive. ConclusionsThe overall sensitivities of the three Ag-RDTs were 68.6%-75.0%, increasing to at least 85.6% after the viral load cut-off was applied. For SD-Biosensor, the diagnostic accuracy with OP-N and NP sampling was comparable. Over 55% of false negative Ag-RDT participants tested positive during follow-up.
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We estimated vaccine effectiveness against onward transmission by comparing secondary attack rates among household members between vaccinated and unvaccinated index cases, based on source and contact tracing data collected when Delta variant was dominant. Effectiveness of full vaccination of the index against transmission to fully vaccinated household contacts was 40% (95% confidence interval (CI) 20-54%), which is in addition to the direct protection of vaccination of contacts against infection. Effectiveness of full vaccination of the index against transmission to unvaccinated household contacts was 63% (95%CI 46-75%). We previously reported effectiveness of 73% (95%CI 65-79%) against transmission to unvaccinated household contacts for the Alpha variant.
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The objective of this study was to estimate vaccine effectiveness (VE) against COVID-19 hospitalization and ICU admission, per period according to dominating SARS-CoV-2 variant (Alpha and Delta), per vaccine and per time since vaccination. To this end, data from the national COVID-19 vaccination register was added to the national register of COVID-19 hospitalizations. For the study period 4 April - 29 August 2021, 15,571 hospitalized people with COVID-19 were included in the analysis, of whom 887 (5.7%) were fully vaccinated. Incidence rates of hospitalizations and ICU admissions per age group and vaccination status were calculated, and VE was estimated as 1-incidence rate ratio, adjusted for calendar date and age group in a negative binomial regression model. VE against hospitalization for full vaccination was 94% (95%CI 93-95%) in the Alpha period and 95% (95%CI 94-95%) in the Delta period. The VE for full vaccination against ICU admission was 93% (95%CI 87-96%) in the Alpha period and 97% (95%CI 97-98%) in the Delta period. VE was high in all age groups and did not show waning with time since vaccination up to 20 weeks after full vaccination.
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BackgroundPre-/asymptomatic close contacts of SARS-CoV-2 infected individuals were tested at day 5 after contact by real-time reverse transcriptase polymerase chain reaction (RT-PCR). Diagnostic accuracy of antigen-detecting rapid diagnostic tests (Ag-RDT) in pre-/asymptomatic close contacts was up till now unknown. MethodsWe performed a prospective cross-sectional diagnostic test accuracy study. Close contacts (e.g. selected via the test-and-trace program or contact tracing app) aged [≥]16 years and asymptomatic when requesting a test, were included consecutively and tested at day 5 at four Dutch public health service test sites. We evaluated two Ag-RDTs (BD Veritor System Ag-RDT (BD), and Roche/SD Biosensor Ag-RDT (SD-B)) with RT-PCR as the reference standard. Virus culture was performed in RT-PCR positive individuals to determine the viral load cut-off above which 95% was culture positive, as a proxy of infectiousness. ResultsOf 2,678 BD-tested individuals, 233 (8.7%) were RT-PCR positive and BD detected 149 (sensitivity 63.9%; 95% confidence interval 57.4%-70.1%). Out of 1,596 SD-B-tested individuals, 132 (8.3%) were RT-PCR positive and SD-B detected 83 (sensitivity 62.9%; 54.0%-71.1%). When applying an infectiousness viral load cut-off [≥] 5.2 log10 gene copies/mL, the sensitivity was 90.1% (84.2%-94.4%) for BD, 86.8% (78.1% to 93.0%) for SD-B overall, and 88.1% (80.5%-93.5%) for BD, 85.1% (74.3%-92.6%) for SD-B for those still asymptomatic at the actual time of sampling. Specificity was >99% for both Ag-RDTs in all analyses. ConclusionsThe sensitivity for detecting SARS-CoV-2 of both Ag-RDTs in pre-/asymptomatic close contacts is over 60%, increasing to over 85% after applying an infectiousness viral load cut-off. Trial registration numberNot applicable. A study protocol is available upon request.
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BackgroundIndoor environments are considered a main setting for transmission of SARS-CoV-2. Households in particular present a close-contact environment with high probability of transmission between persons of different ages and with different roles in society. MethodsComplete households with a laboratory-confirmed SARS-CoV-2 positive case in the Netherlands (March-May 2020) were included. At least three home visits were performed during 4-6 week of follow-up, collecting naso- and oropharyngeal swabs, oral fluid, faeces and blood samples for molecular and serological analyses of all household members. Symptoms were recorded from two weeks before the first visit up to the last visit. Secondary attack rates (SAR) were estimated with logistic regression. A transmission model was used to assess transmission routes in the household. ResultsA total of 55 households with 187 household contacts were included. In 17 households no transmission took place, and in 11 households all persons were infected. Estimated SARs were high, ranging from 35% (95%CI: 24%-46%) in children to 51% (95%CI: 39%-63%) in adults. Estimated transmission rates in the household were high, with reduced susceptibility of children compared to adolescents and adults (0.67; 95%CI: 0.40-1.1). ConclusionEstimated SARs were higher than reported in earlier household studies, presumably owing to a dense sampling protocol. Children were shown to be less susceptible than adults, but the estimated SAR in children was still high. Our results reinforce the role of households as main multiplier of SARS-CoV-2 infection in the population. Key pointsWe analyze data from a SARS-CoV-2 household study and find higher secondary attack rates than reported earlier. We argue that this is due to a dense sampling strategy that includes sampling at multiple time points and of multiple anatomical sites.
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BackgroundDuring the current pandemic of coronavirus (COVID-19) many countries have taken drastic measures to reduce transmission of SARS-CoV2. The measures often include physical distancing that aims to reduce the number of contacts in the population. Little is known about the actual reduction in number of contacts as a consequence of physical distancing measures. MethodsIn the Netherlands, a cross-sectional survey was carried out in 2016/2017 in which 8179 participants retrospectively reported the number, age and gender of different persons they had contacted (spoken to in person or touched) during the previous day. The survey was repeated among 2830 of the original participants, using the same questionnaire, in March and April 2020 after physical distancing measures had been implemented. ResultsThe average number of contacts in the community was reduced from on average 12.5 (interquartile range: 2-17) to 3.7 (interquartile range: 0-4) different persons per participant, a reduction of 71% (95% confidence interval: 71-71). The reduction in the number of community contacts was highest for children and adolescents (between 5 and 20 years) and smallest for elderly persons of 80 years and older. The reduction in the effective number of total contacts, measured as the largest eigenvalue of the matrix with community and household contacts, was 62% (95% confidence interval: 48 - 72). ConclusionThe substantial reduction in contacts has contributed greatly in halting the COVID-19 epidemic. This reduction was unevenly distributed over age groups, household sizes and occupations. These findings offer guidance for the lifting of age-group targeted measures.
