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Non-pharmaceutical interventions minimize social contacts, hence the spread of SARS-CoV-2. We quantified two-day contact patterns among US employees from 2020-2021 during the COVID-19 pandemic. Contacts were defined as face-to-face conversations, involving physical touch or proximity to another individual and were collected using electronic diaries. Mean (standard deviation) contacts reported by 1,456 participants were 2.5 (2.5), 8.2 (7.1), 9.2 (7.1) and 10.1 (9.5) across round 1 (April-June 2020), 2 (November 2020-January 2021), 3 (June-August 2021), and 4 (November-December 2021), respectively. Between round 1 and 2, we report a 3-fold increase in the mean number of contacts reported per participant with no major increases from round 2-4. We modeled SARS-CoV-2 transmission at home, work, and community. The model revealed reduced relative transmission in all settings in round 1. Subsequently, transmission increased at home and in the community but remained very low in work settings. Contact data are important to parameterize models of infection transmission and control.
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BackgroundBeginning in early February 2020, COVID-19 spread across the state of Georgia, leading to 258,354 cumulative cases as of August 25, 2020. The time scale of spreading (i.e., serial interval) and magnitude of spreading (i.e., Rt or reproduction number) for COVID-19, were observed to be heterogenous by demographic characteristics, region and time period. In this study, we examined the COVID-19 transmission in the state of Georgia, United States. MethodsDuring February 1 to July 13, 2020, we identified 4080 transmission pairs using contact information from reports of COVID-19 cases from the Georgia Department of Public Health. We examined how various transmission characteristics were affected by disease symptoms, demographics (age, gender, and race), and time period (during shelter-in-place and after reopening). In addition, we estimated the time course of reproduction numbers during early February-mid-June for all 159 counties in the state of Georgia, using a total of 118,491 reported COVID-19 cases. FindingsOver this period, the serial interval appeared to decrease from 5.97 days in February-April to 4.40 days in June-July. With regard to age, transmission was assortative and patterns of transmission changed over time. COVID-19 mainly spread from adults to all age groups; transmission among and between children and the elderly was found less frequently. Younger adults (20- 50 years old) were involved in the majority of transmissions occurring during or after reopening subsequent to the shelter-in-place period. By mid-July, two waves of COVID-19 transmission were apparent, separated by the shelter-in-place period in the state of Georgia. Counties around major cities and along interstate highways had more intense transmission. InterpretationThe transmission of COVID-19 in the state of Georgia had been heterogeneous by area and changed over time. The shelter-in-place was not long enough to sufficiently suppress COVID-19 transmission in densely populated urban areas connected by major transportation links. Studying local transmission patterns may help in predicting and guiding states in prevention and control of COVID-19 according to population and region. FundingEmory COVID-19 Response Collaborative. Research in context Evidence before this studyThe ongoing COVID-19 pandemic has caused 37,109,581 cases and 1,070,355 deaths worldwide as of October 11, 2020. We searched PubMed for articles published on and before October 11, 2020 using keywords "novel coronavirus", "SARS-nCoV-2", "COVID-19", "transmission", "serial interval", "reproduction number", and "shelter-in-place". Few published studies have estimated the serial interval but no study was found that examined the time-varying serial interval. Few studies have examined the transmission patterns between groups with different characteristics. And no study has examined the timevarying reproduction number for COVID-19 and impact of shelter-in-place order at the county level in the United States. Added value of this studyTo our knowledge, this is the first study showing the multiple aspects of COVID-19 transmission, including serial interval, transmission patterns between age, gender, or race groups, and spatiotemporal patterns, based on data from 118,491 confirmed COVID-19 cases and 4080 tracked pairs of infector and infectee. We found that during February-July the serial interval for symptom onset shortened, and the major contribution to the spread of COVID-19 shifted to younger ages (from 40-70 years old in February-April to 20-50 years old in June-July). We also found three to four weeks of the shelter-in-place slowed transmission but was insufficient to prevent transmission into urban and peri-urban counties connected with major transportation. Implications of all the available evidenceThe contracting serial intervals and increasing spread by younger generation show the COVID-19 transmission at county level changes over time. The spatiotemporal patterns of transmission in county level further provide important evidence to guide effective COVID-19 prevention and control measures (e.g., shelter-in-place) in different areas.
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2.ImportanceDevising control strategies against diseases such as COVID-19 require understanding of contextual social mixing and contact patterns. There has been no standardized multi-site social contact study conducted in workplace settings in the United States that can be used to broadly inform pandemic preparedness policy in these settings. ObjectiveThe study aimed to characterize the patterns of social contacts and mixing across workplace environments, including on-site or when teleworking. DesignThis was a cross-sectional non-probability survey that used standardized social contact diaries to collect data. Employees were requested to record their physical and non-physical contacts in a diary over two consecutive days, documented at the end of each day. Employees from each company were enrolled through email and electronic diaries sent as individual links. Data were collected from April to June 2020. SettingTwo multinational consulting companies and one university administrative department, all located in Georgia, USA. ParticipantsEmployees opted into the study by accepting the invitation on a link sent via email. Main OutcomeThe outcome was median number of contacts per person per day. This was stratified by day of data collection, age, sex, race and ethnicity. ResultsOf 3,835 employees approached, 357 (9.3%) completed the first day of contact diary of which 304 completed both days of contact diary. There was a median of 2 contacts (IQR: 1-4, range: 0-21) per respondent on both day one and two. The majority (55%) of contacts involved conversation only, occurred at home (64%), and cumulatively lasted more than 4 hours (38%). Most contacts were repeated, and within same age groups, though participants aged 30-59 years reported substantial inter-generational mixing with children. ConclusionParticipating employees in 3 surveyed workplaces reported few contacts, similar to studies from the UK and China when shelter-in-place orders were in effect during the pandemic. Many contacts were repeated which may limit the spread of infection. Future rounds are planned to assess changes in contact patterns when employees resume work in the office after the lockdown due to COVID-19 pandemic.
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Many months into the SARS-CoV-2 pandemic, basic epidemiologic parameters describing burden of disease are lacking. To reduce selection bias in current burden of disease estimates derived from diagnostic testing data or serologic testing in convenience samples, we are conducting a national probability-based sample SARS-CoV-2 serosurvey. Sampling from a national address-based frame and using mailed recruitment materials and test kits will allow us to estimate national prevalence of SARS-CoV-2 infection and antibodies, overall and by demographic, behavioral, and clinical characteristics. Data will be weighted for unequal selection probabilities and non-response and will be adjusted to population benchmarks. Due to the urgent need for these estimates, expedited interim weighting of serosurvey responses will be undertaken to produce early release estimates, which will be published on the study website, COVIDVu.org. Here, we describe a process for computing interim survey weights and guidelines for release of interim estimates.
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Significant progress has already been made in development and testing of SARS-CoV-2 vaccines, and Phase III clinical trials have begun for 6 novel vaccine candidates to date. These Phase III trials seek to demonstrate direct benefits of a vaccine on vaccine recipients. However, vaccination is also known to bring about indirect benefits to a population through the reduction of virus circulation. The indirect effects of SARS-CoV-2 vaccination can play a key role in reducing case counts and COVID-19 deaths. To illustrate this point, we show through simulation that a vaccine with strong indirect effects has the potential to reduce SARS-CoV-2 circulation and COVID-19 deaths to a greater extent than an alternative vaccine with stronger direct effects but weaker indirect effects. Protection via indirect effects may be of particular importance in the context of this virus, because elderly individuals are at an elevated risk of death but are also less likely to be directly protected by vaccination due to immune senescence. We therefore encourage ongoing data collection and model development aimed at evaluating the indirect effects of forthcoming SARS-CoV-2 vaccines.
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It is imperative to advance our understanding of heterogeneities in the transmission of SARS-CoV-2 such as age-specific infectiousness and super-spreading. To this end, it is important to exploit multiple data streams that are becoming abundantly available during the pandemic. In this paper, we formulate an individual-level spatio-temporal mechanistic framework to integrate individual surveillance data with geo-location data and aggregate mobility data, enabling a more granular understanding of the transmission dynamics of SARS-CoV-2. We analyze reported cases, between March and early May 2020, in five (urban and rural) counties in the State of Georgia USA. First, our results show that the reproductive number reduced to below 1 in about two weeks after the shelter-in-place order. Super-spreading appears to be widespread across space and time, and it may have a particularly important role in driving the outbreak in rural areas and an increasing importance towards later stages of outbreaks in both urban and rural settings. Overall, about 2% of cases were directly responsible for 20% of all infections. We estimate that the infected non-elderly cases (<60) may be 2.78 [2.10, 4.22] times more infectious than the elderly, and the former tend to be the main driver of super-spreading. Our results improve our understanding of the natural history and transmission dynamics of SARS-CoV-2. More importantly, we reveal the roles of age-specific infectiousness and characterize systematic variations and associated risk factors of super-spreading. These have important implications for the planning of relaxing social distancing and, more generally, designing optimal control measures. Significance StatementThere is still considerable scope for advancing our understanding of the epidemiology and ecology of COVID-19. In particular, much is unknown about individual-level transmission heterogeneities such as super-spreading and age-specific infectiousness. We statistically synthesize multiple valuable datastreams, including surveillance data and mobility data, that are available during the current COVID-19 pandemic. We show that age is an important factor in the transmission of the virus. Super-spreading is ubiquitous over space and time, and has particular importance in rural areas and later stages of an outbreak. Our results improve our understanding of the natural history the virus and have important implications for designing optimal control measures.
