Household secondary attack rate in mild COVID-19.

Background: The coronavirus disease 2019 (COVID-19) pandemic has reached a staggering number of almost 280 million cases worldwide, with over 5.4 million deaths as of 29 December 2021. A further understanding of the factors related to the household spread of the infection might help to bring about specific protocols to curb such transmission. Objective: This study aims to find the secondary attack rate (SAR) and factors affecting SAR among the households of mild COVID-19 cases. Methods: An observational study was designed where data of patients admitted at All India Institute of Medical Sciences, New Delhi due to mild COVID-19 were collected, and outcome was noted after the discharge of the patient. Index cases who were the first in the household to have a positive infection only were included. Based on these data, the overall household SAR, factors related to the index case and contacts that affected transmissibility were noted. Results: A total of 60 index cases having contacts with 184 household members were included in the present study. The household SAR was measured to be 41.85%. At least one positive case was present in 51.67% households. Children below 18 years old had lower odds of getting a secondary infection compared to adults and elderly [odds ratio (OR) = 0.46, 95%CI = 0.22-0.94, p = 0.0383). An exposure period of more than a week was significantly associated with a higher risk of infection (p = 0.029). The rate of transmissibility drastically declined with effective quarantine measures adopted by the index case (OR = 0.13, 95%CI = 0.06-0.26, p < 0.00001). Symptomatic index cases contributed more to the SAR than asymptomatic primaries (OR = 4.74, 95%CI = 1.03-21.82, P = 0.045). Healthcare worker index cases had lower rates of spread (OR = 0.29, 95%CI = 0.15-0.58, P = 0.0003). Conclusion: The high SAR shows the household is a potential high-risk unit for transmissibility of COVID-19. Proper quarantine measures of all those exposed to the index case can mitigate such spread and lead to reduction of risk of COVID-19 within a household.

Incidence of COVID-19 Infection in Nurses and their Families: A Prospective Cohort Study in Lahore

Background: Nurses, as frontline workers, are at a higher risk of contracting COVID19 and serve as a potential source of transmitting this infection to their family members. Estimating risk in this population is critical for controlling the rate of secondary attack rate among their families.Objective: To measure the incidence of COVID-19 in nurses and their families with associated risk factors for transmission of infection.Methods: A prospective cohort study was conducted in Akhtar Saeed Medical and Dental college, Lahore and associated three hospitals. A cohort of 176 nurses was identified in March 2020 by using non-probability convenience sampling technique and was followed till September 2021. During this tenure of one and a half year, incidence was observed by following the cohort and using a self-structured questionnaire. With SPSS 23, Chi square test was applied and p value of < 0.05 was considered significant.Results: In a cohort of 176 nurses, followed for one and a half year, incidence of COVID-19 was 37(21%). Secondary attack rate among household members of these nurses was 3%. A significant association was observed between the nursing staff and exposure to their family members (p = 0.002). Vaccination rate in nurses at the start of the study was only 26%. Nurses who worked directly with COVID-19 patients reported high incidence (p = 0.009). Nurses who used Personal protective equipment (PPE) were less affected (p = 0.003). Nurses with comorbidities showed higher incidence (p =0.02). Rate of psychological distress was high (p =0.007).Conclusion: Incidence of COVID-19 among nurses was 21% with secondary attack rate of 3% in families. Protective use of N 95 mask and comorbidities were associated with incidence.

Infection-Induced Immunity Is Associated With Protection Against Severe Acute Respiratory Syndrome Coronavirus 2 Infection and Decreased Infectivity.

BACKGROUND: The impact of infection-induced immunity on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission has not been well established. Here we estimate the effects of prior infection induced immunity in adults and children on SARS-CoV-2 transmission in households. METHODS: We conducted a household cohort study from March 2020-November 2022 in Managua, Nicaragua; following a housheold SARS-CoV-2 infection, household members are closely monitored for infection. We estimate the association of time period, age, symptoms, and prior infection with secondary attack risk. RESULTS: Overall, transmission occurred in 70.2% of households, 40.9% of household contacts were infected, and the secondary attack risk ranged from 8.1% to 13.9% depending on the time period. Symptomatic infected individuals were more infectious (rate ratio [RR] 21.2, 95% confidence interval [CI]: 7.4-60.7) and participants with a prior infection were half as likely to be infected compared to naïve individuals (RR 0.52, 95% CI:.38-.70). In models stratified by age, prior infection was associated with decreased infectivity in adults and adolescents (secondary attack risk [SAR] 12.3, 95% CI: 10.3, 14.8 vs 17.5, 95% CI: 14.8, 20.7). However, although young children were less likely to transmit, neither prior infection nor symptom presentation was associated with infectivity. During the Omicron era, infection-induced immunity remained protective against infection. CONCLUSIONS: Infection-induced immunity is associated with decreased infectivity for adults and adolescents. Although young children are less infectious, prior infection and asymptomatic presentation did not reduce their infectivity as was seen in adults. As SARS-CoV-2 transitions to endemicity, children may become more important in transmission dynamics.

Active Monitoring of Persons Exposed to Patients with Confirmed COVID-19 in South of Iran

Background: Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) can be transmitted through direct, indirect, or close contact with infected people by contaminated respiratory droplets or saliva. This study aimed to investigate the epidemiology of coronavirus disease 2019 (COVID-19) and the secondary attack rate (SAR) in the cases' close contact. Methods: A total of 431 confirmed COVID-19 patients were randomly selected using systematic random sampling from 15 May to 13 June 2020. The required data were extracted from the CORONALAB database of the Center for Disease Control and Prevention (CDC) at Shiraz University of Medical Sciences. Detection of COVID-19 was performed using Real-Time Polymerase Chain Reaction (RT-PCR) and nasopharyngeal swabs. SAR was also calculated for different groups. Results: Among the index cases, 64.27% were male, 24.80% were public sector employees, and 4.87% were admitted to the intensive care unit. In addition, most of them aged 30-39 years. The SAR was 11.56% (95% CI: 9.86% to 13.25%) in the close contacts. Accordingly, the highest SAR was observed among the friends, 19.05% (95% CI: 7.17% to 30.92%), followed by the spouses of COVID-19 cases, 16.67% (95% CI: 10.81% to 22.51%). Furthermore, diabetes (6.03%) and cardiovascular disease (5.1%) were the most common comorbidities among the index cases. Conclusion: The findings suggested that the SAR was relatively lower among the close contacts. Considering the familial and non-familial relationships between the index cases and their close contacts were the major causes of disease transmission. Therefore, it is crucial to conduct tracing for COVID-19 contacts in all cases with whom patients have had close contact. © 2023 Authors. All rights reserved.

Household Secondary Attack Rate of COVID-19 among Healthcare Workers and Related Factors

Background: The risk for transmission of COVID-19 to people in close contact with infected people, especially healthcare workers, has not been well estimated. Therefore the present study was conducted to assess the household secondary attack rate (SAR) of COVID-19 among healthcare workers and related factors. Material(s) and Method(s): The present prospective case-ascertained study was conducted on 202 healthcare workers with confirmed COVID-19 in Hamadan, diagnosed from March 1, 2020, to August 20, 2020. For households with close contact with the index case, RT-PCR was performed regardless of symptoms. We defined SAR as the proportion of secondary cases from the total contacts that live in the index case household. SAR was reported as a percentage and 95% confidence interval (CI). Multiple logistic regression was performed to explore the predictors of COVID-19 transmission of index cases to their households. Result(s): We found 36 secondary cases out of 391 household contacts with laboratory confirmation (RT-PCR), representing a household SAR of 9.2% (95% CI: 6.3, 12.1). Among factors related to the family member, female gender (OR: 2.9, 95% CI: 1.2, 6.9), being the patient's spouse (OR: 2.2, 95% CI: 1.0, 4.6), and living in the apartment (OR: 2.78, 95% CI: 1.24, 6.23), and among factors related to index cases, hospitalization (OR: 5.9, 95% CI: 1.3, 26.9) and caught (OR: 2.4, 95% CI: 1.1, 5.2) were the significant predictors of disease transmission to other family members (P<0.05). Conclusion(s): The findings of this study suggest that the SAR is remarkable in household contacts of infected healthcare workers. Some characteristics of family members of the index case, including female gender, being the patient's spouse, and living in the apartment, and some characteristics of the index case, including hospitalization and caught, were associated with the increased SAR.Copyright © 2022 NRITLD, National Research Institute of Tuberculosis and Lung Disease, Iran.

Household transmission dynamics of COVID-19 among residents of Delhi, India: a prospective case-ascertained study.

Objective: The aim of this study was to observe the secondary infection rate and transmission dynamics of COVID-19 among household contacts, and their associations with various factors across four dimensions of interaction. Methods: This was a case-ascertained study among unvaccinated household contacts of a laboratory-confirmed COVID-19 case in New Delhi between December 2020 and July 2021. For this study, 99 index cases and their 316 household contacts were interviewed and sampled (blood and oro-nasal swab) on days 1, 7, 14, and 28. Results: The secondary infection rate among unvaccinated household contacts was 44.6% (95% confidence interval (CI) 39.1-50.1). The predictors of secondary infection among individual contact levels were: being female (odds ratio (OR) 2.13), increasing age (OR 1.01), symptoms at baseline (OR 3.39), and symptoms during follow-up (OR 3.18). Among index cases, age of the primary case (OR 1.03) and symptoms during follow-up (OR 6.29) were significantly associated with secondary infection. Among household-level and contact patterns, having more rooms (OR 4.44) and taking care of the index case (OR 2.02) were significantly associated with secondary infection. Conclusion: A high secondary infection rate highlights the need to adopt strict measures and advocate COVID-19-appropriate behaviors. A targeted approach for higher-risk household contacts would efficiently limit infections among susceptible contacts.

An early return-to-work program for COVID-19 close contacts in healthcare during the Omicron wave in Japan

During the coronavirus disease 2019 (COVID-19) pandemic, maintaining adequate staffing in healthcare facilities is important to provide a safe work environment for healthcare workers (HCWs). Japan's early return-to-work (RTW) program may be a rational strategy at a time when there is an increased demand for the services of HCWs. We assessed whether the early RTW program for HCWs who have been in close contact with a COVID-19 case in our hospital was justified. Close contacts were identified according to the guidance document of the World Health Organization. HCWs who met all of the following conditions were eligible to apply to an early RTW program: (1) difficult to replace with another HCW, (2) received the third dose of a COVID-19 mRNA vaccine, (3) a negative COVID-19 antigen test before each work shift, and (4) consent from relevant HCWs and their managers to participate in the program. Between January and March 2022, 256 HCWs were identified as close contacts (median age, 35 years;192 female). Thirty-seven (14%) secondary attack cases of COVID-19 were detected. Among 141 HCWs (55%) who applied to the early RTW program, nurses and physicians comprised about three-quarters of participants, with a higher participation rate by physicians (78%) than nurses (59%). Eighteen HCWs tested positive for COVID-19 by the sixth day after starting the early RTW program. No COVID-19 infection clusters were reported during the observation period. These findings suggest that the early RTW program for COVID-19 close contacts was a reasonable strategy for HCWs during the Omicron wave. © 2022 Japanese Society of Chemotherapy and The Japanese Association for Infectious Diseases

Limited role of children in transmission of SARS-CoV-2 virus in households-Immunological analysis of 26 familial clusters.

BACKGROUND: The impact of children on the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) remains uncertain. This study provides an insight into distinct patterns of SARS-CoV-2 household transmission in case of pediatric and adult index cases as well as age-dependent susceptibility to SARS-CoV-2 infection. METHODS: Immune analysis, medical interviewing, and contact tracing of 26 families with confirmed SARS-CoV-2 infection cases have been conducted. Blood samples were analyzed serologically with the use of a SARS-CoV-2-specific IgG assay and virus neutralization test (VNT). Uni- and multivariable linear regression and mixed effect logistic regression models were used to describe potential risk factors for higher contagiousness and susceptibility to SARS-CoV-2 infection. RESULTS: SARS-CoV-2 infection could be confirmed in 67 of 124 family members. Fourteen children and 11 adults could be defined as index cases in their households. Forty of 82 exposed family members were defined as secondarily infected. The mean secondary attack rate in households was 0.48 and was significantly higher in households with adult than with pediatric index cases (0.85 vs 0.19; p < 0.0001). The age (grouped into child and adult) of index case, severity of disease, and occurrence of lower respiratory symptoms in index cases were significantly associated with secondary transmission rates in households. Children seem to be equally susceptible to acquire a SARS-CoV-2 infection as adults, but they suffer milder courses of the disease or remain asymptomatic. CONCLUSION: SARS-CoV-2 transmission from infected children to other household members occurred rarely in the first wave of the pandemic, despite close physical contact and the lack of hygienic measures.

Association Between Population-Level Factors and Household Secondary Attack Rate of SARS-CoV-2: A Systematic Review and Meta-analysis.

Background: Accurate estimation of household secondary attack rate (SAR) is crucial to understand the transmissibility of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The impact of population-level factors, such as transmission intensity in the community, on SAR estimates is rarely explored. Methods: In this study, we included articles with original data to compute the household SAR. To determine the impact of transmission intensity in the community on household SAR estimates, we explored the association between SAR estimates and the incidence rate of cases by country during the study period. Results: We identified 163 studies to extract data on SARs from 326 031 cases and 2 009 859 household contacts. The correlation between the incidence rate of cases during the study period and SAR estimates was 0.37 (95% CI, 0.24-0.49). We found that doubling the incidence rate of cases during the study period was associated with a 1.2% (95% CI, 0.5%-1.8%) higher household SAR. Conclusions: Our findings suggest that the incidence rate of cases during the study period is associated with higher SAR. Ignoring this factor may overestimate SARs, especially for regions with high incidences, which further impacts control policies and epidemiological characterization of emerging variants.

Lower transmissibility of SARS-CoV-2 among asymptomatic cases: evidence from contact tracing data in Oslo, Norway.

BACKGROUND: Asymptomatic COVID-19 cases have complicated the surveillance and tracking of the pandemic. Previous studies have estimated that 15-25% of all infectees remain asymptomatic. METHODS: Based on contact tracing data from Oslo, Norway, we estimated transmission and susceptibility dynamics among symptomatic and asymptomatic cases and their contacts as identified by manual contact tracing between September 1, 2020, and September 1, 2021. RESULTS: Among 27,473 indexes and 164,153 registered contacts, the secondary attack rate (SAR-14) was estimated to be 28% lower through asymptomatic exposure (13%) compared to symptomatic exposure (18%). Furthermore, those infected by asymptomatic cases were almost three times more likely to be asymptomatic compared to those infected by symptomatic cases. CONCLUSIONS: Symptomatic cases spread the virus to a greater extent than asymptomatic, and infectees are more likely to be asymptomatic if their assumed infector was asymptomatic.

Reasons for SARS-CoV-2 infection in children and their role in the transmission of infection according to age: a case-control study.

BACKGROUND: The locations where children get exposed to SARS-CoV-2 infection and their contribution in spreading the infection are still not fully understood. Aim of the article is to verify the most frequent reasons for SARS-CoV-2 infection in children and their role in the secondary transmission of the infection. METHODS: A case-control study was performed in all SARS-CoV-2 positive children (n = 81) and an equal number of age- and sex- matched controls who were referred to the S. Camillo-Forlanini Pediatric Walk-in Center of Rome. The results of all SARS-CoV-2 nasopharyngeal swabs performed in children aged < 18 years from October 16 to December 19, 2020 were analyzed. RESULTS: School contacts were more frequent in controls than in cases (OR 0.49; 95% CI: 0.3-0.9), while household contacts were higher in cases (OR 5.09; 95% CI: 2.2-12.0). In both cases and controls, school contacts were significantly less frequent, while on the contrary household contacts seemed to be more frequent in nursery school children compared to primary school or middle/high school children. A multivariate logistic regression showed that the probability of being positive to SARS-CoV-2 was significantly lower in children who had school contacts or who had flu symptoms compared to children who had household contacts. Results showed a 30.6% secondary attack rate for household contacts. CONCLUSION: In our study population, the two most frequent reasons for SARS-CoV-2 infection were school and home contacts. The risk of being positive was 5 times lower in children who had school contacts than in children who had household contacts.

Symptomatic survey of COVID-19 in the rural health and demographic surveillance site of Maharashtra, India.

Introduction: The COVID-19 epidemic is fast-evolving and restrictions to contain it changes quickly. The secondary attack rate (SAR) indicates the infectiousness at the household level, and it provides a clue of load of infections in the community. Rapid implementation of a large scale, but brief survey provides a nationally representative view of the pandemic's progression and impact as well as the effect of the measures taken to control the spread. Materials and Methods: This cross-sectional study was done for a period of 2 months July-August 2020, in one of the rural health and demographic surveillance system (HDSS) site in India among 11,507 households having 46,571 individuals with the objective to determine the prevalence of the COVID-19 infection and to estimate the family SAR. The data were collected using the mobile phone by calling the head of the households registered under the HDSS. The research tool was created using questions based on the guidelines provided by the Ministry of Health and Family Welfare, India. The interviewers were first trained in data collection. Results: The households of 33,780 individuals were contacted and 33 patients were diagnosed as COVID positive, prevalence rate of 0.1% was thus documented through telephonic survey. The mean age of COVID patients was found to be 37 ± 16 years, and 63% of the diagnosed patients were males, majority of them belonged to above poverty line households. During the survey, 46 individuals reported to have the symptoms suggestive of COVID, fever and cough were the most commonly reported symptoms. The family SAR of 17.2% was documented. Conclusion: Thus to conclude a low COVID-19 prevalence rate of 0.1%, a higher family SAR of 17.2% was reported in the current study. Timely diagnosis and quarantine of close contacts should be continued to be implemented rigorously to prevent the spread of the COVID-19 virus. These efforts will be useful to contain the epidemic before it reaches an alarming level in the rural areas.

Résumé Introduction: L'épidémie de COVID-19 évolue rapidement et les restrictions pour la contenir changent rapidement. Le taux d'attaque secondaire (SAR) indique l'infectiosité au niveau du ménage et fournit un indice de la charge d'infections dans la communauté. La mise en œuvre rapide d'une enquête à grande échelle mais brève fournit une vue représentative à l'échelle nationale de la progression et de l'impact de la pandémie ainsi que de l'effet des mesures prises pour contrôler la propagation. Matériels et Méthodes: cette étude transversale a été réalisée sur une période de 2 mois de juillet à août 2020, dans l'un des sites du système de surveillance sanitaire et démographique en milieu rural (HDSS) en Inde auprès de 11 507 ménages comptant 46 571 individus dans le but de déterminer prévalence de l'infection au COVID-19 et d'estimer le DAS familial. Les données ont été collectées à l'aide du téléphone mobile en appelant le chef de ménage inscrit au HDSS. L'outil de recherche a été créé à l'aide de questions basées sur les directives fournies par le ministère indien de la Santé et du Bien-être familial. Les enquêteurs ont d'abord été formés à la collecte de données. Résultats: les ménages de 33 780 individus ont été contactés et 33 patients ont été diagnostiqués positifs au COVID, un taux de prévalence de 0,1 % a ainsi été documenté par une enquête téléphonique. L'âge moyen des patients COVID était de 37 ± 16 ans, et 63 % des patients diagnostiqués étaient des hommes, la majorité d'entre eux appartenant à des ménages au-dessus du seuil de pauvreté. Au cours de l'enquête, 46 personnes ont déclaré avoir des symptômes évocateurs de COVID, la fièvre et la toux étaient les symptômes les plus fréquemment signalés. Le SAR familial de 17,2 % a été documenté. Conclusion: Ainsi, pour conclure à un faible taux de prévalence de la COVID-19 de 0,1 %, un DAS familial supérieur de 17,2 % a été rapporté dans la présente étude. Le diagnostic rapide et la mise en quarantaine des contacts étroits doivent être poursuivis et mis en œuvre de manière rigoureuse pour empêcher la propagation du virus COVID-19. Ces efforts seront utiles pour contenir l'épidémie avant qu'elle n'atteigne un niveau alarmant dans les zones rurales. Mots-clés: COVID 19, système de surveillance sanitaire et démographique, téléphone portable, prévalence, milieu rural, taux d'attaque secondaire.

SARS-CoV-2 Secondary Attack Rates in Vaccinated and Unvaccinated Household Contacts during Replacement of Delta with Omicron Variant, Spain.

We performed a prospective, cross-sectional study of household contacts of symptomatic index case-patients with SARS-CoV-2 infection during the shift from Delta- to Omicron-dominant variants in Spain. We included 466 household contacts from 227 index cases. The secondary attack rate was 58.2% (95% CI 49.1%-62.6%) during the Delta-dominant period and 80.9% (95% CI 75.0%-86.9%) during the Omicron-dominant period. During the Delta-dominant period, unvaccinated contacts had higher probability of infection than vaccinated contacts (odds ratio 5.42, 95% CI 1.6-18.6), but this effect disappeared at ≈20 weeks after vaccination. Contacts showed a higher relative risk of infection (9.16, 95% CI 3.4-25.0) in the Omicron-dominant than Delta-dominant period when vaccinated within the previous 20 weeks. Our data suggest vaccine evasion might be a cause of rapid spread of the Omicron variant. We recommend a focus on developing vaccines with long-lasting protection against severe disease, rather than only against infectivity.

High secondary attack rate and persistence of SARS-CoV-2 antibodies in household transmission study participants, Finland 2020-2021.

Background: Household transmission studies offer the opportunity to assess both secondary attack rate (SAR) and persistence of SARS-CoV-2 antibodies over time. Methods: In Spring 2020, we invited confirmed COVID-19 cases and their household members to four visits, where we collected nasopharyngeal and serum samples over 28 days after index case onset. We calculated SAR based on the presence of SARS-CoV-2 neutralizing antibodies (NAb) and assessed the persistence of NAb and IgG antibodies (Ab) against SARS-CoV-2 spike glycoprotein and nucleoprotein. Results: SAR was 45% (39/87), including 35 symptomatic secondary cases. During the initial 28-day follow-up, 62% (80/129) of participants developed NAb. Of those that seroconverted, 90% (63/70), 85% (63/74), and 78% (45/58) still had NAb to early B-lineage SARS-CoV-2 3, 6, and 12 months after the onset of the index case. Anti-spike IgG Ab persisted in 100% (69/69), 97% (72/74), and 93% (55/59) of seroconverted participants after 3, 6, and 12 months, while anti-nucleoprotein IgG Ab levels waned faster, persisting in 99% (68/69), 78% (58/74), and 55% (39/71) of participants, respectively. Conclusion: Following detection of a COVID-19 case in a household, other members had a high risk of becoming infected. NAb to early B-lineage SARS-CoV-2 persisted for at least a year in most cases.

Transmission of SARS-CoV-2 among children and staff in German daycare centres.

In daycare centres, the close contact of children with other children and employees favours the transmission of infections. The majority of children <6 years attend daycare programmes in Germany, but the role of daycare centres in the SARS-CoV-2 pandemic is unclear. We investigated the transmission risk in daycare centres and the spread of SARS-CoV-2 to associated households. 30 daycare groups with at least one recent laboratory-confirmed SARS-CoV-2 case were enrolled in the study (10/2020-06/2021). Close contact persons within daycare and households were examined over a 12-day period (repeated SARS-CoV-2 PCR tests, genetic sequencing of viruses, symptom diary). Households were interviewed to gain comprehensive information on each outbreak. We determined primary cases for all daycare groups. The number of secondary cases varied considerably between daycare groups. The pooled secondary attack rate (SAR) across all 30 daycare centres was 9.6%. The SAR tended to be higher when the Alpha variant was detected (15.9% vs. 5.1% with evidence of wild type). The household SAR was 53.3%. Exposed daycare children were less likely to get infected with SARS-CoV-2 than employees (7.7% vs. 15.5%). Containment measures in daycare programmes are critical to reduce SARS-CoV-2 transmission, especially to avoid spread to associated households.

Symptomatic, Presymptomatic, and Asymptomatic Transmission of SARS-CoV-2 in a University Student Population, August-November 2020.

OBJECTIVES: The impact and risk of SARS-CoV-2 transmission from asymptomatic and presymptomatic hosts remains an open question. This study measured the secondary attack rates (SARs) and relative risk (RR) of SARS-CoV-2 transmission from asymptomatic and presymptomatic index cases as compared with symptomatic index cases. METHODS: We used COVID-19 test results, daily health check reports, and contact tracing data to measure SARs and corresponding RRs among close contacts of index cases in a cohort of 12 960 young adults at the University of Notre Dame in Indiana for 103 days, from August 10 to November 20, 2020. Further analysis included Fisher exact tests to determine the association between symptoms and COVID-19 infection and z tests to determine statistical differences between SARs. RESULTS: Asymptomatic rates of transmission of SARS-CoV-2 were higher (SAR = 0.19; 95% CI, 0.14-0.24) than was estimated in prior studies, producing an RR of 0.75 (95% CI, 0.54-1.07) when compared with symptomatic transmission. In addition, the transmission rate associated with presymptomatic cases (SAR = 0.25; 95% CI, 0.21-0.30) was approximately the same as that for symptomatic cases (SAR = 0.25; 95% CI, 0.19-0.31). Furthermore, different symptoms were associated with different transmission rates. CONCLUSIONS: Asymptomatic and presymptomatic hosts of SARS-CoV-2 are a risk for community spread of COVID-19, especially with new variants emerging. Moreover, typical symptom checks may easily miss people who are asymptomatic or presymptomatic but still infectious. Our study results may be used as a guide to analyze the spread of SARS-CoV-2 variants and help inform appropriate public health measures as they relate to asymptomatic and presymptomatic cases.

Increased Secondary Attack Rates among the Household Contacts of Patients with the Omicron Variant of the Coronavirus Disease 2019 in Japan.

This study investigated the household secondary attack rate (HSAR) of patients with coronavirus disease (COVID-19) during the omicron variant-dominant period. The HSAR of COVID-19 cases during the omicron variant-dominant period (4-20 January 2022) was calculated and compared with the delta variant-dominant period (20 August to 7 November 2021) in Itako, Japan. In Itako, all 47 and 119 samples tested during the omicron and delta variant-dominant periods were negative and positive, respectively, for the L452R mutation. We used a generalized estimating equation regression model. The HSAR was 31.8% (95% confidence interval (CI) 27.7-36.2) for 456 household contacts during the omicron variant-dominant period; it was higher than that during the delta variant-dominant period (25.2%) (adjusted risk ratio [aRR] 1.61, CI 1.13-2.28). During the omicron variant-dominant period, HSAR was lower for the household contacts of completely vaccinated index patients (27.3%) than for contacts of other index patients (41.2%) (vaccine effectiveness for infectee 0.43, 95% CI 0.16-0.62) and was significantly higher for female contacts than for male contacts (36.2% vs. 26.1%; aRR 1.29, 95% CI 1.01-1.65). The HSAR was significantly higher during the omicron variant-dominant period than the delta variant-dominant period. The vaccination of index patients might protect household contacts.

Transmission dynamics of COVID-19 in household and community settings in the United Kingdom, January to March 2020.

BackgroundHouseholds appear to be the highest risk setting for COVID-19 transmission. Large household transmission studies in the early stages of the pandemic in Asia reported secondary attack rates ranging from 5 to 30%.AimWe aimed to investigate the transmission dynamics of COVID-19 in household and community settings in the UK.MethodsA prospective case-ascertained study design based on the World Health Organization FFX protocol was undertaken in the UK following the detection of the first case in late January 2020. Household contacts of cases were followed using enhanced surveillance forms to establish whether they developed symptoms of COVID-19, became confirmed cases and their outcomes. We estimated household secondary attack rates (SAR), serial intervals and individual and household basic reproduction numbers. The incubation period was estimated using known point source exposures that resulted in secondary cases.ResultsWe included 233 households with two or more people with 472 contacts. The overall household SAR was 37% (95% CI: 31-43%) with a mean serial interval of 4.67 days, an R0 of 1.85 and a household reproduction number of 2.33. SAR were lower in larger households and highest when the primary case was younger than 18 years. We estimated a mean incubation period of around 4.5 days.ConclusionsRates of COVID-19 household transmission were high in the UK for ages above and under 18 years, emphasising the need for preventative measures in this setting. This study highlights the importance of the FFX protocol in providing early insights on transmission dynamics.

Increased Secondary Attack Rate among Unvaccinated Household Contacts of Coronavirus Disease 2019 Patients with Delta Variant in Japan.

This study aimed to elucidate the household secondary attack rate (HSAR) of the Delta variant in comparison to the Alpha variant, and evaluate the risk factors among unvaccinated household contacts of patients with coronavirus disease 2019 (COVID-19). We studied household contacts of index cases of COVID-19 infected with Delta (L452R mutation), Alpha (N501Y mutation), and wild strain from December 2020 through November 2021 in Itako, Japan. The HSARs of the entire household contact, and the contact of index case with Delta variant were calculated and compared across the risk factors. We used a generalized estimating equation regression model for the multivariate analysis. We enrolled 1257 unvaccinated contacts from 580 households. The HSAR was higher in household contacts of index patients with Delta (48.5%) than with Alpha variant (21.7%) (aOR = 3.34, p = 0.000). In Delta variants, the HSAR was higher in household contacts with spousal relationships to index patients (63.4%) than contacts with other relationships (45.5%) (aOR 1.94, p = 0.026), and was lower in household contacts of index patients aged ≤19 (33.1%) than for contacts of index cases aged 20-59 years (52.6%) (aOR = 0.50, p = 0.027). The result of our study can be used to devise informed strategy to prevent transmission within households.

Secondary Attack Rate, Transmission and Incubation Periods, and Serial Interval of SARS-CoV-2 Omicron Variant, Spain.

Contact tracing data of SARS-CoV-2 Omicron variant cases during December 2021 in Cantabria, Spain, showed increased transmission (secondary attack rate 39%) compared with Delta cases (secondary attack rate 26%), uninfluenced by vaccination status. Incubation and serial interval periods were also reduced. Half of Omicron transmissions happened before symptom onset in the index case-patient.