SARS-CoV-2, a coronavirus that emerged in late 2019, has spread rapidly worldwide, and information about the modes of transmission of SARS-CoV-2 among humans is critical to apply appropriate infection control measures and to slow its spread. Here we show that SARS-CoV-2 is transmitted efficiently via direct contact and via the air (via respiratory droplets and/or aerosols) between ferrets, 1 to 3 days and 3 to 7 days after exposure respectively. The pattern of virus shedding in the direct contact and indirect recipient ferrets is similar to that of the inoculated ferrets and infectious virus is isolated from all positive animals, showing that ferrets are productively infected via either route. This study provides experimental evidence of robust transmission of SARS-CoV-2 via the air, supporting the implementation of community-level social distancing measures currently applied in many countries in the world and informing decisions on infection control measures in healthcare settings.

bioRxiv preprint SARS-CoV-2, a coronavirus that newly emerged in China in late 2019 1,2 and spread rapidly 12 worldwide, caused the first witnessed pandemic sparked by a coronavirus. As the pandemic 13 progresses, information about the modes of transmission of SARS-CoV-2 among humans is critical 14 to apply appropriate infection control measures and to slow its spread. Here we show that SARS-15 CoV-2 is transmitted efficiently via direct contact and via the air (via respiratory droplets and/or 16 aerosols) between ferrets. Intranasal inoculation of donor ferrets resulted in a productive upper 17 respiratory tract infection and long-term shedding, up to 11 to 19 days post-inoculation. SARS-18 CoV-2 transmitted to four out of four direct contact ferrets between 1 and 3 days after exposure 19 and via the air to three out of four independent indirect recipient ferrets between 3 and 7 days 20 after exposure. The pattern of virus shedding in the direct contact and indirect recipient ferrets 21 was similar to that of the inoculated ferrets and infectious virus was isolated from all positive 22 animals, showing that ferrets were productively infected via either route. This study provides 23 experimental evidence of robust transmission of SARS-CoV-2 via the air, supporting the 24 implementation of community-level social distancing measures currently applied in many 25 countries in the world and informing decisions on infection control measures in healthcare 26 settings 3 . In late December 2019, clusters of patients in China presenting with pneumonia of unknown etiology 29 were reported to the World Health Organization (WHO) 1 . The causative agent was rapidly identified 30 as being a virus from the Coronaviridae family, closely related to the severe acute respiratory 31 syndrome coronavirus (SARS-CoV) 2,4,5 . The SARS-CoV epidemic affected 26 countries and resulted in 32 more than 8000 cases in 2003. The newly emerging coronavirus, named SARS-CoV-2 6 , rapidly spread 33 worldwide and was declared pandemic by the WHO on March 11, 2020 7 . The first evidence 34 suggesting human-to-human transmission came from the descriptions of clusters among the early 35 cases 8,9 . Based on epidemiological data from China before measures were taken to control the 36 spread of the virus, the reproductive number R0 (the number of secondary cases directly generated 37 from each case) was estimated to be between 2 and 3 10-12 . In order to apply appropriate infection 38 control measures to reduce the R0, the modes of transmission of SARS-CoV-2 need to be elucidated.39 Respiratory viruses can be transmitted via direct and indirect contact (via fomites), and through the 40 air via respiratory droplets and/or aerosols. Transmission via respiratory droplets (> 5 μm) is 41 mediated by expelled particles that have a propensity to settle quickly and is therefore reliant on 42 close proximity between infected and susceptible individuals, usually within 1 m of the site of 43 expulsion. Transmission via aerosols (< 5 μm) is mediated by expelled parti...

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A major cause of respiratory failure during influenza A virus (IAV) infection is damage to the epithelial-endothelial barrier of the pulmonary alveolus. Damage to this barrier results in flooding of the alveolar lumen with proteinaceous oedema fluid, erythrocytes and inflammatory cells. To date, the exact roles of pulmonary epithelial and endothelial cells in this process remain unclear.Here, we used an in vitro co-culture model to understand how IAV damages the pulmonary epithelialendothelial barrier. Human epithelial cells were seeded on the upper half of a transwell membrane while human endothelial cells were seeded on the lower half. These cells were then grown in co-culture and IAV was added to the upper chamber.We showed that the addition of IAV (H1N1 and H5N1 subtypes) resulted in significant barrier damage. Interestingly, we found that, while endothelial cells mounted a pro-inflammatory/pro-coagulant response to a viral infection in the adjacent epithelial cells, damage to the alveolar epithelial-endothelial barrier occurred independently of endothelial cells. Rather, barrier damage was associated with disruption of tight junctions amongst epithelial cells, and specifically with loss of tight junction protein claudin-4.Taken together, these data suggest that maintaining epithelial cell integrity is key in reducing pulmonary oedema during IAV infection. @ERSpublications Influenza A virus damages tight junctions, and specifically claudin-4, of respiratory epithelial cells

Influenza A viruses are amongst the most challenging viruses that threaten both human and animal health. Influenza A viruses are unique in many ways. Firstly, they are unique in the diversity of host species that they infect. This includes waterfowl (the original reservoir), terrestrial and aquatic poultry, swine, humans, horses, dog, cats, whales, seals and several other mammalian species. Secondly, they are unique in their capacity to evolve and adapt, following crossing the species barrier, in order to replicate and spread to other individuals within the new species. Finally, they are unique in the frequency of inter-species transmission events that occur. Indeed, the consequences of novel influenza virus strain in an immunologically naïve population can be devastating. The problems that influenza A viruses present for human and animal health are numerous. For example, influenza A viruses in humans represent a major economic and disease burden, whilst the poultry industry has suffered colossal damage due to repeated outbreaks of highly pathogenic avian influenza viruses. This review aims to provide a comprehensive overview of influenza A viruses by shedding light on interspecies virus transmission and summarising the current knowledge regarding how influenza viruses can adapt to a new host.

Human influenza A viruses are known to be transmitted via the air from person to person. It is unknown from which anatomical site of the respiratory tract influenza A virus transmission occurs. Here, pairs of genetically tagged and untagged influenza A/H1N1, A/H3N2 and A/H5N1 viruses that are transmissible via the air are used to co-infect donor ferrets via the intranasal and intratracheal routes to cause an upper and lower respiratory tract infection, respectively. In all transmission cases, we observe that the viruses in the recipient ferrets are of the same genotype as the viruses inoculated intranasally, demonstrating that they are expelled from the upper respiratory tract of ferrets rather than from trachea or the lower airways. Moreover, influenza A viruses that are transmissible via the air preferentially infect ferret and human nasal respiratory epithelium. These results indicate that virus replication in the upper respiratory tract, the nasal respiratory epithelium in particular, of donors is a driver for transmission of influenza A viruses via the air.

Influenza A virus (IAV) infection is an important cause of respiratory disease in humans. The original reservoirs of IAV are wild waterfowl and shorebirds, where virus infection causes limited, if any, disease. Both in humans and in wild waterbirds, epithelial cells are the main target of infection. However, influenza virus can spread from wild bird species to terrestrial poultry. Here, the virus can evolve into highly pathogenic avian influenza (HPAI). Part of this evolution involves increased viral tropism for endothelial cells. HPAI virus infections not only cause severe disease in chickens and other terrestrial poultry species but can also spread to humans and back to wild bird populations. Here, we review the role of the endothelium in the pathogenesis of influenza virus infection in wild birds, terrestrial poultry and humans with a particular focus on HPAI viruses. We demonstrate that whilst the endothelium is an important target of virus infection in terrestrial poultry and some wild bird species, in humans the endothelium is more important in controlling the local inflammatory milieu. Thus, the endothelium plays an important, but species-specific, role in the pathogenesis of influenza virus infection.

Many respiratory viruses of humans originate from animals. For instance, there are now eight paramyxoviruses, four coronaviruses and four orthomxoviruses that cause recurrent epidemics in humans but were once confined to other hosts. In the last decade, several members of the same virus families have jumped the species barrier from animals to humans. Fortunately, these viruses have not become established in humans, because they lacked the ability of sustained transmission between humans. However, these outbreaks highlighted the lack of understanding of what makes a virus transmissible. In part triggered by the relatively high frequency of occurrence of influenza A virus zoonoses and pandemics, the influenza research community has started to investigate the viral genetic and biological traits that drive virus transmission via aerosols or respiratory droplets between mammals. Here we summarize recent discoveries on the genetic and phenotypic traits required for airborne transmission of zoonotic influenza viruses of subtypes H5, H7 and H9 and pandemic viruses of subtypes H1, H2 and H3. Increased understanding of the determinants and mechanisms of respiratory virus transmission is not only key from a basic scientific perspective, but may also aid in assessing the risks posed by zoonotic viruses to human health, and preparedness for such risks.