Transmission and Infectivity
Our knowledge and understanding of this novel coronavirus that has caused the COVID-19 pandemic continues to grow and evolve. I am going to weave together the results of three recent studies that give us a much better understanding regarding the transmissibility and infectivity of this virus.
While I characterize these new findings as surprises, and they will be for the vast majority of my readers, but others, who have studied this virus and its behavior closely, will be much less surprised and have some long-held suspicions confirmed.
Let’s start with a review of how respiratory viruses are transmitted and infect people. Airborne transmission of respiratory viruses | Science.
Long-time readers of my blog will remember our discussions early on in the pandemic about the main mechanisms of transmission of respiratory viruses – droplets and aerosols. If you talk to aerosol scientists or air engineers you will get into technical discussions about the size of each in microns (1 micron = 1 one thousandth of a millimeter or 0.00003937ths of an inch), but few people can imagine that in relative terms, e.g., 1 micron would be roughly 1/3rd – 1/8th the diameter of a strand of a spider’s web or if you held up a sheet of paper and examined the thickness, a micron would be roughly 1/70th to 1/180th of that). My guess is that most of you are still struggling other than to realize, it is really small and too small to be seen with the naked eye. Okay, you have that right. While the cut-off in size between a droplet (>5 microns) and an aerosolized molecule (<5 microns) is a bit arbitrary and of some debate, I don’t think we need to dwell on this. Take-away these points, and then let me explain why any of this makes a difference:
- When viruses are carried in droplets or aerosols, they are small, and generally not visible to the naked eye.
- Droplets are bigger than aerosols.
- Droplets place those in close contact (<6 feet away from the infected person).
- Aerosols place everyone in the room, hallway or other indoor space that is served by a common ventilation system at risk.
What I find is much more helpful to explain these concepts is to use an analogy. We all have experienced ourselves or witnessed in someone else, especially when they are talking fast, loud and/or for a long time, little drops come out of their mouths while talking, leading to the tease often used by children: “Say it, don’t spray it!” Think about these as droplets. We also have experienced spraying our hair, spraying bug sprays while camping, spraying sun tan lotion on ourselves and our kids at the pool or the beach, or even seen those sprays used in movies by art thieves who are trying to detect laser beams that trigger alarms in museums. Think of those mists as aerosols.
For some time, it has been thought that most respiratory pathogens (viruses and bacteria that cause human disease) are spread between people via large droplets produced by coughing, talking, singing, shouting, etc. Droplets are large because they consist of mucus and saliva, which in turn carry virus within them. Because they are larger and have the accompanying secretions, they tend to travel only 3 – 6 feet from the mouth before falling to the floor or a surface, including someone else’s face who happens to be standing within that amount of distance from the person expelling the virus. Now, exactly how far the droplet travels can be affected by the ambient temperature (by impacting the rate of evaporation), the relative humidity, whether there is a breeze or other increase in air currents, by UV light, and by how hard the person coughs, sneezes or projects their voice. When the person coughs or sneezes with a mask on or into a tissue), then far fewer droplets will threaten those within close distance to the person who is infected.
Many viruses and bacteria are sensitive to light, heat and moisture and therefore may not survive very long when droplets land on surfaces, however, there are some that can retain their infectivity for hours or even for days, and this latter group of viruses and bacteria can then be a source of potential infection when someone touches the surface and then places their hand to their mouth, e.g., in eating. When people are infected through contact with surfaces or other inanimate objects such as bed linens (examples include norovirus, the monkeypox virus and Ebola virus), we refer to that as fomite transmission.
However, with development of aerosol science and new techniques for studying both droplets and aerosols, we have learned that aerosols play an important role in the transmission of a number or respiratory viruses, and for some viruses, may even be the predominant mode of transmission.
As opposed to droplets, aerosols (fine mists containing viruses) can float and travel in air streams because without the large amount of surrounding mucus and saliva, they are much lighter and do not fall to the ground so quickly.
Of course, like SARS-CoV-2, a virus that can infect you by aerosol transmission when you are at the back of a classroom, as an example, can also infect those in the front row by droplet transmission. Generally, for SARS-CoV-2 transmission is predominantly aerosol in nature. Droplet transmission is predominant only when people are in very close contact (<8 inches apart).
Viruses for which there is convincing evidence for aerosol transmission include:
- Severe Acute Respiratory Syndrome coronavirus (SARS-CoV),
- Severe Acute Respiratory Syndrome coronavirus – 2 (SARS-CoV-2)
- Middle East Respiratory Syndrome (MERS)–CoV,
- Influenza A & B viruses,
- Human rhinovirus, and
- Respiratory Syncytial Virus (RSV).
Except for those viruses and bacteria that are able to survive on surfaces for some period of time while maintaining their infectivity, droplets will seldom pose a risk to others unless you are in close proximity to someone who is infected. On the other hand, aerosols, because they travel on air streams, can pose a risk to anyone in any part of the room or hallway in which air is circulating, and, depending on what rate of air changes the ventilation system is set for, these aerosols can remain in a large room or hallway for minutes to perhaps an hour after the infected person has left the room or hallway. One of the surprising things I found on my walk-through of schools early in the pandemic, was how often the teacher’s desk was directly under the air return, which means all the air streams carrying any virus that might be expelled in that room were in essence directed right at the teacher.
Most recently, we have seen transmission in elevators in which it appears that virus remained suspended in the air after the infected person(s) left the elevator, infecting a person who later rode the same elevator.
Further, droplet transmission alone could not explain observations made relatively early in the pandemic:
- The clear difference in transmission rates that were observed between indoor and outdoor settings;
- Superspreader events; and
- A nicely done study of an outbreak in a school that showed that not only were children in the back of a classroom infected by their teacher, but some children in a classroom across the hall were also infected, as well as another study that examined an outbreak of COVID-19 in a high-rise apartment building that occurred along vertically aligned units that were connected by a single air duct.
But droplets vs. aerosols are not just an important distinction in terms of how close of contact is required for exposure and infection (transmissibility) and how long the virus may remain suspended in air, but the size also determines how far down the respiratory tree viruses are deposited. For example, droplets mostly land in the nose and throat of the person who is exposed, whereas aerosols are small enough that the exposed person can breathe them down into the distant portions of the lungs.
So let’s jump into the recent articles that provide us with new information. The first is, Viral infectivity in paediatric SARS-CoV-2 clinical samples does not vary by age | Microbiology Society (microbiologyresearch.org). There has been so much misinformation put out into the public about children and their relationship to SARS-CoV-2 and COVID-19. Much has been related to either coordinated efforts to remove or not offer infection control measures for children, often under the misguided notion that if we get children infected, they will not experience anything other than a “cold” while promoting the ill-conceived notion that it would advance the development of herd immunity or to efforts by antivax groups to dissuade parents from getting their children vaccinated.
One such topic of misinformation was the mantra that children, even if infected, do not produce high levels of virus in their upper respiratory tracts, and thus pose little threat of forward transmission of infection. This study’s investigators found that age had no impact on the infectiousness of SARS-CoV-2 within our cohort, with children of all ages able to produce high levels of infectious virus. In other words, children of all ages, from infants to teen agers to those attaining the age of majority, had equally high viral loads of infectious virus. (Another study has shown that viral load correlates with the risk of transmission within families and their community: SARS-CoV-2 viral load is associated with risk of transmission to household and community contacts | BMC Infectious Diseases | Full Text (biomedcentral.com)).
The second study (https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2805468) with surprises found that in households, 70.4% of infections began with an infected child. This was not terribly surprising to experts. We have long known that the major transmission pathway for influenza is from children in schools to their family members. In this case, many of us warned that by not implementing any significant infection control measures in schools, we were likely establishing a pathway for SARS-CoV-2 to infect school-age children who would then take the virus home to, in many cases, multi-generational families and infect high-risk individuals.
The third study (Viral emissions into the air and environment after SARS-CoV-2 human challenge: a phase 1, open label, first-in-human study – The Lancet Microbe) revealed that superspreaders have higher viral emissions from their noses and that viral emissions correlate to some degree with viral loads on nasal swabs, however some individuals with low viral loads in their nose still emitted high levels of virus.
Viral emissions did not correlate with how symptomatic the infected person was, however, 90% of the viral emissions do occur during the symptomatic phase of infection.
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