scholarly journals Surveillance of SARS-CoV-2 transmission in educational institutions, August to December 2020, Germany

2021 ◽  
Vol 149 ◽  
Author(s):  
Anja Schoeps ◽  
Dietmar Hoffmann ◽  
Claudia Tamm ◽  
Bianca Vollmer ◽  
Sabine Haag ◽  
...  
Keyword(s):  
High Risk ◽  
Day Care ◽  
Index Case ◽  
Transmission Risk ◽  
Contact Tracing ◽  
Educational Institutions ◽  
Individual Level ◽  
Mandatory Notification ◽  
Day Care Centres ◽  
Index Cases

Abstract This study aims at providing estimates on the transmission risk of SARS-CoV-2 in schools and day-care centres. We calculated secondary attack rates (SARs) using individual-level data from state-wide mandatory notification of index cases in educational institutions, followed by contact tracing and PCR-testing of high-risk contacts. From August to December 2020, every sixth of overall 784 independent index cases was associated with secondary cases in educational institutions. Monitoring of 14 594 institutional high-risk contacts (89% PCR-tested) of 441 index cases during quarantine revealed 196 secondary cases (SAR 1.34%, 0.99–1.78). SARS-CoV-2 infection among high-risk contacts was more likely around teacher-indexes compared to student-/child-indexes (incidence rate ratio (IRR) 3.17, 1.79–5.59), and in day-care centres compared to secondary schools (IRR 3.23, 1.76–5.91), mainly due to clusters around teacher-indexes in day-care containing a higher mean number of secondary cases per index case (142/113 = 1.26) than clusters around student-indexes in schools (82/474 = 0.17). In 2020, SARS-CoV-2 transmission risk in educational settings was low overall, but varied strongly between setting and role of the index case, indicating the chance for targeted intervention. Surveillance of SARS-CoV-2 transmission in educational institutions can powerfully inform public health policy and improve educational justice during the pandemic.

scholarly journals COVID-19 transmission in educational institutions August to December 2020 in Germany: a study of index cases and close contact cohorts

2021 ◽  
Author(s):  
Anja Schoeps ◽  
Dietmar Hoffmann ◽  
Claudia Tamm ◽  
Bianca Vollmer ◽  
Sabine Haag ◽  
...  
Keyword(s):  
High Risk ◽  
Day Care ◽  
Index Case ◽  
Incidence Rates ◽  
Transmission Risk ◽  
Educational Institutions ◽  
School Closures ◽  
Preventative Measures ◽  
Day Care Centres ◽  
Index Cases

AbstractBackgroundThe lack of precise estimates on transmission risk hampers rational decisions on closure of educational institutions during the COVID-19 pandemic.MethodsSecondary attack rates (SARs) for schools and day-care centres were calculated using data from state-wide mandatory notification of SARS-CoV-2 index cases in educational institutions and information on routine contact tracing and PCR-testing.FindingsFrom August to December 2020, every sixth of overall 784 independent index cases caused a transmission in educational institutions (risk 0·17, 95% CI 0·14–0·19). In a subgroup, monitoring of 14,594 institutional high-risk contacts (89% PCR-tested) of 441 index cases revealed 196 secondary cases (SAR 1·34%, 1·16–1·54). Transmission was more likely from teachers than from students/children (incidence risk ratio [IRR] 3·17, 1·79–5·59), and from index cases in day-care centres (IRR 3·23, 1·76–5·91) than from those in secondary schools. In 748 index cases, teachers caused four times more secondary cases than children (1·08 vs. 0·25 secondary cases per index, IRR 4·39, 2·67–7·21). This difference was mainly due to a large number of teacher-to-teacher transmissions in day-care centres (mean number of secondary cases 0.66) and a very low number of student/child-to-teacher transmissions in schools (mean number of secondary cases 0.004).InterpretationIn educational institutions, the risk of infection for contacts to a confirmed COVID-19 case is one percent, but varies depending on type of institution and index case. Hygiene measures and vaccination targeting the day-care setting and teacher-to-teacher transmission are priorities in reducing the burden of infection and may promote educational justice during the pandemic.FundingNo particular funding was received for this study.Research in contextEvidence before this studyWe searched PubMed on Jan 27, 2021, without any language restrictions for all articles in which the title or abstract contained the search terms “COVID 19” or “corona”, and “school”, “education*”, or “daycare”, and “transmission”, and “risk”, “attack rate”, or “SAR”, and screened 175 results for original research or reviews on COVID-19 transmission risk in the educational setting. Following a similar strategy, we also searched Google Scholar, SSRN, medRxiv, and the reference lists of identified literature. We found five cohort studies on transmission risk looking at overall 171 index cases and their 6,910 contact persons in Australian, Italian, Irish, Singaporean, and German schools and reporting attack rates between 0% and 3% percent. These five studies were conducted before October 2020 and thus looked at COVID-19 transmission risk in schools before the second wave in Europe.A number of modelling studies from the first wave of COVID-19 provide inconclusive guidance to policy makers. While two publications, one from several countries and one from Switzerland, concluded that school closures contributed markedly to the reduction of SARS-CoV-2 transmission and individual mobility, two other studies, one using cross-country data and one from Japan rated school closures among the least effective measures to reduce COVID-19 incidence rates.Added value of this studyBased on a large data set that emerged from the current public health practice in Germany, which incorporates routine PCR-testing during active follow-up of asymptomatic high-risk contacts to index cases, this study provides a precise estimate of the true underlying SARS-CoV-2 transmission risk in schools and day-care centres. Its analysis also allows for a meaningful examination of differences in the risk of transmission with respect to the characteristics of the index case. We found that the individual risk of acquiring SARS-CoV-2 among high-risk contacts in the educational setting is 1.3%, but that this risk rises to 3.2% when the index case is a teacher and to 2.5% when the index case occurs in a day-care centre. Furthermore, we could show that, on average, teacher index cases produced about four times as many secondary cases as student/child index cases. Despite the relatively small proportion of teachers among index cases (20%), our study of transmission pathways revealed that the majority of all secondary cases (54%), and the overwhelming majority of secondary cases in teachers (78%) were caused by teacher index cases. Of note, most cases of teacher-to-teacher transmission (85%) occurred in day-care centres.Implications of all the available evidenceIn this setting, where preventative measures are in place and COVID-19 incidence rates were rising sharply in the population, we found a low and stable transmission risk in educational institutions over time, which provides evidence for the effectiveness of current preventative measures to control the spread of COVID-19 in schools. The identification of a substantial teacher-to-teacher transmission risk in day-care, but a clearly mitigated child/student–to-teacher transmission risk in schools, indicates the need to shift the focus to hygiene among day-care teachers, including infection prevention during staff-meetings and in break rooms. These findings also strongly support the re-prioritization of vaccination against SARS-CoV-2 to educational staff in day-care.

scholarly journals Prevalence and risk factors for in-school transmission of SARS-CoV-2 in Massachusetts K-12 public schools, 2020-2021

2021 ◽  
Author(s):  
Sandra B Nelson ◽  
Caitlin Dugdale ◽  
Alyssa Bilinski ◽  
Duru Cosar ◽  
Nira L Pollock ◽  
...  
Keyword(s):  
Public Schools ◽  
Index Case ◽  
Transmission Risk ◽  
Mitigation Measures ◽  
Contact Tracing ◽  
Convenience Sample ◽  
School Based ◽  
Increased Risk ◽  
K 12 ◽  
Index Cases

Introduction The SARS-CoV-2 secondary attack rate (SAR) in schools is low when mitigation measures are adopted, Data on the relative impact of such strategies are limited. We evaluated the SARS-CoV-2 SAR in Massachusetts schools during 2020-21 and factors associated with transmission risk. Methods: In a convenience sample of 25 Massachusetts public K-12 school districts, de-identified information about SARS-CoV-2 cases and their school-based contacts was reported using a standardized contact-tracing tool. Index cases were included if they were in school while infectious. SAR was defined as the proportion of in-school contacts acquiring SARS-CoV-2 infection and designated as possible or probable in-school transmission by school-based teams. We compared exposure-specific SAR using unadjusted risk ratios (RR) with 95% confidence intervals (CI); p-values were calculated using Fishers exact tests. Results Eight districts (70 schools with >33,000 enrolled students) participated. There were 435 index cases and 1,771 school-based contacts (Table 1). Most contacts (1327/1771 [75%]) underwent SARS-CoV-2 testing and 39/1327 (2.9%) contacts tested positive. Of 39 positive contacts, 10 (25.6%) had clear out-of-school exposures and were deemed not in-school transmissions, so were excluded from further calculations. Twenty-nine (74.4%) contacts were deemed possible or probable in-school transmissions, resulting in an in-school SAR of 2.2%. Of the 29 in-school transmissions, 6 (20.7%) were staff-to-staff, 7 (24.1%) were staff-to-student, 3 (10.3%) were student-to-staff, and 13 (44.8%) were student-to-student; 6 (20.7%) occurred from index cases attending work/school while symptomatic. The unadjusted SAR (Table 2) was significantly higher if the index case was a staff member versus a student (RR 2.18, 95% CI 1.06-4.49; p=0.030), if the index case was identified via in-school contact tracing versus via school-based asymptomatic testing (RR 8.44, 95% CI 1.98-36.06; p=0.001), if the exposure occurred at lunch versus elsewhere (RR 5.74, 95% CI 2.11-15.63; p<0.001; all lunch transmissions were staff-to-staff), and if both parties were unmasked versus both masked (RR 6.98, 95% CI 3.09-15.77; p<0.001). For students, SAR did not differ by grade level. Conclusions Secondary attack rates for SARS-CoV-2 were low in public school settings with comprehensive mitigation measures in place before the emergence of the delta variant; lack of masking and staff-to-staff dining were associated with increased risk.

scholarly journals Investigation of Four Clusters of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in Rwanda, 2020

2021 ◽  
Vol 18 (13) ◽  
pp. 7018
Author(s):  
Olivier Nsekuye ◽  
Edson Rwagasore ◽  
Marie Aime Muhimpundu ◽  
Ziad El-Khatib ◽  
Daniel Ntabanganyimana ◽  
...  
Keyword(s):  
High Risk ◽  
Severe Acute Respiratory Syndrome ◽  
Symptom Onset ◽  
Attack Rate ◽  
Positive Sample ◽  
Contact Tracing ◽  
Face To Face ◽  
Laboratory Confirmation ◽  
Closed Environment ◽  
Index Cases

We reported the findings of the first Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) four clusters identified in Rwanda. Case-investigations included contact elicitation, testing, and isolation/quarantine of confirmed cases. Socio-demographic and clinical data on cases and contacts were collected. A confirmed case was a person with laboratory confirmation of SARS-CoV-2 infection (PCR) while a contact was any person who had contact with a SARS-CoV-2 confirmed case within 72 h prior, to 14 days after symptom onset; or 14 days before collection of the laboratory-positive sample for asymptomatic cases. High risk contacts were those who had come into unprotected face-to-face contact or had been in a closed environment with a SARS-CoV-2 case for >15 min. Forty cases were reported from four clusters by 22 April 2020, accounting for 61% of locally transmitted cases within six weeks. Clusters A, B, C and D were associated with two nightclubs, one house party, and different families or households living in the same compound (multi-family dwelling). Thirty-six of the 1035 contacts tested were positive (secondary attack rate: 3.5%). Positivity rates were highest among the high-risk contacts compared to low-risk contacts (10% vs. 2.2%). Index cases in three of the clusters were imported through international travelling. Fifteen of the 40 cases (38%) were asymptomatic while 13/25 (52%) and 8/25 (32%) of symptomatic cases had a cough and fever respectively. Gatherings in closed spaces were the main early drivers of transmission. Systematic case-investigations contact tracing and testing likely contributed to the early containment of SARS-CoV-2 in Rwanda.

scholarly journals Contact Tracing of COVID-19 in Karnataka, India: Superspreading and Determinants of Infectiousness and Symptomaticity

2020 ◽  
Author(s):  
Mohak Gupta ◽  
Giridara G Parameswaran ◽  
Manraj S Sra ◽  
Rishika Mohanta ◽  
Devarsh Patel ◽  
...  
Keyword(s):  
Symptom Onset ◽  
Attack Rate ◽  
Clinical Severity ◽  
Contact Tracing ◽  
Significant Heterogeneity ◽  
Symptomatic Infection ◽  
Individual Level ◽  
Serial Interval ◽  
Index Cases ◽  
Degree Of Heterogeneity

Brief AbstractWe analysed SARS-CoV-2 surveillance and contact tracing data from Karnataka, India up to 21 July 2020. We estimated metrics of infectiousness and the tendency for superspreading (overdispersion), and evaluated potential determinants of infectiousness and symptomaticity in COVID-19 cases. Among 956 cases confirmed to be forward-traced, 8.7% of index cases had 14.4% of contacts but caused 80% of all secondary cases, suggesting significant heterogeneity in individual-level transmissibility of SARS-CoV-2 which could not be explained by the degree of heterogeneity in underlying number of contacts. Secondary attack rate was 3.6% among 16715 close contacts. Transmission was higher when index case was aged >18 years, or was symptomatic (adjusted risk ratio, aRR 3.63), or was lab-confirmed ≥4 days after symptom onset (aRR 3.01). Probability of symptomatic infection increased with age, and symptomatic infectors were 8.16 times more likely to generate symptomatic secondaries. This could potentially cause a snowballing effect on infectiousness and clinical severity across transmission generations; further studies are suggested to confirm this. Mean serial interval was 5.4 days. Adding backward contact tracing and targeting control measures to curb super-spreading may be prudent. Due to low symptomaticity and infectivity, interventions aimed at children might have a relatively small impact on reducing transmission.Structured AbstractBackgroundIndia has experienced the second largest outbreak of COVID-19 globally, yet there is a paucity of studies analysing contact tracing data in the region. Such studies can elucidate essential transmission metrics which can help optimize disease control policies.MethodsWe analysed contact tracing data collected under the Integrated Disease Surveillance Programme from Karnataka, India between 9 March and 21 July 2020. We estimated metrics of disease transmission including the reproduction number (R), overdispersion (k), secondary attack rate (SAR), and serial interval. R and k were jointly estimated using a Bayesian Markov Chain Monte Carlo approach. We evaluated the effect of age and other factors on the risk of transmitting the infection, probability of asymptomatic infection, and mortality due to COVID-19.FindingsUp to 21 July, we found 111 index cases that crossed the super-spreading threshold of ≥8 secondary cases. R and k were most reliably estimated at R 0.75 (95% CI, 0.62-0.91) and k 0.12 (0.11-0.15) for confirmed traced cases (n=956); and R 0.91 (0.72-1.15) and k 0.22 (0.17-0.27) from the three largest clusters (n=394). Among 956 confirmed traced cases, 8.7% of index cases had 14.4% of contacts but caused 80% of all secondary cases. Among 16715 contacts, overall SAR was 3.6% (3.4-3.9) and symptomatic cases were more infectious than asymptomatic cases (SAR 7.7% vs 2.0%; aRR 3.63 [3.04-4.34]). As compared to infectors aged 19-44 years, children were less infectious (aRR 0.21 [0.07-0.66] for 0-5 years and 0.47 [0.32-0.68] for 6-18 years). Infectors who were confirmed ≥4 days after symptom onset were associated with higher infectiousness (aRR 3.01 [2.11-4.31]). Probability of symptomatic infection increased with age, and symptomatic infectors were 8.16 (3.29-20.24) times more likely to generate symptomatic secondaries. Serial interval had a mean of 5.4 (4.4-6.4) days with a Weibull distribution. Overall case fatality rate was 2.5% (2.4-2.7) which increased with age.ConclusionWe found significant heterogeneity in the individual-level transmissibility of SARS-CoV-2 which could not be explained by the degree of heterogeneity in the underlying number of contacts. To strengthen contact tracing in over-dispersed outbreaks, testing and tracing delays should be minimised, retrospective contact tracing should be considered, and contact tracing performance metrics should be utilised. Targeted measures to reduce potential superspreading events should be implemented. Interventions aimed at children might have a relatively small impact on reducing SARS-CoV-2 transmission owing to their low symptomaticity and infectivity. There is some evidence that symptomatic cases produce secondary cases that are more likely to be symptomatic themselves which may potentially cause a snowballing effect on infectiousness and clinical severity across transmission generations; further studies are needed to confirm this finding.FundingGiridhara R Babu is funded by an Intermediate Fellowship by the Wellcome Trust DBT India Alliance (Clinical and Public Health Research Fellowship); grant number: IA/CPHI/14/1/501499.

scholarly journals Risk of SARS-CoV-2 Transmission among Air Passengers in China

2021 ◽  
Author(s):  
Maogui Hu ◽  
Jinfeng Wang ◽  
Hui Lin ◽  
Corrine W Ruktanonchai ◽  
Chengdong Xu ◽  
...  
Keyword(s):  
Relative Risk ◽  
Travel Time ◽  
Index Case ◽  
Epidemiological Data ◽  
Transmission Risk ◽  
Upper And Lower Bounds ◽  
Wuhan City ◽  
Upper Limit ◽  
Index Cases ◽  
Close Contacts

Abstract Background Modern transportation plays a key role in the spread of SARS-CoV-2 and new variants. However, little is known about the exact transmission risk of the virus on airplanes. Methods Using the itinerary and epidemiological data of COVID-19 cases and close contacts on domestic airplanes departing from Wuhan city in China before the lockdown on January 23, 2020, we estimated the upper and lower bounds of overall transmission risk of COVID-19 among travellers. Results 175 index cases were identified among 5797 passengers on 177 airplanes. The upper and lower attack rates (ARs) of a seat were 0.60% (34/5622, 95%CI 0.43%-0.84%) and 0.33% (18/5400, 95%CI 0.21%-0.53%), respectively. In the upper- and lower-bound risk estimates, each index case infected 0.19 (SD 0.45) and 0.10 (SD 0.32) cases respectively. The seats immediately adjacent to the index cases had an AR of 9.2% (95%CI 5.7%-14.4%), with a relative risk 27.8 (95%CI 14.4-53.7) compared to other seats in the upper limit estimation. The middle seat had the highest AR (0.7%, 95%CI 0.4%-1.2%). The upper-bound AR increased from 0.7% (95%CI 0.5%-1.0%) to 1.2% (95%CI 0.4%-3.3%) when the co-travel time increased from 2.0 hours to 3.3 hours. Conclusions The ARs among travellers varied by seat distance from the index case and joint travel time, but the variation was not significant between the types of aircraft. The overall risk of SARS-CoV-2 transmission during domestic travel on planes was relatively low. These findings can improve our understanding of COVID-19 spread during travel and inform response efforts in the pandemic.

scholarly journals Role of children in household transmission of COVID-19

2020 ◽  
pp. archdischild-2020-319910 ◽  
Author(s):  
Jieun Kim ◽  
Young June Choe ◽  
Jin Lee ◽  
Young Joon Park ◽  
Ok Park ◽  
...  
Keyword(s):  
Attack Rate ◽  
Index Case ◽  
Clinical Findings ◽  
International Travel ◽  
Contact Tracing ◽  
Secondary Case ◽  
Household Transmission ◽  
Household Members ◽  
Index Cases

ObjectiveTransmissibility of COVID-19 by children in the household is not clear. Herein, we describe children’s role in household transmission of COVID-19.Design and settingAll paediatric COVID-19 index cases and their household members reported from 20 January to 6 April 2020 in South Korea were reviewed. The secondary attack rate (SAR) from child index case to household secondary case was calculated. Epidemiological and clinical findings of child index case-household secondary case pair was assessed.ResultsA total of 107 paediatric COVID-19 index cases and 248 of their household members were identified. One pair of paediatric index-secondary household case was identified, giving a household SAR of 0.5% (95% CI 0.0% to 2.6%). The index case was self-quarantined at home after international travel, stayed in her room, but shared a meal table with the secondary case.ConclusionThe SAR from children to household members was low in the setting of social distancing, underscoring the importance of rigorous contact tracing and early isolation in limiting transmission within households.

scholarly journals The secondary transmission pattern of COVID-19 based on contact tracing in Rwanda

2021 ◽  
Vol 6 (6) ◽  
pp. e004885
Author(s):  
Muhammed Semakula ◽  
FranÇois Niragire ◽  
Angela Umutoni ◽  
Sabin Nsanzimana ◽  
Vedaste Ndahindwa ◽  
...  
Keyword(s):  
Risk Factors ◽  
High Risk ◽  
Disease Transmission ◽  
Index Case ◽  
Conditional Logistic Regression ◽  
Contact Tracing ◽  
African Countries ◽  
Targeted Interventions ◽  
The Impact ◽  
Mapping Models

IntroductionCOVID-19 has shown an exceptionally high spread rate across and within countries worldwide. Understanding the dynamics of such an infectious disease transmission is critical for devising strategies to control its spread. In particular, Rwanda was one of the African countries that started COVID-19 preparedness early in January 2020, and a total lockdown was imposed when the country had only 18 COVID-19 confirmed cases known. Using intensive contact tracing, several infections were identified, with the majority of them being returning travellers and their close contacts. We used the contact tracing data in Rwanda for understanding the geographic patterns of COVID-19 to inform targeted interventions.MethodsWe estimated the attack rates and identified risk factors associated to COVID-19 spread. We used Bayesian disease mapping models to assess the spatial pattern of COVID-19 and to identify areas characterised by unusually high or low relative risk. In addition, we used multiple variable conditional logistic regression to assess the impact of the risk factors.ResultsThe results showed that COVID-19 cases in Rwanda are localised mainly in the central regions and in the southwest of Rwanda and that some clusters occurred in the northeast of Rwanda. Relationship to the index case, being male and coworkers are the important risk factors for COVID-19 transmission in Rwanda.ConclusionThe analysis of contact tracing data using spatial modelling allowed us to identify high-risk areas at subnational level in Rwanda. Estimating risk factors for infection with SARS-CoV-2 is vital in identifying the clusters in low spread of SARS-CoV-2 subnational level. It is imperative to understand the interactions between the index case and contacts to identify superspreaders, risk factors and high-risk places. The findings recommend that self-isolation at home in Rwanda should be reviewed to limit secondary cases from the same households and spatiotemporal analysis should be introduced in routine monitoring of COVID-19 in Rwanda for policy making decision on real time.

scholarly journals A comparison of COVID-19 secondary attack rate in household and close contacts compared to current risk stratification guidelines of the Kerala government

2021 ◽  
pp. 004947552110020
Author(s):  
Balram Rathish ◽  
Arun Wilson ◽  
Sonya Joy
Keyword(s):  
High Risk ◽  
Risk Stratification ◽  
Attack Rate ◽  
Low Risk ◽  
Contact Tracing ◽  
Index Cases ◽  
Current Risk ◽  
Close Contacts

COVID-19 has been found to be highly infectious with a high secondary attack rate with a R0 of 3.3. However, the secondary attack rate based on risk stratification is sparsely reported, if ever. We studied the contact tracing data for two index cases of COVID-19 with some overlap of contacts. We found that 60% of high-risk contacts and 0% of low-risk contacts of symptomatic COVID-19 patients contracted the infection, in keeping with the Kerala government contact risk stratification guidelines.

scholarly journals Transmission of COVID-19 in 282 clusters in Catalonia, Spain: a cohort study

2020 ◽  
Author(s):  
Michael Marks ◽  
Pere Millat-Martinez ◽  
Dan Ouchi ◽  
Chrissy h. Roberts ◽  
Andrea Alemany ◽  
...  
Keyword(s):  
Viral Load ◽  
Quantitative Pcr ◽  
Incubation Time ◽  
Attack Rate ◽  
Index Case ◽  
Transmission Risk ◽  
Dependent Manner ◽  
Symptomatic Disease ◽  
Dose Dependent ◽  
Index Cases

ABSTRACTBackgroundThere remains limited data on what variables affect risk of transmission of SARS-CoV-2 and developing symptomatic Covid-19 and in particular the relationship to viral load (VL). We analysed data from linked index cases and their contacts to explore factors associated with transmission of SARS-CoV-2.MethodsPatients were recruited as part of a randomized control trial, conducted between March to April 2020, that aimed to assess if hydroxychloroquine reduced transmission of SARS-CoV-2. Non-hospitalised Covid-19 cases and their contacts were identified through the local surveillance system. VL, measured by quantitative PCR from a nasopharyngeal swab, was assessed at enrollment, at day 14, and whenever the participant reported Covid-19-like symptoms. Risk of transmission, developing symptomatic disease and incubation dynamics were evaluated using regression analysis.FindingsWe identified 314 cases, 282 of which had at least one contact (753 contacts in total). Ninety (33%) of 282 clusters had at least one transmission event. The secondary attack rate was 16% (125/753), with a variation from 12% to 24% for VL of the index case of <106, and >109 copies/mL, respectively (OR per log10 increase in VL 1.3 95%CI 1.1–1.6). Increased risk of transmission was also associated with household contact (OR 2.7; 1.4–5.06) and age of the contact (OR 1.02 per year; 1.01–1.04). The proportion of PCR positive contacts who developed symptomatic Covid-19 was 40.3% (181/449), with a variation from 25% to 60% for VL of the contact <107, and >109 copies/mL (HR log10 increase in VL 1.12; 95% CI 1.05 – 1.2). Time to onset of symptomatic disease decreased from a median of 7 days (IQR 5–10) for individuals with an initial viral load <107 to 6 days (4–8) and 5 days (3–8) for individuals with an initial viral load of 107–109 and >109, respectively.InterpretationViral load of index cases is a leading driver of SARS-CoV-2 transmission. The risk of symptomatic Covid-19 is strongly associated with viral load of contacts at baseline and shortens the incubation time in a dose-dependent manner.FundingCrowdfunding campaign YoMeCorono (http://www.yomecorono.com/), and Generalitat de Catalunya. Support for laboratory equipment from Foundation Dormeur.Research in contextEvidence before this studyIn September 2020, we searched PubMed database for articles reporting on factors influencing transmission and the risk of developing symptomatic disease. Search terms included “Covid-19”, “SARS-CoV-2”, “transmission”, “incubation time”, and “risk”, with no language restrictions. By 20th September, various authors had reported on retrospective analyses of clusters of index cases and their corresponding contacts, as well as series of patients who developed symptomatic Covid-19 disease after PCR positive result. Besides describing the secondary attack rate, various authors identified risk factors for transmission associated with the place and duration of exposure and the lack of use of personal protective equipment. A single study suggested that symptomatic individuals might be more likely to transmit than asymptomatic cases but we found no clear evidence regarding the influence of viral load of the index case on transmission risk. Similarly, although various retrospective series of patients with positive PCR results had reported incubation times elsewhere, the characteristics of index case and contacts that may influence the risk of developing symptomatic Covid-19 and the time to this event had been barely addressed.Added value of this studyWe analyzed data from a large cluster-randomized clinical trial on post-exposure therapy for Covid-19 that provide new information on SARS-CoV-2 transmission dynamics. Several design components add value to this dataset. Notably, quantitative PCR was available for the index cases to estimate risk of transmission. Furthermore, quantitative PCR was also performed on asymptomatic contacts at the time of enrollment allowing to investigate the dynamics of symptomatic disease onset among them. We found that the viral load of the index case was the leading determinant of the risk of SARS-CoV-2 PCR positivity among contacts. Among contacts who were SARS-CoV-2 PCR positive at baseline, viral load significantly influenced the risk of developing the symptomatic disease in a dose-dependent manner. This influence also became apparent in the incubation time, which shortened with increasing baseline viral loads.Implication of all the available evidenceOur results provide important insights into the knowledge regarding the risk of SARS-CoV-2 transmission and Covid-19 development. The fact that the transmission risk is primarily driven by the viral load of index cases, more than other factors such as their symptoms or age, suggests that all cases should be considered potential transmitters irrespective of their presentation and encourages assessing viral load in cases with a larger number of close contacts. Similarly, our results regarding the risk and expected time to developing symptomatic Covid-19 encourage risk stratification of newly diagnosed SARS-CoV-2 infections based on the initial viral load.

scholarly journals Secondary attack rates of COVID-19 in Norwegian families: a nation-wide register-based study

2021 ◽  
Author(s):  
Kjetil Telle ◽  
Silje B. Jørgensen ◽  
Rannveig Hart ◽  
Margrethe Greve-Isdahl ◽  
Oliver Kacelnik
Keyword(s):  
Young Children ◽  
Index Case ◽  
Family Members ◽  
Age Group ◽  
Older Children ◽  
Index Child ◽  
Individual Level ◽  
The Family ◽  
Index Cases ◽  
Index Family

AbstractTo characterize the family index case for detected SARS-CoV-2 and describe testing and secondary attack rates in the family, we used individual-level administrative data of all families and all PCR tests for SARS-CoV-2 in Norway in 2020. All families with at least one parent and one child below the age of 20 who lived at the same address (N = 662,582), where at least one member, i.e. the index case, tested positive for SARS-CoV-2 in 2020, were included. Secondary attack rates (SAR7) were defined as the share of non-index family members with a positive PCR test within 7 days after the date when the index case tested positive. SARs were calculated separately for parent- and child-index cases, and for parent- and child-secondary cases. We identified 7548 families with an index case, comprising 26,991 individuals (12,184 parents, 14,808 children). The index was a parent in 66% of the cases. Among index children, 42% were in the age group 17–20 and only 8% in the age group 0–6. When the index was a parent, SAR7 was 24% (95% CI 24–25), whilst SAR7 was 14% (95% CI 13–15) when the index was a child. However, SAR7 was 24% (95% CI 20–28) when the index was a child aged 0–6 years and declined with increasing age of the index child. SAR7 from index parent to other parent was 35% (95% CI 33–36), and from index child to other children 12% (95% CI 11–13). SAR7 from index child aged 0–6 to parents was 27% (95% CI 22–33). The percent of non-index family members tested within 7 days after the index case, increased from about 20% in April to 80% in December, however, SAR7 stabilized at about 20% from May. We conclude that parents and older children are most often index cases for SARS-CoV-2 in families in Norway, while parents and young children more often transmit the virus within the family. This study suggests that whilst the absolute infection numbers are low for young children because of their low introduction rate, when infected, young children and parents transmit the virus to the same extent within the family.

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