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The SARS‐CoV‐2 virus is highly contagious, as demonstrated by numerous well‐documented superspreading events. The infection commonly starts in the upper respiratory tract (URT) but can migrate to the lower respiratory tract (LRT) and other organs, often with severe consequences. Whereas LRT infection can lead to shedding of virus via breath and cough droplets, URT infection enables shedding via abundant speech droplets. Their viral load can be high in carriers with mild or no symptoms, an observation linked to the abundance of SARS‐CoV‐2‐susceptible cells in the oral cavity epithelium. Expelled droplets rapidly lose water through evaporation, with the smaller ones transforming into long‐lived aerosol. Although the largest speech droplets can carry more virions, they are few in number, fall to the ground rapidly and therefore play a relatively minor role in transmission. Of more concern is small speech aerosol, which can descend deep into the LRT and cause severe disease. However, since their total volume is small, the amount of virus they carry is low. Nevertheless, in closed environments with inadequate ventilation, they can accumulate, which elevates the risk of direct LRT infection. Of most concern is the large fraction of speech aerosol that is intermediate‐sized because it remains suspended in air for minutes and can be transported over considerable distances by convective air currents. The abundance of this speech‐generated aerosol, combined with its high viral load in pre‐ and asymptomatic individuals, strongly implicates airborne transmission of SARS‐CoV‐2 through speech as the primary contributor to its rapid spread.

Oral Fluid Droplets Generated during Speech In this experiment, investigators used a laser in a darkened box to visualize droplets that were generated during speech. The droplets decreased in numbe...

During a pandemic, identifying modes of transmission is paramount to devise effective and practical mitigation strategies. Mohamed Abbas and Didier Pittet1Abbas M Pittet D Surfing the COVID-19 scientific wave.Lancet Infect Dis. 2020; (published online June 30.)https://doi.org/10.1016/S1473-3099(20)30558-2Summary Full Text Full Text PDF Scopus (0) Google Scholar challenge the conclusions of our reports that normal speaking might be an important mode of transmission for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), involving small particles that remain airborne for minutes.2Anfinrud P Stadnytskyi V Bax CE Bax A Visualizing speech-generated oral fluid droplets with laser light scattering.N Engl J Med. 2020; 382: 2061-2063Crossref PubMed Scopus (50) Google Scholar, 3Stadnytskyi V Bax CE Bax A Anfinrud P The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission.Proc Natl Acad Sci USA. 2020; 117: 11875-11877Crossref PubMed Scopus (41) Google Scholar Whereas the opening remarks in Abbas and Pittet's correspondence are irrelevant to our work, we eagerly welcome an intellectual debate on the scientific merits of our research. In their correspondence, they claim that our “findings have no immediate implications”.1Abbas M Pittet D Surfing the COVID-19 scientific wave.Lancet Infect Dis. 2020; (published online June 30.)https://doi.org/10.1016/S1473-3099(20)30558-2Summary Full Text Full Text PDF Scopus (0) Google Scholar Nothing could be further from the truth. While we refer readers to the appendix (pp 1–5) for a detailed response to all issues raised, we here address two of Abbas and Pittet's more pertinent concerns.

Speech droplets generated by asymptomatic carriers of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are increasingly considered to be a likely mode of disease transmission. Highly sensitive laser light scattering observations have revealed that loud speech can emit thousands of oral fluid droplets per second. In a closed, stagnant air environment, they disappear from the window of view with time constants in the range of 8 to 14 min, which corresponds to droplet nuclei of ca. 4 μm diameter, or 12- to 21-μm droplets prior to dehydration. These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments.

It is well-known that tetrameric hemoglobin binds ligands cooperatively by undergoing a ligand-induced T → R quaternary structure transition, a structure-function relationship that has long served as a model system for understanding allostery in proteins. However, kinetic studies of the reverse, R → T quaternary structure transition following photolysis of carbonmonoxyhemoglobin (HbCO) reveal complex behavior that may be better explained by the presence of two different R quaternary structures coexisting in thermal equilibrium. Indeed, we report here time-resolved small- and wide-angle X-ray scattering (SAXS/WAXS) patterns of HbCO following a temperature jump that not only provide unambiguous evidence for more than one R state, but also unveil the time scale for interconversion between them. Since the time scale for the photolysis-induced R → T transition is likely different for different R-states, this structural heterogeneity must be accounted for to properly explain the kinetic heterogeneity observed in time-resolved spectroscopic studies following photolysis of HbCO.

The intramembrane cytochrome bc1 complex of the photosynthetic bacterium Rhodobacter capsulatus and the cytochrome b6f complex, which functions in oxygenic photosynthesis, utilize two pairs of b-hemes in a symmetric dimer to accomplish proton-coupled electron transfer. The transmembrane electron transfer pathway in each complex was identified through the novel use of heme Soret band excitonic circular dichroism (CD) spectra, for which the responsible heme–heme interactions were determined from crystal structures. Kinetics of heme reduction and CD amplitude change were measured simultaneously. For bc1, in which the redox potentials of the transmembrane heme pair are separated by 160 mV, heme reduction occurs preferentially to the higher-potential intermonomer heme pair on the electronegative (n) side of the complex. This contrasts with the b6f complex, where the redox potential difference between transmembrane intramonomer p- and n-side hemes is substantially smaller and the n–p pair is preferentially redu...