With increasing evidence of community and hospital-acquired transmission of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), the virus that causes Coronavirus Disease 2019 (COVID-19), there is still uncertainty regarding the exact modes of SARS-Cov-2 transmission. It is imperative to understand the risks associated with particular modes of transmission, especially to ensure that healthcare workers are able to protect themselves with proper personal protective equipment (PPE). I have seen confusion surrounding the appropriate PPE to wear for possible suspected patients in the emergency room setting, where it is important to correctly triage and isolate patients to limit exposure. There have been news stories of healthcare workers expressing their own concerns about the availability and types of PPE, and there have been international reports of shortages of masks such as N95 respirators (1).
In order to understand SARS-CoV-2 transmission, it is important to identify the different modalities of virus transmission. The main routes of transmission for various viruses include direct contact, fomite, droplet, and aerosol/airborne (2). Certain viruses are more effectively transmitted in different ways, which depends on many factors such as how the virus infects a host, how it is shed in bodily secretions, and how stable it is in the environment. Transmission via direct contact requires a susceptible person coming into contact with infected secretions or bodily fluids. This mode of transmission is common to many viruses and limited by using gloves, gowns, and washing hands. Fomite transmission refers to contact with surfaces that have been contaminated with infected secretions, where the virus is able to persist and remain infectious. Experimental studies can be designed to determine how long a particular virus lasts on different surfaces and under different environmental conditions (3). PPE for fomite transmission is similar to that for direct contact with the additional need to decontaminate surfaces. The most confusing and controversial modes of transmission are droplet and aerosol/airborne transmission. Often these terms are used interchangeably and there is still some scientific uncertainty to their definitions as well.
The term aerosol refers to particles suspended in air, and many divide aerosols into small droplets (also exclusively called aerosols) and large droplets. Small droplets may desiccate and travel long distances as droplet nuclei, whereas large droplets do not travel as far before landing on a surface. The transmission of small droplets is often called airborne or aerosol transmission, while the term droplet transmission refers to large droplets (2). There is no precise cutoff for small and large droplets, some refer to ≤5 µm in diameter for small droplets, while another possible cutoff between aerosol types is 20 µm, since aerosols ≤20 µm in diameter can form droplet nuclei, and aerosols ≥20 µm do not deposit substantially in the lower respiratory tract (3). The location of deposition is important since certain viruses preferentially infect and replicate in the upper or lower respiratory tract. The Infectious Disease Society of America uses the terms respirable and inspirable particles to refer to those ≤10 µm and those between 10 µm and 100 µm respectively, divided by whether the particle is deposited in the upper or lower respiratory tract (4). Others suggest a 10 µm cutoff based on factors such as suspension time and airway penetration (5). N95 respirators or powered air purifying respirators (PAPRs) are used for aerosol/airborne transmission, while surgical masks with eye protection are used for droplet transmission. Additionally, aerosol generating medical procedures (AGMPs) such as intubation or ventilation can create infectious aerosols requiring the use of N95 respirators or PAPRs (6).
While the epidemiological and experimental data surrounding SARS-CoV-2 transmission continue to be analyzed, many wonder whether the virus is transmitted primarily via droplets and whether aerosol/airborne transmission is also possible. Evidence from other emerging coronaviruses, the original SARS-CoV and MERS-CoV, point to primarily droplet transmission, but experts still acknowledge the possibility of aerosol/airborne transmission (5, 7). Early data show that SARS-CoV-2 has some similar characteristics but is also different in that it replicates efficiently in the upper respiratory tract and the virus is shed early on (8). In order to protect healthcare workers, one could use the precautionary principle, which is to presume airborne/aerosol transmission is possible unless there is sufficient evidence to show otherwise. This is the approach that the US Centers for Disease Control and Prevention (CDC) has been taking in recommending that healthcare workers wear gowns, gloves, N95 respirators with eye protection or a PAPR when treating suspected COVID-19 patients (9). Additionally, these measures are recommended when performing AGMPs. However, this differs from other recommendations, for example the WHO recommended primarily droplet/contact precautions with airborne precautions only when performing AGMPs (10). The Canadian Public Health Service had similar recommendations (11). This may be due to varying interpretations of evidence as well as resource availability. As hospital resources for airborne precautions have become limited and logistical challenges have arisen for patient care, many hospital protocols have shifted to match those of the WHO. Regardless of recommendations, I have seen different masks and modalities of PPE be confused for one another in the clinical setting. While we need more experimental and epidemiological evidence in order to elucidate SARS-CoV-2 transmission, we also need clear messages and guidance in the clinical setting to ensure that healthcare workers and patients are protected as we prepare for this pandemic.
References:
1. https://time.com/5785223/medical-masks-coronavirus-covid-19/
2. https://www.mdpi.com/1999-4915/7/2/511/htm
3. https://wwwnc.cdc.gov/eid/article/12/11/06-0426_article
4. https://www.nap.edu/read/13027/chapter/4#30
5. https://bmcinfectdis.biomedcentral.com/articles/10.1186/s12879-019-3707-y
6. https://www.mdpi.com/1999-4915/11/10/940
7. https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(14)60852-7/fulltext
9. https://www.cdc.gov/coronavirus/2019-ncov/hcp/clinical-criteria.html