Here we are months into a world-overturning epidemic and scientists can’t decide if or how much aerosol transmission there is. An article in the New England Journal of Medicine gives a quick review of the state of the evidence. (NEJM Article) This is a thoughtful examination of the evidence. While noting the evidence suggesting that coronavirus may possibly be spread by aerosols, the authors further examine the data on whether this could be a significant source of transmission. They find that the low rate of transmissibility suggests that droplets rather than aerosols are the predominant source of transmission. Similarly the secondary attack rate, or how often a contact of an infected person gets infected is low, with only about 5% of even close contacts becoming infected. That rate is highest in households, where there is relatively constant and heavy exposure. Cluster events such as among choir or church attendees or restaurant patrons could also reflect droplet or other transmission, rather than aerosols. So the authors conclude that aerosols are not likely the dominant mode of transmission. If true, this would suggest that masks have more benefit in slowing transmission, since they have a greater impact on droplets than aerosols.
But, speaking of masks, while these articles tend to be on the more certain side than I think is the case, there are harms to consider and they are real. Reducing oxygen content and increasing carbon dioxide concentration are not good things, nor is creating a virus and bacteria-favorable environment at the entrance to your respiratory system. So read these with a dose of skepticism, but also as a counterweight to the near-hysteria about the value of mask-wearing. (Mask Article) (Mask Article 2) And again, I am not in any manner discouraging the use of masks.
Now I want to take a couple of minutes to talk about “infection”, and how that relates to acquired immunity, whether from a prior infection or a vaccine. It is an intriguing question to think about what it means to be “infected”. Does it mean any presence of the active virus in or on a person’s body? Does it mean presence of the virus coupled with some symptoms of illness? Does it mean presence of the virus, active replication of the virus inside the person’s cells and expulsion of active virus from the person into the environment? The latter criteria would be a situation where the person would also be considered “infectious”, capable of transmitting the disease to others. I suspect there are many situations in which a person meets the first potential definition but not the other two. Seems likely that someone who meets the second definition probably meet the third as well. And some in the third may not meet the second. Depending on how a test is performed and where the sample is taken from, some people in the first group might test positive, but aren’t either sick themselves or infectious. From a public health perspective, the second and third category would be of most concern, unless people in the first category, while not infected in the sense of replicating virus, are expelling what they have in the form of sneezes or touching.
When the virus either lands on a person’s outer services or is inhaled into the nose or throat, an immune response is prompted in most people. If some part of the virus is recognized by the sentinel immune system cells in the periphery of the body, it may be quickly dealt with by an antibody, T cell or other immune component, and eliminated before it ever has a chance to gain entrance to cells and begin replicating. Many people to whom this happens may never be tested or test positive by infection test. In some the immune response may be due to sentinel cells resulting from prior coronavirus or even other virus infection, if the virus has similar protein makeups. In some cases, the generalized immune response to perceived foreign proteins may be responsible. While people who quickly repel the virus might be considered “infected” in some sense, they weren’t really susceptible to serious infection. But to what extent could this be dose dependent–a certain amount of virus could be handled by the immune system, but if a much larger dose occurred, a real infection in the sense of illness and infectiousness would ensue.
Not sure we know all the answers at this point. But it is pretty apparent to me that many people come into contact with the virus and dispose of it very quickly. This occurs even among the old, who seem so disproportionately vulnerable.