[In particular,] how the virus can be present but not necessarily in an amount that would cause someone to get sick. I’m thinking about how much is aerosolized and how long someone would have to be breathing it, or how much would need to be on a countertop and then picked up on hands then put into eyes/nose/mouth. Can you compare this to more familiar viruses, so laypeople might put the SARS-CoV-2 virus into context?
Chris Kemp, M.S.:
Viruses consist of little particles (singularly known as virions), and a certain number of them (the infectious dose) need to enter our system for us to get sick. At the moment, the infectious dose for SARS-CoV-2 is not known, but virologists are assuming the number is quite low, since it seems very transmissable. For context, the infectious dose for Hepatitis A is estimated at 10-100 particles, and the estimates for Norovirus are also in the 10-100 particle range. Most likely, SARS-CoV-2 is in this range too. But it's simply not known. When you think about infection, a sustained period of time in an enclosed space with an infected person dramatically increases the chance of infection. It's likely that, at some point, you've been around SARS-CoV-2 virions, but you need to get that infectious dose to become infected.
Alison Bernstein, Ph.D.:
There are many pieces to this question so I’m going to break it down into the individual topics.
Every pathogen has an infectious dose. For a virus, this is the number of individual viruses needed to infect someone. So for the virus to infect you, you need to be exposed to at least this amount of virus for the virus to take hold in your cells.
Viral load is a measure of how much virus is present in a person. It is the amount of virus present once a person has been infected and the virus has had time to replicate in their cells. This changes over time - going up as infection progresses and then declining as you, hopefully, recover.
In general, the higher your viral load, the worse your symptoms and the more contagious you are because you shed more whole viruses.
However, there is a lot about this we don’t know for SARS-CoV-2 yet. Scientists expect the infectious dose is low, based on how easily it seems to spread through interpersonal contact.
Answers to questions about viral load and how it relates to how infectious this is and how contagious someone is at any given point in the disease course are still unclear. However, uncertainty on those details doesn’t change what we do know about this virus, or the recommendations that we know can stop the spread of the disease: masks, diligent hygiene and physical distancing.
All of the issues raised in the question about aerosols, droplets, and airborne go into how contagious this is and how specifically it spreads. The droplets and aerosols come from infected people and go into the air. Then they either get breathed in by another person or land on a surface. If the other person breathes in enough of this, they might be infected. If someone were to touch one of those surfaces, then touch their face, they might be infected. (The answer in the question about fomites explains how viruses on surfaces might be contagious.)
Scientists now think that spread is mostly through close contact with people and not from surfaces. Surface transmission is still possible, but isn’t its primary mode of transmission. For people living through this, the specific level of how much spread is from surfaces vs direct contact doesn’t change recommendations for diligent hygiene and masking.
Fortunately, taking these precautions gets around all the uncertainty about how long SARS-CoV-2 can survive on surfaces and what the mode of transmission is. No matter what those details are, we still need to take these precautions and always wash your hands well after coming into contact with something that may be contaminated.
The airborne, aerosol, droplet question is important scientifically, and another example where scientists use these words differently than the public. In the very specific way that scientists mean it, SARS-CoV-2 is not airborne but is spread through aerosols and droplets. But again, the specifics don’t change what we need to do to stop the spread - wear masks and wash hands often.
Resources:
https://www.statnews.com/2020/04/14/how-much-of-the-coronavirus-does-it-take-to-make-you-sick/