Reducing Transmission of Airborne Respiratory Pathogens: A New Beginning as the COVID-19 Emergency Ends.

BACKGROUND: In response to the COVID-19 pandemic, new evidence-based strategies have emerged for reducing transmission of respiratory infections through management of indoor air. OBJECTIVES: This paper reviews critical advances that could reduce the burden of disease from inhaled pathogens and describes challenges in their implementation. DISCUSSION: Proven strategies include assuring sufficient ventilation, air cleaning by filtration, and air disinfection by germicidal ultraviolet (UV) light. Layered intervention strategies are needed to maximize risk reduction. Case studies demonstrate how to implement these tools while also revealing barriers to implementation. Future needs include standards designed with infection resilience and equity in mind, buildings optimized for infection resilience among other drivers, new approaches and technologies to improve ventilation, scientific consensus on the amount of ventilation needed to achieve a desired level of risk, methods for evaluating new air-cleaning technologies, studies of their long-term health effects, workforce training on ventilation systems, easier access to federal funds, demonstration projects in schools, and communication with the public about the importance of indoor air quality and actions people can take to improve it. https://doi.org/10.1289/EHP13878.

Alert for pharynx-centered gastrointestinal and respiratory cross-infection and cryptic cross-transmission routes of 2019-nCoV.

We proposed that the pharynx, as a common organ of the respiratory and digestive tracts, may be a respiratory and digestive tract cross cryptic transmission pathway for 2019-nCoV infection from the nasal cavities to the pharynx and lung, then to nasal cavities by aerosol (respiratory route) to the pharynx and the gastrointestinal tract, then to the oral cavity by feces (fecal-oral route) and to pharynx, lungs, or gastrointestinal tract.

Evaluating Seasonal Variations in Human Contact Patterns and Their Impact on the Transmission of Respiratory Infectious Diseases.

BACKGROUND: Human contact patterns are a key determinant driving the spread of respiratory infectious diseases. However, the relationship between contact patterns and seasonality as well as their possible association with the seasonality of respiratory diseases is yet to be clarified. METHODS: We investigated the association between temperature and human contact patterns using data collected through a cross-sectional diary-based contact survey in Shanghai, China, between December 24, 2017, and May 30, 2018. We then developed a compartmental model of influenza transmission informed by the derived seasonal trends in the number of contacts and validated it against A(H1N1)pdm09 influenza data collected in Shanghai during the same period. RESULTS: We identified a significant inverse relationship between the number of contacts and the seasonal temperature trend defined as a spline interpolation of temperature data (p = 0.003). We estimated an average of 16.4 (95% PrI: 15.1-17.5) contacts per day in December 2017 that increased to an average of 17.6 contacts (95% PrI: 16.5-19.3) in January 2018 and then declined to an average of 10.3 (95% PrI: 9.4-10.8) in May 2018. Estimates of influenza incidence obtained by the compartmental model comply with the observed epidemiological data. The reproduction number was estimated to increase from 1.24 (95% CI: 1.21-1.27) in December to a peak of 1.34 (95% CI: 1.31-1.37) in January. The estimated median infection attack rate at the end of the season was 27.4% (95% CI: 23.7-30.5%). CONCLUSIONS: Our findings support a relationship between temperature and contact patterns, which can contribute to deepen the understanding of the relationship between social interactions and the epidemiology of respiratory infectious diseases.

Incidence and transmission associated with respiratory viruses in an acute care facility: An observational study.

We estimated the extent of respiratory virus transmission over three pre-COVID-19 seasons. Of 16,273 assays, 22.9% (3,726) detected ≥1 respiratory virus. The frequency of putatively hospital-acquired infection ranged from 6.9% (influenza A/B) to 24.7% (adenovirus). The 176 clusters were most commonly associated with rhinovirus/enterovirus (70) and influenza A/B (62).

Simulating respiratory disease transmission within and between classrooms to assess pandemic management strategies at schools.

The global spread of coronavirus disease 2019 (COVID-19) has emphasized the need for evidence-based strategies for the safe operation of schools during pandemics that balance infection risk with the society's responsibility of allowing children to attend school. Due to limited empirical data, existing analyses assessing school-based interventions in pandemic situations often impose strong assumptions, for example, on the relationship between class size and transmission risk, which could bias the estimated effect of interventions, such as split classes and staggered attendance. To fill this gap in school outbreak studies, we parameterized an individual-based model that accounts for heterogeneous contact rates within and between classes and grades to a multischool outbreak data of influenza. We then simulated school outbreaks of respiratory infectious diseases of ongoing threat (i.e., COVID-19) and potential threat (i.e., pandemic influenza) under a variety of interventions (changing class structures, symptom screening, regular testing, cohorting, and responsive class closures). Our results suggest that interventions changing class structures (e.g., reduced class sizes) may not be effective in reducing the risk of major school outbreaks upon introduction of a case and that other precautionary measures (e.g., screening and isolation) need to be employed. Class-level closures in response to detection of a case were also suggested to be effective in reducing the size of an outbreak.

Hybrid measurement of respiratory aerosol reveals a dominant coarse fraction resulting from speech that remains airborne for minutes.

Accurate measurements of the size and quantity of aerosols generated by various human activities in different environments are required for efficacious mitigation strategies and accurate modeling of respiratory disease transmission. Previous studies of speech droplets, using standard aerosol instrumentation, reported very few particles larger than 5 µm. This starkly contrasts with the abundance of such particles seen in both historical slide deposition measurements and more recent light scattering observations. We have reconciled this discrepancy by developing an alternative experimental approach that addresses complications arising from nucleated condensation. Measurements reveal that a large volume fraction of speech-generated aerosol has diameters in the 5- to 20-µm range, making them sufficiently small to remain airborne for minutes, not hours. This coarse aerosol is too large to penetrate the lower respiratory tract directly, and its relevance to disease transmission is consistent with the vast majority of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections initiating in the upper respiratory tract. Our measurements suggest that in the absence of symptoms such as coughing or sneezing, the importance of speech-generated aerosol in the transmission of respiratory diseases is far greater than generally recognized.

A video-based analysis of situations bearing the risk of respiratory disease transmission during football matches.

We aimed to analyze the number and type of contacts involving the risk of respiratory disease transmission during football match play. We analysed 50 matches from different playing levels. Two reviewers evaluated the contacts of all players in each match. We focused on between-player contacts, crowding, actions with potentially increased aerosol and droplet production and within-player hand-to-head contacts. We categorized the duels with direct contact into frontal and other ones and measured contact duration. The number of between-player contacts were similar between playing levels (median 28.3 [IQR 22.6, 33] contacts per player-hour). Frontal contacts summed up to 8% of all contacts. Contacts involving the head occurred less than once per player and match with none lasting longer than 3 s. Crowding included between two and six players and the duration was mostly less than 10 s. Aerosol and droplet producing activities were three to four times more frequent in adult compared to youth players. Our results suggest that the risk of respiratory pathogen transmission is low during football matches. This conclusion is based on the finding that most close contact situations are of short duration and on the fact that it is an outdoor sport.

Poor ventilation worsens short-range airborne transmission of respiratory infection.

To explain the observed phenomenon that most SARS-CoV-2 transmission occurs indoors whereas its outdoor transmission is rare, a simple macroscopic aerosol balance model is developed to link short- and long-range airborne transmission. The model considers the involvement of exhaled droplets with initial diameter ≤50 µm in the short-range airborne route, whereas only a fraction of these droplets with an initial diameter within 15 µm or equivalently a final diameter within 5 µm considered in the long-range airborne route. One surprising finding is that the room ventilation rate significantly affects the short-range airborne route, in contrast to traditional belief. When the ventilation rate in a room is insufficient, the airborne infection risks due to both short- and long-range transmission are high. A ventilation rate of 10 L/s per person provides a similar concentration vs distance decay profile to that in outdoor settings, which provides additional justification for the widely adopted ventilation standard of 10 L/s per person. The newly obtained data do not support the basic assumption in the existing ventilation standard ASHRAE 62.1 (2019) that the required people outdoor air rate is constant if the standard is used directly for respiratory infection control. Instead, it is necessary to increase the ventilation rate when the physical distance between people is less than approximately 2 m.

Health Care-Acquired Viral Respiratory Diseases.

Health care-acquired viral respiratory infections are common and cause increased patient morbidity and mortality. Although the threat of viral respiratory infection has been underscored by the coronavirus disease 2019 (COVID-19) pandemic, respiratory viruses have a significant impact in health care settings even under normal circumstances. Studies report decreased nosocomial transmission when aggressive infection control measures are implemented, with more success noted when using a multicomponent approach. Influenza vaccination of health care personnel furthers decrease rates of transmission; thus, mandatory vaccination is becoming more common. This article discusses the epidemiology, transmission, and control of health care-associated respiratory viral infections.

Current Insights Into Respiratory Virus Transmission and Potential Implications for Infection Control Programs : A Narrative Review.

Policies to prevent respiratory virus transmission in health care settings have traditionally divided organisms into Droplet versus Airborne categories. Droplet organisms (for example, influenza) are said to be transmitted via large respiratory secretions that rapidly fall to the ground within 1 to 2 meters and are adequately blocked by surgical masks. Airborne pathogens (for example, measles), by contrast, are transmitted by aerosols that are small enough and light enough to carry beyond 2 meters and to penetrate the gaps between masks and faces; health care workers are advised to wear N95 respirators and to place these patients in negative-pressure rooms. Respirators and negative-pressure rooms are also recommended when caring for patients with influenza or SARS-CoV-2 who are undergoing "aerosol-generating procedures," such as intubation. An increasing body of evidence, however, questions this framework. People routinely emit respiratory particles in a range of sizes, but most are aerosols, and most procedures do not generate meaningfully more aerosols than ordinary breathing, and far fewer than coughing, exercise, or labored breathing. Most transmission nonetheless occurs at close range because virus-laden aerosols are most concentrated at the source; they then diffuse and dilute with distance, making long-distance transmission rare in well-ventilated spaces. The primary risk factors for nosocomial transmission are community incidence rates, viral load, symptoms, proximity, duration of exposure, and poor ventilation. Failure to appreciate these factors may lead to underappreciation of some risks (for example, overestimation of the protection provided by medical masks, insufficient attention to ventilation) or misallocation of limited resources (for example, reserving N95 respirators and negative-pressure rooms only for aerosol-generating procedures or requiring negative-pressure rooms for all patients with SARS-CoV-2 infection regardless of stage of illness). Enhanced understanding of the factors governing respiratory pathogen transmission may inform the development of more effective policies to prevent nosocomial transmission of respiratory pathogens.

Within and between classroom transmission patterns of seasonal influenza among primary school students in Matsumoto city, Japan.

Schools play a central role in the transmission of many respiratory infections. Heterogeneous social contact patterns associated with the social structures of schools (i.e., classes/grades) are likely to influence the within-school transmission dynamics, but data-driven evidence on fine-scale transmission patterns between students has been limited. Using a mathematical model, we analyzed a large-scale dataset of seasonal influenza outbreaks in Matsumoto city, Japan, to infer social interactions within and between classes/grades from observed transmission patterns. While the relative contribution of within-class and within-grade transmissions to the reproduction number varied with the number of classes per grade, the overall within-school reproduction number, which determines the initial growth of cases and the risk of sustained transmission, was only minimally associated with class sizes and the number of classes per grade. This finding suggests that interventions that change the size and number of classes, e.g., splitting classes and staggered attendance, may have a limited effect on the control of school outbreaks. We also found that vaccination and mask-wearing of students were associated with reduced susceptibility (vaccination and mask-wearing) and infectiousness (mask-wearing), and hand washing was associated with increased susceptibility. Our results show how analysis of fine-grained transmission patterns between students can improve understanding of within-school disease dynamics and provide insights into the relative impact of different approaches to outbreak control.

Rapid transmission of respiratory infections within but not between mountain gorilla groups.

Minimizing disease transmission between humans and wild apes and controlling outbreaks in ape populations is vital to both ape conservation and human health, but information on the transmission of real infections in wild populations is rare. We analyzed respiratory outbreaks in a subpopulation of wild mountain gorillas (Gorilla beringei beringei) between 2004 and 2020. We investigated transmission within groups during 7 outbreaks using social networks based on contact and proximity, and transmission between groups during 15 outbreaks using inter-group encounters, transfers and home range overlap. Patterns of contact and proximity within groups were highly predictable based on gorillas' age and sex. Disease transmission within groups was rapid with a median estimated basic reproductive number (R0) of 4.18 (min = 1.74, max = 9.42), and transmission was not predicted by the social network. Between groups, encounters and transfers did not appear to have enabled disease transmission and the overlap of groups' ranges did not predict concurrent outbreaks. Our findings suggest that gorilla social structure, with many strong connections within groups and weak ties between groups, may enable rapid transmission within a group once an infection is present, but limit the transmission of infections between groups.

Universal risk phenotype of US counties for flu-like transmission to improve county-specific COVID-19 incidence forecasts.

The spread of a communicable disease is a complex spatio-temporal process shaped by the specific transmission mechanism, and diverse factors including the behavior, socio-economic and demographic properties of the host population. While the key factors shaping transmission of influenza and COVID-19 are beginning to be broadly understood, making precise forecasts on case count and mortality is still difficult. In this study we introduce the concept of a universal geospatial risk phenotype of individual US counties facilitating flu-like transmission mechanisms. We call this the Universal Influenza-like Transmission (UnIT) score, which is computed as an information-theoretic divergence of the local incidence time series from an high-risk process of epidemic initiation, inferred from almost a decade of flu season incidence data gleaned from the diagnostic history of nearly a third of the US population. Despite being computed from the past seasonal flu incidence records, the UnIT score emerges as the dominant factor explaining incidence trends for the COVID-19 pandemic over putative demographic and socio-economic factors. The predictive ability of the UnIT score is further demonstrated via county-specific weekly case count forecasts which consistently outperform the state of the art models throughout the time-line of the COVID-19 pandemic. This study demonstrates that knowledge of past epidemics may be used to chart the course of future ones, if transmission mechanisms are broadly similar, despite distinct disease processes and causative pathogens.

SARS-CoV-2: lessons from both the history of medicine and from the biological behavior of other well-known viruses.

SARS-CoV-2 is the etiological agent of the current pandemic worldwide and its associated disease COVID-19. In this review, we have analyzed SARS-CoV-2 characteristics and those ones of other well-known RNA viruses viz. HIV, HCV and Influenza viruses, collecting their historical data, clinical manifestations and pathogenetic mechanisms. The aim of the work is obtaining useful insights and lessons for a better understanding of SARS-CoV-2. These pathogens present a distinct mode of transmission, as SARS-CoV-2 and Influenza viruses are airborne, whereas HIV and HCV are bloodborne. However, these viruses exhibit some potential similar clinical manifestations and pathogenetic mechanisms and their understanding may contribute to establishing preventive measures and new therapies against SARS-CoV-2.

Efficacy of face masks against respiratory infectious diseases: a systematic review and network analysis of randomized-controlled trials.

During the ongoing COVID-19 pandemic, face masks are among the most common and practical control measures used globally in reducing the risk of infection and disease transmission. Although several studies have investigated the efficacy of various face masks and respirators in preventing infection, the results have been inconsistent. Therefore, we performed a systematic review and network meta-analysis (NMA) of the randomized-controlled trials (RCTs) to assess the actual efficacy of face masks in preventing respiratory infections. We searched nine electronic databases up to July 2020 to find potential articles. We accepted trials reporting the protective efficacy of face masks against respiratory infections, of which the primary endpoint was the presence of respiratory infections. We used the ROB-2 Cochrane tool to grade the trial quality. We initially registered the protocol for this study in PROSPERO (CRD42020178516). Sixteen RCTs involving 17 048 individuals were included for NMA. Overall, evidence was weak, lacking statistical power due to the small number of participants, and there was substantial inconsistency in our findings. In comparison to those without face masks, participants with fit-tested N95 respirators were likely to have lesser infection risk (RR 0.67, 95% CI 0.38-1.19,P-score 0.80), followed by those with non-fit-tested N95 and non-fit-tested FFP2 respirators that shared the similar risk, (RR 0.73, 95% CI 0.12-4.36,P-score 0.63) and (RR 0.80, 95% CI 0.38-1.71,P-score 0.63), respectively. Next, participants who donned face masks with and without hand hygiene practices showed modest risk improvement alike (RR 0.89, 95% CI 0.67-1.17,P-score 0.55) and (RR 0.92, 95% CI 0.70-1.22,P-score 0.51). Otherwise, participants donning double-layered cloth masks were prone to infection (RR 4.80, 95% CI 1.42-16.27,P-score 0.01). Eleven out of 16 RCTs that underwent a pairwise meta-analysis revealed a substantially lower infection risk in those donning medical face masks (MFMs) than those without face masks (RR 0.83 95% CI 0.71-0.96). Given the body of evidence through a systematic review and meta-analyses, our findings supported the protective benefits of MFMs in reducing respiratory transmissions, and the universal mask-wearing should be applied-especially during the COVID-19 pandemic. More clinical data is required to conclude the efficiency of cloth masks; in the short term, users should not use cloth face masks in the outbreak hot spots and places where social distancing is impossible.

Airborne transmission of respiratory viruses.

The COVID-19 pandemic has revealed critical knowledge gaps in our understanding of and a need to update the traditional view of transmission pathways for respiratory viruses. The long-standing definitions of droplet and airborne transmission do not account for the mechanisms by which virus-laden respiratory droplets and aerosols travel through the air and lead to infection. In this Review, we discuss current evidence regarding the transmission of respiratory viruses by aerosols-how they are generated, transported, and deposited, as well as the factors affecting the relative contributions of droplet-spray deposition versus aerosol inhalation as modes of transmission. Improved understanding of aerosol transmission brought about by studies of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection requires a reevaluation of the major transmission pathways for other respiratory viruses, which will allow better-informed controls to reduce airborne transmission.

Pediatric Household Transmission of Severe Acute Respiratory Coronavirus-2 Infection-Los Angeles County, December 2020 to February 2021.

This brief report presents transmission rates from a prospective study of 15 households with pediatric index cases of severe acute respiratory coronavirus-2 in Los Angeles County from December 2020 to February 2021. Our findings support ongoing evidence that transmission from pediatric index cases to household contacts is frequent but can be mitigated with practicing well-documented control measures at home, including isolation, masking and good hand hygiene.