COVID-19 Transmission Dynamics
Thanks to large-scale social distancing efforts, we're all moving a little more slowly these days – including SARS-CoV-2. As you read these words in an environment that's largely controlled and physically distant from society, you may feel like a passive soldier in the fight against COVID-19, but recent reports have shown evidence that social distancing is working.
In a broad sense, the virus depends on the movement of its host to spread. As the nation seeks to "reopen," we have to recognize that resuming activity will expose a vulnerable world to possible resurgences of COVID-19. Managing future waves will therefore require targeted identification of the origins of outbreaks (contact tracing), along with an increased knowledge of the transmission dynamics of SARS-CoV-2. If we hope to shut down the virus without shutting down our own lives in the process, we need a better handle on where the virus came from, where it's going and how it "plans" to get there.
Modes of Transmission
Transmission dynamics of SARS-CoV-2 have generated a significant amount of debate since the emergence of the virus in late 2019. One reason for this is simply a lack of evidence. Everything we know about how this pathogen behaves is based on data that's been collected in a short couple of months or has been hypothesized from the behavior of its relatives (SARS-CoV and MERS-CoV). Initial research indicated that SARS-CoV-2 was primarily spread through person-to-person contact and respiratory droplet transmission. New research suggests that it may also be spread through indirect routes, such as contaminated surfaces and airborne transmission. In all of these cases, before SARS-CoV-2 can spread to new hosts, it must be relseased to the environment. Let's take a closer look.
Step 1: Release
SARS-CoV-2 is a respiratory virus. That means it infects the respiratory tract and is released through respiration. When air passes over a layer of fluid in a person's respiratory tract, small particles, called droplets, are naturally created. Virus particles cohabitating the respiratory tract hitch a ride on those droplets and are released to the environment when an infected person talks, coughs, sneezes or simply exhales. It's important to recognize this process is not unique to SARS-CoV-2, but is common to other respiratory viruses like influenza viruses, respiratory syncytial virus (RSV), respiratory adenoviruses and other coronaviruses like SARS-CoV and MERS-CoV.
Step 2: Spread
How the expelled droplets impact disease transmission depends on the number of droplets produced, size of the droplets and concentration of pathogenic hitchhikers on board.
Droplets that are > 5-10 μm in diameter are called respiratory droplets. They're larger, heavier and are more likely to fall from the air before evaporating. Transmission from respiratory droplets occurs when a person touches a contaminated surface or gets caught directly in the spray zone of an infected patient. The 6 ft. guidelines of social distancing originated from preliminary evidence that SARS-CoV-2 is primarily spread through respiratory droplet transmission and that heavy droplets do not typically make it more than 6 ft. before landing.
Analysis of 75,465 COVID-19 cases in China, in which airborne transmission was not reported, has been cited as evidence that SARS-CoV-2 is primarily spread through direct person-to-person contact.
Droplets that are < 5 μm in diameter are called droplet nuclei or aerosols. These particles are so small and buoyant that they remain airborne for long periods of time and often float very long distances before evaporating. Transmission occurs when a person inhales droplet nuclei that are suspended in the air.
Initial research (not yet peer reviewed) conducted at the University of Nebraska Medical Center indicates that SARS-CoV-2 might also be transmitted via airborne particles. Thirteen individuals with confirmed SARS-CoV-2 infection from the Diamond Princess cruise ship were cared for and evaluated in 2 separate isolation centers, the Nebraska Biocontainment Unit and the National Quarantine Center. Surface and air samples were taken from the rooms of these patients and analyzed by RT-PCR for the presence of viral RNA. 75% of all sampled personal items and 63% of in room air samples were determined to be positive for SARS-CoV-2. Importantly, the detection of viral RNA did not tell whether the virus was viable or transmissible in the environmental samples that were collected, and further research is needed to determine whether COVID-19 can definitively be spread via airborne transmission.
Furthermore, the New England Journal of Medicine recently reported that SARS-CoV-2 remained viable in aerosols for 3 hours in a laboratory setting. We still need to know if this is true in clinical and environmental settings outside of the lab.
It should be noted that droplet type might not be as black and white as the descriptions above. Atmospheric conditions that impact the rate of evaporation and distance of droplet spread might create a range in droplet size and behavior that impacts transmission.
Recommended nonpharmeceutical interventions for airborne viruses include avoiding close contact with people who have active symptoms, staying home when ill, wearing a face mask to prevent the spread of germs and practicing good hand hygiene. While these strategies are the same interventions that we're currently practicing to prevent the spread of SARS-CoV-2, the spread of airborne viruses is more difficult to control than the spread of viruses through person-to-person contact alone. The distance that airborne particles can travel and the length of time they remain suspended in the air is much greater. Therefore, protocols for the isolation of individuals who are sick are much more important with airborne viruses.
More data are needed to determine the transmission dynamics of SARS-CoV-2. Such knowledge is necessary to accurately drive national and international guidelines for public health interventions, as well as the allocation of proper resources when the world begins to move again.