Sophie Korzan, MD
Virus Transmission: The size of the SARS-CoV-2 virus is about 100-125 nm (0.1-125 μm- micrometers).
- Human-to-human spread of the virus via respiratory droplets is currently considered to be the main route of transmission.[i]
- There is concern for an element of airborne or limited-distance airborne transmission via bio-aerosols.
- What is an aerosol? Particles formed by solid or liquid particles dispersed and suspended in the air. The definition of an aerosol varies but most authorities characterize this as consisting of droplets of <5µm.
- When a person, who was infected with the virus, coughs, sneezes, breathes vigorously, or speaks loudly, the virus will be excreted from the body and may dissolve with the aerosol and become the bio-aerosols.[ii]
- The particles in a bio-aerosol are generally 0.3 to 100 μm in diameter; however, the respirable size fraction of 1 to 10 μm is of primary concern. 4
- Bio-aerosols ranging in size from 1.0 to 5.0 μm generally remain in the air, whereas larger particles are deposited on surfaces. 4
- Droplets are droplets of saliva discharged by people sneezing or coughing, and their particle size is generally 1 to 5 mm. They spread in a space of about 1 to 2 meters from the source of infection. However, the aerosol can travel hundreds of meters or more. More importantly, current researches have proven that aerosols are involved in the spread of SARS, MERS, H1N1, and some other diseases. [iii]
- It has been suggested aerosols stay in the air for up to three hours if not more, and on surfaces up to days (depending on the inoculum shed) 5
- Coughing and sneezing patients can shed relatively large particles (>10µm) that travel short distances and may contaminate the bedside environment. Smaller droplets or aerosols will remain airborne for longer periods and disseminate over greater distances. [iv]
- Pathogen transmission can occur via inhalation of infected respiratory droplets, in particular if the exposure of droplets is within close proximity (6 feet) and also includes contact with mucous membranes.[v]
Nosocomial Exposure and Transmission:
- When oxygen is delivered through nasal catheter, mask or non-invasive ventilation (NIV), substantial exhaled air is released into the air, which can increase dispersion of the virus, and subsequently increase the risk of nosocomial infection [vi]
- Prior studies have suggested that the application of high-flow nasal cannula (HFNC), NIV through the specific mask with optimized vent holes or the helmet with a double-limb circuit may lower the risk of airborne transmission [vii]
- Performing non-invasive respiratory support in a single, well-ventilated, negative pressure ward is also considered a safe option. Currently, however, the majority of patients are still receiving respiratory support through nasal catheter or common mask in general wards or emergency departments with limited medical resources. The potential for airborne transmission in this population has not received enough attention. [viii]
- Does aerosolization during NIPPV/HFNC matter if you are in a negative pressure room? It technically shouldn’t if you are wearing appropriate PPE as though it is airborne.[ix]
- Distance of cough aerosols with HFNC vs without exhaled air dispersion during non-invasive respiratory support may increase the risk of coronavirus transmission, and requires more attention from medical personnel and patients.10
- Based on prior NIV and chest physiotherapy are droplet (not aerosol)-generating procedures, producing droplets of >10µm in size. Due to their large mass, most fall out onto local surfaces within 1m.
- These findings suggest that health-care workers providing NIV and chest physiotherapy, working within 1m of an infected patient should have a higher level of respiratory protection, but that infection control measures designed to limit aerosol spread may have less relevance for these procedures.
- These results may have infection control implications for other airborne infections, such as severe acute respiratory syndrome and tuberculosis, as well as for pandemic influenza infection. 6
- Exhaled air dispersion during HFNC and CPAP via different interfaces is limited provided there is good mask interface fitting. However, exhaled air leakage to 620 mm laterally occurs when the connection between HFNC and the interface tube becomes loose.[x]
Other Modalities of Transmission:
- Fomite Transmission: The virus persists on fomites in the environment. Depending on the type of surface, it may persist for roughly four days 5
- Fecal-oral transmission: There is evidence to suggest prolonged presence of the virus in fecal samples even after respiratory symptoms subsided with viral shedding for nearly five weeks after respiratory samples tested negative. Further investigation is needed with regard to transmission.[xi]
- The occurrence of less common symptoms, such as nausea, vomiting, abdominal discomfort, and diarrhea, differs significantly depending on the study population; however, gastrointestinal symptoms can also precede typical respiratory presentation.[xii]
- With the increase in permeability of the intestinal barrier to foreign pathogens after infection with the virus, intestinal symptoms such as diarrhoea appear due to enterocyte absorption disorders, which theoretically indicates that the digestive system may be susceptible to COVID-19 infection.[xiii]
What about conjunctiva? Not likely a mode of transmission but rather a coincidence when found in conjunctival secretions.[xiv]
What can we learn from outbreaks on Cruise ships?
More than 800 cases of laboratory-confirmed COVID-19 cases occurred during outbreaks on three cruise ship voyages, and cases linked to several additional cruises have been reported across the United States. Transmission occurred across multiple voyages from ship to ship by crew members; both crew members and passengers were affected; 10 deaths associated with cruise ships have been reported to date. The results of testing of passengers and crew on board the Diamond Princess demonstrated a high proportion (46.5%) of asymptomatic infections at the time of testing. Available statistical models of the Diamond Princess outbreak suggest that 17.9% of infected persons never developed symptoms.[xv]
A high proportion of asymptomatic infections could partially explain the high attack rate among cruise ship passengers and crew.[xvi]
Asymptomatic and presymptomatic virus shedding:
- High number of patients with mild symptoms
- Evidence for superspreading events
Based on previous studies of influenza viruses and community-acquired human coronaviruses, it is predicted the viral loads in asymptomatic carriers are relatively low.[xvii] If this is also the case for SARS-CoV-2, the risk should remain low though unfortunately asymptomatic transmission appears to be possible.
- Current estimates of the incubation period range from 1 to 14 days with a median of 5 to 6 days, although recent case reports suggest that the incubation period may be as long as 24 days.[xviii]
- Furthermore, a significant proportion of suspected patients with mild to no symptoms who are managing at home may also require long-term home oxygen or NIV treatment (e.g. patients with advanced staged COPD). These patients may increase the risk of family cluster infections by the widespread dispersion of exhaled air in their homes. 10
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Masks (PPE vs Surgical Mask vs Cloth Mask)
Standard surgical masks can be used for most community-level clinical interactions with possible COVID-19 patients though they are most effective when used in combination with other PPE measures based off meta-analysis by the Chinese Cochrane Centre evaluating N95 vs FFP in a total of 9171 participants with influenza-like illnesses. There was no statistical significant difference in efficacy in preventing lab-confirmed viral illness, but respirators appeared to protect against bacterial colonization.[xix]
BMJ 2015 RCT of cloth mask vs medical masks vs control group (medical masks/standard of care) in 1607 healthcare workers (Hanoi, Vietnam)[xx]
- The first clinical efficacy data of cloth masks, which suggest HCWs should not use cloth masks as protection against respiratory infection. Reuse of cloth masks and poor filtration may result in increased risk of infection.
- Cloth masks also had significantly higher rates of ILI compared with the control arm.
- Further research is needed to inform the widespread use of cloth masks globally. However, as a precautionary measure, cloth masks should not be recommended for HCWs, particularly in high-risk situations given the penetration of cloth masks by particles was increased compared to medical masks along with increased moisture retention. 22
That being said there is some discussion about incorporating HEPA filters into cloth masks. Further investigation is needed.
- In another study looking at surgeons wearing masks in the OR, cloth masks have the lowest filtering efficiency and highest airflow resistance compared to surgical mask and medical masks. Cloth masks are not recommended to be used, especially considering they may not be washed daily.[xxi]
- Compared with cloth masks, disposable surgical masks are more effective in reducing particulate exposure; cloth masks are only marginally beneficial in protecting individuals from particles<2.5 μm.[xxii]
N95 masks fulfill the filtering efficiency criteria of the National Institute for Occupational Safety and Health (NIOSH) and are approved for protection against droplet and airborne transmission of 95% of particles greater than 0.3 microns in size.[xxiii]
CDC Frequently asked Questions about PPE: https://www.cdc.gov/coronavirus/2019-ncov/hcp/respirator-use-faq.html
What about the basic reproduction number (R0) of COVID-19?
R0 is the average number of secondary infections produced by an infectious case in a population where everyone is susceptible and it is used to measure the transmission potential of a communicable disease.[xxiv]
Varying numbers based off different studies, one estimated the range to be between 1.5-6.68.[xxv]
There may be discrepancies among studies aimed to estimate the R0 because it is calculated through multiple parameters including the duration of contagiousness; the likelihood of infection per contact between; and the contact rate, along with economical, social and environmental factors.[xxvi]
R0 does not tell us about the transmission
ability of asymptomatic carriers or what the proportion of infected cases are
missed at the tracing and control procedures. 28
[i] Lili Guan, Luqian Zhou, Jinnong Zhang, Wei Peng, Rongchang Chen. More awareness is needed for severe acute respiratory syndrome coronavirus 2019 transmission through exhaled air during non-invasive respiratory support: experience from China. European Respiratory Journal Mar 2020, 55 (3) 2000352; DOI: 10.1183/13993003.00352-2020
[ii] Wang, J., Du, G. COVID-19 may transmit through aerosol. Ir J Med Sci (2020). https://doi.org/10.1007/s11845-020-02218-2
[iii] Doremalen, Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1. NEJM (3/17/20). Doi: 10.1056/NEJMc2004973)
[iv] Simonds A, Hanak A, Chatwin M, Morrell M, Hall A. Evaluation of droplet dispersion during non-invasive ventilation, oxygen therapy, nebuliser treatment and chest physiotherapy in clinical practice: implications for management of pandemic influenza and other airborne infections. Health Technol Assess 2010;14(46 Article 2).
[v] Wang D, Hu B, Hu C, et al. Clinical Characteristics of 138 Hospitalized Patients With 2019 Novel Coronavirus–Infected Pneumonia in Wuhan, China. JAMA. 2020;323(11):1061–1069. doi:10.1001/jama.2020.1585
[vi] Simonds AK, Hanak A, Chatwin M, et al. Evaluation of droplet dispersion during non-invasive ventilation, oxygen therapy, nebuliser treatment and chest physiotherapy in clinical practice: implications for management of pandemic influenza and other airborne infections. Health Technol Assess 2010; 14: 131–172. doi:10.3310/hta14460-02
[vii] Hui DS, Chow BK, Lo T, et al. Exhaled air dispersion during high-flow nasal cannula therapy versus CPAP via different masks. Eur Respir J 2019; 53: 1802339. doi:10.1183/13993003.02339-2018] [Hui DS, Chow BK, Lo T, et al. Exhaled air dispersion during noninvasive ventilation via helmets and a total facemask. Chest 2015; 147: 1336–1343. doi:10.1378/chest.14-1934
[viii] Lili Guan, Luqian Zhou, Jinnong Zhang, Wei Peng, Rongchang Chen. More awareness is needed for severe acute respiratory syndrome coronavirus 2019 transmission through exhaled air during non-invasive respiratory support: experience from China European Respiratory Journal Mar 2020, 55 (3) 2000352; DOI: 10.1183/13993003.00352-2020
[ix] Loh, N.W., Tan, Y., Taculod, J. et al. The impact of high-flow nasal cannula (HFNC) on coughing distance: implications on its use during the novel coronavirus disease outbreak. Can J Anesth/J Can Anesth (2020). https://doi.org/10.1007/s12630-020-01634-3
[x] David S. Hui, Benny K. Chow, Thomas Lo, Owen T.Y. Tsang, Fanny W. Ko, Susanna S. Ng, Tony Gin, Matthew T.V. Chan. Exhaled air dispersion during high-flow nasal cannula therapy versus CPAP via different masks. European Respiratory Journal Apr 2019, 53 (4) 1802339; DOI: 10.1183/13993003.02339-2018
[xi] Yongjian Wu,Cheng Guo,Lantian Tang,Zhongsi Hong,Jianhui Zhou,Xin Dong,Huan Yin,Qiang Xiao,Yanping Tang,Xiujuan Qu,Liangjian Kuang,Xiaomin Fang,Nischay Mishra,Jiahai Lu,Hong Shan,Guanmin Jiang,Xi Huang Prolonged Presence of SARS-CoV-2 viral RNA in faecal samples. The Lancet Gastroenterology & Hepatology; March 19, 2020 https://doi.org/10.1016/ S2468-1253(20)30083-2
[xii] Wang D, Hu B, Hu C, et al. Clinical characteristics of 138 hospitalized patients with 2019 novel coronavirus-infected pneumonia in Wuhan, China. JAMA 2020. doi: 10.1001/jama.2020.1585
[xiii] Kotfis K, Skonieczna-Żydecka K. COVID-19: gastrointestinal symptoms and potential sources of 2019-nCoV transmission. Anaesthesiology Intensive Therapy. 2020. doi:10.5114/ait.2020.93867
[xiv] Yuzhu Peng Yi‐Hua Zhou. Is novel coronavirus disease (COVID‐19) transmitted through conjunctiva? Journal of Medical Virology. 2020
[xv] Mizumoto, K., Kagaya, K., Zarebski, A. and Chowell, G. Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020. Eurosurveillance 2020;25.
[xvi] Moriarty LF, Plucinski MM, Marston BJ, et al. Public Health Responses to COVID-19 Outbreaks on Cruise Ships — Worldwide, February–March 2020. MMWR Morb Mortal Wkly Rep 2020;69:347-352. DOI: http://dx.doi.org/10.15585/mmwr.mm6912e3external icon
[xvii] Heimdal I, Moe N, Krokstad S, Christensen A, Skanke LH, Nordbø SA, Døllner H. Human coronavirus in hospitalized children with respiratory tract infections: a 9-year population-based study from Norway. J infect Dis. 2019;219(8):1198–206.
[xviii] Bai Y, Yao L, Wei T, et al. Presumed asymptomatic carrier transmission of COVID-19. JAMA. Published online February 21, 2020. doi:10.1001/jama.2020.2565
[xix] Oxford COVID-19 Evidence Service 3/23/20
[xx] MacIntyre CR, Seale H, Dung TC, et al. A cluster randomised trial of cloth masks compared with medical masks in healthcare workers. BMJ Open 2015;5: e006577. doi:10.1136/ bmjopen-2014-006577
[xxi] Liu Z, Yu D, Ge Y, et al. Understanding the factors involved in determining the bioburdens of surgical masks. Ann Transl Med. 2019;7(23):754. doi:10.21037/atm.2019.11.91
[xxii] Shakya, K., Noyes, A., Kallin, R. et al. Evaluating the efficacy of cloth facemasks in reducing particulate matter exposure. J Expo Sci Environ Epidemiol 27, 352–357 (2017). https://doi.org/10.1038/jes.2016.42
[xxiii] Anesthesia Patient Safety Foundation Perioperative Considerations for the 2019 Novel Coronavirus (COVID-19) Feb 12, 2020
[xxiv] Delamater PL, Street EJ, Leslie TF, et al. Complexity of the Basic Reproduction Number (R0). Emerg Infect Dis 2019;25:1-4.
[xxv] Liu Y, Gayle AA, Wilder-Smith A, Rocklöv J. The reproductive number of COVID-19 is higher compared to SARS coronavirus. J Travel Med 2020 Feb 13
[xxvi] Viceconte, G., & Petrosillo, N. (2020). COVID-19 R0: Magic number or conundrum?. Infectious Disease Reports, 12(1). https://doi.org/10.4081/idr.2020.8516