COVID-19: transmission

source: emcrit.org

large droplet transmission

• COVID-19 transmission can occur via large droplet transmission (with a risk limited to ~6 feet from the patient)(Carlos del Rio 2/28).
• This is typical for respiratory viruses such as influenza.
• Transmission via large droplet transmission can be prevented by using a standard surgical-style mask.

airborne transmission ??

• It's controversial whether COVID19 can be transmitted via an airborne route (small particles which remain aloft in the air for longer periods of time).  Airborne transmission would imply the need for N95 masks (“FFP2” in Europe), rather than surgical masks.  This controversy is explored further in Shiu et al 2019.
• Airborne precautions started being used with MERS and SARS out of an abundance of caution (rather than any clear evidence that coronaviruses are transmitted via an airborne route).  This practice has often been carried down to COVID19.
• Guidelines disagree about whether to use airborne precautions:
  • The Canadian Guidelines and World Health Organization guidelines both recommend using only droplet precautions for routine care of COVID19 patients.  However, both of these guidelines recommend airborne precautions for procedures which generate aerosols (e.g. intubation, noninvasive ventilation, CPR, bag-mask ventilation, and bronchoscopy).
  • The United States CDC recommends using airborne precautions all the time when managing COVID19 patients.
• Using airborne precautions for all patients who are definitely or potentially infected with COVID19 will likely result in rapid depletion of N95 masks.  This will leave healthcare providers unprotected when they actually need these masks for aerosol-generating procedures.
• In the context of a pandemic, the Canadian and WHO guidelines may be more sensible in countries with finite resources (i.e. most locales).  However, infection control is ultimately local, so be sure to follow your hospital's guidance regarding this.

contact transmission (“fomite-to-face”)

• This mode of transmission has a tendency to get overlooked, but it may be incredibly important.  This is how it works:
  • (i)  Someone with coronavirus coughs, emitting large droplets containing the virus.  Droplets settle on surfaces in the room, creating a thin film of coronavirus.  The virus may be shed in nasal secretions as well, which could be transmitted to the environment.
  • (ii) The virus persists on fomites in the environment.  Human coronaviruses can survive on surfaces for up to about a week (Kampf et al 2020).  It's unknown how long COVID-19 can survive in the environment, but it might be even longer (some animal coronaviruses can survive for weeks!).
  • (iii)  Someone else touches the contaminated the surface hours or days later, transferring the virus to their hands.
  • (iv)  If the hands touch a mucous membrane (eyes, nose, or mouth), this may transmit the infection.
• Any effort to limit spread of the virus must block contact transmission.  The above chain of events can be disrupted in a variety of ways:
  • (a) Regular cleaning of environmental surfaces (e.g. using 70% ethanol or 0.5% sodium hypochlorite solutions; for details see Kampf et al 2020 and CDC guidelines).
  • (b) Hand hygiene (high concentration ethanol neutralizes the virus and is easy to perform, so this might be preferable if hands aren't visibly soiled)(Kampf 2017).
  • (c) Avoidance of touching your face.  This is nearly impossible, as we unconsciously touch our faces constantly.  The main benefit of wearing a surgical mask could be that the mask acts as a physical barrier to prevent touching the mouth or nose.
• Any medical equipment could become contaminated with COVID-19 and potentially transfer virus to providers (e.g. stethoscope earpieces and shoes).  A recent study found widespread deposition of COVID-19 in one patient's room, but fortunately this seems to be removable by cleaning with sodium dichloroisocyanurate (Ong et al 2020).

when can transmission occur?

• (#1) Asymptomatic transmission (in people with no or minimal symptoms) appears to be possible (Carlos del Rio 2/28).
• (#2) Transmission appears to occur over roughly ~8 days following the initiation of illness.
  • Patients may continue to have positive pharyngeal PCR for weeks after convalescence (Lan 2/27).  However, virus culture methods are unable to recover viable virus after ~8 days of clinical illness (Wolfel 2020).  This implies that prolonged PCR positivity probably doesn't correlate with clinical virus transmission.  However, all subjects in Wolfel et al. had mild illness, so it remains possible that prolonged transmission could occur in more severe cases.
  • CDC guidance is vague on how long patients with known COVID-19 should be isolated.  It may be advisable to obtain two paired RT-PCR tests (one of the nasopharynx and one of the pharynx), with each pair collected >24 hours apart, prior to discontinuing precautions.

R⌀

• R⌀ is the average number of people that an infected person transmits the virus to.
  • If R⌀ is <1, the epidemic will burn out.
  • If R⌀ = 1, then epidemic will continue at a steady pace.
  • If R⌀ >1, the epidemic will increase exponentially.
• Current estimates put R⌀ at ~2.5-2.9 (Peng PWH et al, 2/28).  This is a bit higher than seasonal influenza.
• R⌀ is a reflection of both the virus and also human behavior.  Interventions such as social distancing and improved hygiene will decrease R⌀.
  • Control of spread of COVID-19 in China proves that R⌀ is a modifiable number that can be reduced by effective public health interventions.
  • The R⌀ on board the Diamond Princess cruise ship was 15 – illustrating that cramped quarters with inadequate hygiene will increase R⌀ (Rocklov 2/28).