Transmission of SARS-CoV2

Reflections on viral spread.

This is a brief update based on studies of SARS-CoV2 transmission over the course of the pandemic. They mostly nail down features that we already know, and offer a comparison with influenza, which has interesting differences in its transmission. One observation is that influenza has been eradicated to an astonishing degree by our efforts to prevent SARS-CoV2 transmission, a testament not only to the lower transmissibility of influenza, but also to the regular round of death and illness that we have been putting up with for millennia without much complaint.

After all the hand-wringing about hand-washing, we gradually learned that this new virus is almost exclusively passed by aerosols through the air, with limited range in space and time. Also that, despite the infinitesimal size of the virus, that face masks of many kinds are effective in knocking down both emission and reception of viral innocula by several-fold. This is doubtless because both the viruses with their lipid coats, and the moist aerosols they reside in, are quite sticky, prone to capture by even rough cloth filters with channels many times the size of a viral particle. The notorious superspreader events are characterized by 

• indoors, close physical proximity to others
• limited air circulation
• an infected person, typically asymptomatic, engaging in
• vocal activity, like singing or loud talking
• with no mask

Scale drawing of surgical mask fibers, against viral and aerosol particle sizes.

Meditation is not conducive to transmission, nor do most infected people transmit their infection. Superspreaders seem to have a very high viral load in key areas of their vocal or respiratory tracts that leads to abundant aerosol emissions with high viral counts. For recipients, it takes numerous viruses to establish an infection- something like 300 for influenza, and something similar for SARS-CoV2. This is a live virus count, not counting inactive viruses, which are always part of the produced and transmitted population of particles. The reason is probably due to our various innate clearance mechanisms, both physical and molecular, meaning that only one virus may get through to successfully infect someone, out of a population of thousands that that person breathed in. 

SARS-CoV2 transmission vs influenza. SARS-CoV2 seems to survive longer in air, leading to more infections in enclosed spaces. Being outdoors subjects the aerosols to getting blown away, and to purifying UV light. This graph does not show it, but SARS-CoV2 also differs in having high viral loads prior to symptom onset, or sometimes without any symptoms, making isolation and contact tracing very difficult.

Additionally, infection by one or few viruses may present a speed problem, where they can not grow an infection fast enough relative to the ability of the immune system to respond and put out small fires. Only if the inoculum immediately generates a large conflagration (think Molotov cocktail) is the fire department overwhelmed, at least for a few days. This leads in turn to the fascinating prospect of mass inoculation with small doses of the virus. Understandably, this is not a popular idea, with its similarity to playing roulette. It resembles the old-fashioned method of small pox inoculation, which used to be done with small doses of actual small pox, not cow pox as was later introduced by Jenner. 

But it may be a significant explanation behind the enormous conundrum of the low impact of Covid-19 on tropical and low-income countries. These countries (India, Central America, Nigeria) show quite high seropositive rates, indicating wide-spread infection. But their death rates and hospitalization rates are very low, and they have escaped this pandemic with relative ease. While reporting issues and pre-existing immune exposure are possible explanations, so is a possible warmer outdoor culture with lower innocula and lower-severity infections. An interesting aspect of inoculum size is that it can have far-reaching consequences, with lower-level infections leading to smaller viral counts in the aerosols emitted, thereby causing smaller, less-severe infections in the next recipients.

The study of viral transmission and infectivity could have profound effects on how we deal with this and similar diseases, and one has to say that it has been frustrating that our knowledge of it remains haphazard, and has been so slow in coming, with such mediocre experiments, false starts and poor messaging.

• Restaurants? Don't do it.
• Reclining on an airplane? Don't bet on it.
• Future nuclear power.