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Little is known about the amount and infectiousness of influenza virus shed into exhaled breath. This contributes to uncertainty about the importance of airborne influenza transmission. We screened 355 symptomatic volunteers with acute respiratory illness and report 142 cases with confirmed influenza infection who provided 218 paired nasopharyngeal (NP) and 30-minute breath samples (coarse >5-μm and fine ≤5-μm fractions) on days 1–3 after symptom onset. We assessed viral RNA copy number for all samples and cultured NP swabs and fine aerosols. We recovered infectious virus from 52 (39%) of the fine aerosols and 150 (89%) of the NP swabs with valid cultures. The geometric mean RNA copy numbers were 3.8 × 10⁴/30-minutes fine-, 1.2 × 10⁴/30-minutes coarse-aerosol sample, and 8.2 × 10⁸ per NP swab. Fine- and coarse-aerosol viral RNA were positively associated with body mass index and number of coughs and negatively associated with increasing days since symptom onset in adjusted models. Fine-aerosol viral RNA was also positively associated with having influenza vaccination for both the current and prior season. NP swab viral RNA was positively associated with upper respiratory symptoms and negatively associated with age but was not significantly associated with fine- or coarse-aerosol viral RNA or their predictors. Sneezing was rare, and sneezing and coughing were not necessary for infectious aerosol generation. Our observations suggest that influenza infection in the upper and lower airways are compartmentalized and independent.

The CDC recommends that healthcare settings provide influenza patients with facemasks as a means of reducing transmission to staff and other patients, and a recent report suggested that surgical masks can capture influenza virus in large droplet spray. However, there is minimal data on influenza virus aerosol shedding, the infectiousness of exhaled aerosols, and none on the impact of facemasks on viral aerosol shedding from patients with seasonal influenza. We collected samples of exhaled particles (one with and one without a facemask) in two size fractions (\"coarse\">5 µm, \"fine\"≤5 µm) from 37 volunteers within 5 days of seasonal influenza onset, measured viral copy number using quantitative RT-PCR, and tested the fine-particle fraction for culturable virus. Fine particles contained 8.8 (95% CI 4.1 to 19) fold more viral copies than did coarse particles. Surgical masks reduced viral copy numbers in the fine fraction by 2.8 fold (95% CI 1.5 to 5.2) and in the coarse fraction by 25 fold (95% CI 3.5 to 180). Overall, masks produced a 3.4 fold (95% CI 1.8 to 6.3) reduction in viral aerosol shedding. Correlations between nasopharyngeal swab and the aerosol fraction copy numbers were weak (r = 0.17, coarse; r = 0.29, fine fraction). Copy numbers in exhaled breath declined rapidly with day after onset of illness. Two subjects with the highest copy numbers gave culture positive fine particle samples. Surgical masks worn by patients reduce aerosols shedding of virus. The abundance of viral copies in fine particle aerosols and evidence for their infectiousness suggests an important role in seasonal influenza transmission. Monitoring exhaled virus aerosols will be important for validation of experimental transmission studies in humans.

The current practice for diagnosis of COVID-19, based on SARS-CoV-2 PCR testing of pharyngeal or respiratory specimens in a symptomatic patient at high epidemiologic risk, likely underestimates the true prevalence of infection. Serologic methods can more accurately estimate the disease burden by detecting infections missed by the limited testing performed to date. Here, we describe the validation of a coronavirus antigen microarray containing immunologically significant antigens from SARS-CoV-2, in addition to SARS-CoV, MERS-CoV, common human coronavirus strains, and other common respiratory viruses. A comparison of antibody profiles detected on the array from control sera collected prior to the SARS-CoV-2 pandemic versus convalescent blood specimens from virologically confirmed COVID-19 cases demonstrates near complete discrimination of these two groups, with improved performance from use of antigen combinations that include both spike protein and nucleoprotein. This array can be used as a diagnostic tool, as an epidemiologic tool to more accurately estimate the disease burden of COVID-19, and as a research tool to correlate antibody responses with clinical outcomes. COVID-19 diagnosis is commonly performed by PCR testing, however, serologic methods are more accurate and versatile for monitoring disease burden and epidemiology. Here the authors report a protein microarray with antigens from SARS-CoV-2, SARS-CoV, MERS-CoV as well as common human respiratory viruses.

Background Nasally inoculated influenza cases reported milder symptoms and shed lower viral RNA load in exhaled breath aerosols (EBA) than people with classic influenza‐like illness in a previous study. Whether nasally inoculated influenza is representative of mild natural influenza infection is unknown. We extend previous analyses to include a broader range of community‐acquired cases. Methods We previously studied (A) volunteers intranasally inoculated with a dose of 5.5 log10TCID50 of influenza A/Wisconsin/67/2005 (H3N2) and (B) cases with classic influenza‐like illness including fever recruited in 2013. We now add (C) cases from a 2017–2019 surveillance cohort of college dormitory residents and their contacts and (D) cases from a university health center in 2019. All cases had an influenza A(H3) infection. We collected 30‐min EBA samples using a Gesundheit‐II sampler. Results Community‐acquired cases from the surveillance cohort (C) shed more EBA viral RNA and were more symptomatic than the inoculated cases (A) but shed less viral RNA than the symptom‐selected natural cases (B) from 2013, but not (D) from 2019. Despite similar symptoms to the 2013 selected cases (B), the 2019 community‐acquired cases (D) recruited post‐infection had lower fine aerosol viral RNA. Conclusions Nasal inoculation of influenza virus did not reproduce EBA viral RNA shedding or symptoms observed in mild natural infection. Circulating strains of influenza A(H3) may differ year‐to‐year in the extent to which symptomatic cases shed virus into fine aerosols. New models, including possibly aerosol inoculation, are needed to study viral aerosol shedding from the human respiratory tract.

Background The SARS‐CoV‐2 pandemic focused attention on airborne‐inhalation transmission and building ventilation. However, investment in solutions lags because few epidemiologic studies demonstrate a causal effect of ventilation on acute respiratory infection (ARI) transmission. This highlights a need for improved study designs to support causal inference. Methods To investigate the potential for causal inference, we analyzed prospective cohorts residing in a high‐ventilation (HVent, ≥ 5 L/s per person) or a neighboring low‐ventilation (LVent, < 5 L/s per person) college residence hall during two spring semesters (2018 and 2019). Swab samples, analyzed using a PCR panel for respiratory pathogens, were collected based on self‐reported symptoms and contacts. Our analysis focused on roommate pairs where both had been tested within a 2‐week period. Roommate pairs with concordant positive PCR results were categorized as possible transmission events. We used genetic sequencing and phylogenetic analysis to identify probable transmission clusters and events. Results We analyzed data from 368 cohort participants (82 HVent and 286 LVent), including 60 person‐infections, with a trend toward 54% lower ARI risk among students living in HVent versus LVent residence halls. We identified 97 roommate pairs, 64 two‐week intervals where both members were tested, 36 (2 HVent and 34 LVent) intervals with ≥ 1 infection, and four possible transmission events (all LVent). Sequence data available for two of the four events confirmed one probable transmission cluster and one probable transmission event. Conclusions Future college dorm transmission studies should prioritize enrolling roommates rather than individuals, measuring ventilation, and confirming transmission events through whole genome sequencing.

Recent studies suggest that humans exhale fine particles during tidal breathing but little is known of their composition, particularly during infection. We conducted a study of influenza infected patients to characterize influenza virus and particle concentrations in their exhaled breath. Patients presenting with influenza-like-illness, confirmed influenza A or B virus by rapid test, and onset within 3 days were recruited at three clinics in Hong Kong, China. We collected exhaled breath from each subject onto Teflon filters and measured exhaled particle concentrations using an optical particle counter. Filters were analyzed for influenza A and B viruses by quantitative polymerase chain reaction (qPCR). Twelve out of thirteen rapid test positive patients provided exhaled breath filter samples (7 subjects infected with influenza B virus and 5 subjects infected with influenza A virus). We detected influenza virus RNA in the exhaled breath of 4 (33%) subjects--three (60%) of the five patients infected with influenza A virus and one (14%) of the seven infected with influenza B virus. Exhaled influenza virus RNA generation rates ranged from <3.2 to 20 influenza virus RNA particles per minute. Over 87% of particles exhaled were under 1 microm in diameter. These findings regarding influenza virus RNA suggest that influenza virus may be contained in fine particles generated during tidal breathing, and add to the body of literature suggesting that fine particle aerosols may play a role in influenza transmission.

Uncertainty about the importance of influenza transmission by airborne droplet nuclei generates controversy for infection control. Human challenge-transmission studies have been supported as the most promising approach to fill this knowledge gap. Healthy, seronegative volunteer 'Donors' (n = 52) were randomly selected for intranasal challenge with influenza A/Wisconsin/67/2005 (H3N2). 'Recipients' randomized to Intervention (IR, n = 40) or Control (CR, n = 35) groups were exposed to Donors for four days. IRs wore face shields and hand sanitized frequently to limit large droplet and contact transmission. One transmitted infection was confirmed by serology in a CR, yielding a secondary attack rate of 2.9% among CR, 0% in IR (p = 0.47 for group difference), and 1.3% overall, significantly less than 16% (p<0.001) expected based on a proof-of-concept study secondary attack rate and considering that there were twice as many Donors and days of exposure. The main difference between these studies was mechanical building ventilation in the follow-on study, suggesting a possible role for aerosols.

Influenza is a highly contagious respiratory illness that commonly causes outbreaks among human communities. Details about the exact nature of the droplets produced by human respiratory activities such as breathing, and their potential to carry and transmit influenza A and B viruses is still not fully understood. The objective of our study was to characterize and quantify influenza viral shedding in exhaled aerosols from natural patient breath, and to determine their viral infectivity among participants in a university cohort in tropical Singapore. Using the Gesundheit-II exhaled breath sampling apparatus, samples of exhaled breath of two aerosol size fractions (“coarse” > 5 µm and “fine” ≤ 5 µm) were collected and analyzed from 31 study participants, i.e., 24 with influenza A (including H1N1 and H3N2 subtypes) and 7 with influenza B (including Victoria and Yamagata lineages). Influenza viral copy number was quantified using reverse transcription-quantitative polymerase chain reaction (RT-qPCR). Infectivity of influenza virus in the fine particle fraction was determined by culturing in Madin–Darby canine kidney cells. Exhaled influenza virus RNA generation rates ranged from 9 to 1.67 × 105 and 10 to 1.24 × 104 influenza virus RNA copies per minute for the fine and coarse aerosol fractions, respectively. Compared to the coarse aerosol fractions, influenza A and B viruses were detected more frequently in the fine aerosol fractions that harbored 12-fold higher viral loads. Culturable virus was recovered from the fine aerosol fractions from 9 of the 31 subjects (29%). These findings constitute additional evidence to reiterate the important role of fine aerosols in influenza transmission and provide a baseline range of influenza virus RNA generation rates.

What does it mean to describe an infection as having airborne transmission, and what are the clinical implications? There is a fitting symmetry between the report by Yu et al. about airborne transmission of the severe acute respiratory syndrome (SARS) in this issue of the Journal (pages 1731–1739) and John Snow's investigation of a cholera epidemic 150 years ago. Snow's independent investigation tested the hypothesis that cholera was waterborne. The official investigation by the General Board of Health in England, however, concluded that transmission in the epidemic was airborne, caused by nocturnal vapors emanating from the Thames River — a . . .

We identified seasonal human coronaviruses, influenza viruses and rhinoviruses in exhaled breath and coughs of children and adults with acute respiratory illness. Surgical face masks significantly reduced detection of influenza virus RNA in respiratory droplets and coronavirus RNA in aerosols, with a trend toward reduced detection of coronavirus RNA in respiratory droplets. Our results indicate that surgical face masks could prevent transmission of human coronaviruses and influenza viruses from symptomatic individuals. A study of 246 individuals with seasonal respiratory virus infections randomized to wear or not wear a surgical face mask showed that masks can significantly reduce detection of coronavirus and influenza virus in exhaled breath and may help interrupt virus transmission.