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Coronavirus Illness 2019 (COVID-19)


The principal mode by which persons are contaminated with SARS-CoV-2 (the virus that causes COVID-19) is thru publicity to respiratory droplets carrying infectious virus.

Respiratory droplets are produced throughout exhalation (e.g., respiratory, talking, singing, coughing, sneezing) and span a large spectrum of sizes which may be divided into two fundamental classes primarily based on how lengthy they’ll stay suspended within the air:

  • Bigger droplets a few of that are seen and that fall out of the air quickly inside seconds to minutes whereas near the supply.
  • Smaller droplets and particles (fashioned when small droplets dry in a short time within the airstream) that may stay suspended for a lot of minutes to hours and journey removed from the supply on air currents.

As soon as respiratory droplets are exhaled and as they transfer outward from the supply, their focus decreases by way of fallout from the air (largest droplets first, smaller later) mixed with dilution of the remaining smaller droplets and particles into the rising quantity of air they encounter.

Respiratory viruses are transmitted in a number of methods

Infections with respiratory viruses are principally transmitted by way of three modes: contact, droplet, and airborne.

  • Contact transmission is an infection unfold by way of direct contact with an infectious particular person (e.g., touching throughout a handshake) or with an article or floor that has turn into contaminated. The latter is usually known as “fomite transmission.”
  • Droplet transmission is an infection unfold by way of publicity to virus-containing respiratory droplets (i.e., bigger and smaller droplets and particles) exhaled by an infectious particular person. Transmission is most certainly to happen when somebody is near the infectious particular person, typically inside about 6 ft.
  • Airborne transmission is an infection unfold by way of publicity to these virus-containing respiratory droplets comprised of smaller droplets and particles that may stay suspended within the air over lengthy distances (normally better than 6 ft) and time (usually hours). 

Droplet transmission consists of publicity to bigger droplets, smaller droplets, and particles when an individual is near an contaminated particular person. Airborne transmission consists of publicity to smaller droplets and particles at better distances or over longer occasions.

These modes of transmission will not be mutually unique. For example, “shut contact” refers to transmission that may occur by both contact or droplet transmission whereas an individual is inside about 6 ft of an contaminated particular person.

The time period “aerosol” has been utilized in numerous methods to explain small particles that may transfer by way of the air

Aerosol has been used each to outline respiratory droplets of a sure dimension (e.g., smaller droplets and particles), in addition to to explain the gathering or cloud of those respiratory droplets within the air. Within the healthcare setting, aerosol is used with respect to “aerosol-generating procedures” (e.g., intubation, bronchoscopy) that produce small droplets and particles and require distinct engineering controls to forestall occupational transmission of infectious pathogens like SARS-CoV-2. In group settings, aerosol was the time period used to explain the sewage system-generated cloud of small droplets and particles that was believed to have unfold SARS (brought on by the virus SARS-CoV-1) throughout the 2003 Amoy Gardens outbreak in Hong Kong.

The time period “airborne transmission” has a specialised which means in public well being observe

Airborne can be utilized to explain any dimension particle (e.g., droplet, mud, pollen) able to journey by way of the air. For respiratory droplets, that may embrace droplets which might be near the supply and people who have moved farther away. Nonetheless, most infectious illness and public well being consultants reserve the time period airborne particularly to be used within the context of airborne transmission to explain infections able to being transmitted by way of publicity to infectious, pathogen-containing, small droplets and particles suspended within the air over lengthy distances and that persist within the air for lengthy occasions.

Airborne transmission shouldn’t be equally environment friendly for all respiratory microbes

For some viruses and micro organism, airborne transmission is a extremely environment friendly mode for spreading an infection. Examples embrace Mycobacterium tuberculosis (the bacterium that causes tuberculosis), rubeola (the virus that causes measles), and varicella-zoster (the virus that causes hen pox). Though these infections may be transmitted at shut vary, they’re additionally effectively and ceaselessly transmitted over longer distances (i.e., greater than six ft) or over longer occasions (i.e., to folks passing by way of an air house during which the infectious particular person was current minutes to hours earlier). It’s particularly necessary to manage pathogens that readily infect via airborne transmission in healthcare and different occupational settings the place particular engineering controls are required to forestall unfold (e.g., negative-pressure airborne an infection isolation rooms, excessive effectivity respirators).

The epidemiology of SARS-CoV-2 signifies that almost all infections are unfold by way of shut contact, not airborne transmission

Illnesses which might be unfold effectively by way of airborne transmission are likely to have excessive assault charges as a result of they’ll rapidly attain and infect many individuals in a brief time period. We all know {that a} important proportion of SARS-CoV-2 infections (estimated 40-45%) happen with out signs and that an infection may be unfold by folks displaying no signs. Thus, had been SARS-CoV-2 unfold primarily by way of airborne transmission like measles, consultants would count on to have noticed significantly extra fast international unfold of an infection in early 2020 and better percentages of prior an infection measured by serosurveys. Out there knowledge point out that SARS-CoV-2 has unfold extra like most different widespread respiratory viruses, primarily by way of respiratory droplet transmission inside a brief vary (e.g., lower than six ft). There isn’t a proof of environment friendly unfold (i.e., routine, fast unfold) to folks distant or who enter an area hours after an infectious particular person was there.

Airborne transmission of SARS-CoV-2 can happen below particular circumstances

Pathogens which might be primarily transmitted by way of shut contact (i.e., contact transmission and droplet transmission) can typically even be unfold through airborne transmission below particular circumstances. There are a number of well-documented examples during which SARS-CoV-2 seems to have been transmitted over lengthy distances or occasions. These transmission occasions seem unusual and have usually concerned the presence of an infectious particular person producing respiratory droplets for an prolonged time (>half-hour to a number of hours) in an enclosed house. Sufficient virus was current within the house to trigger infections in individuals who had been greater than 6 ft away or who handed by way of that house quickly after the infectious particular person had left. Circumstances below which airborne transmission of SARS-CoV-2 seems to have occurred embrace:

  • Enclosed areas inside which an infectious particular person both uncovered vulnerable folks on the identical time or to which vulnerable folks had been uncovered shortly after the infectious particular person had left the house.
  • Extended publicity to respiratory particles, usually generated with expiratory exertion (e.g., shouting, singing, exercising) that elevated the focus of suspended respiratory droplets within the air house.
  • Insufficient air flow or air dealing with that allowed a build-up of suspended small respiratory droplets and particles.

Prevention of COVID-19 by airborne transmission

Present interventions to forestall the unfold of SARS-CoV-2 seem enough to deal with transmission each by way of shut contact and below the particular circumstances favorable to potential airborne transmission. Amongst these interventions, which embrace social distancing, use of masks locally, hand hygiene, and floor cleansing and disinfection, air flow and avoidance of crowded indoor areas are particularly related for enclosed areas, the place circumstances can enhance the focus of suspended small droplets and particles carrying infectious virus. At the moment, there isn’t any indication of a basic group want to make use of particular engineering controls, akin to these required to guard towards airborne transmission of infections, like measles or tuberculosis, within the healthcare setting.

SARS-CoV-2 is a brand new virus, and we’re nonetheless studying about the way it behaves.

There are a number of vital questions that must be answered to refine steering for prevention of COVID-19, together with

  • How efficient are mitigation efforts to forestall SARS-CoV-2 unfold, particularly air flow and masking?
  • What quantity of SARS-CoV-2 infections are acquired by way of airborne transmission?
  • What are the circumstances that facilitate airborne transmission?
  • What’s the infectious dose for SARS-CoV-2 (what number of virions are required for an infection to happen)?
  • Do inoculum dimension and route of inoculation have an effect on danger of an infection and illness severity?

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