Transmission of COVID -19 Viruses
The three main factors which govern the spread of the COVID-19 virus are: its own biology (how contagious) the susceptibility of the populace (how vaccinated) and the population’s behaviour (how people protect against the virus and interact with others).
Appropriately, when this pandemic began, there was a huge focus on public health measures that should prevent virus transmission. These measures are well known from experience with other infectious diseases. These well-known basics are wearing face masks, cleaning surfaces including the hands, physical distancing from adjacent persons, and not gathering in large groups. These four public health measures are based on the expected behaviors of the virus as it infects people. However, our understanding of the infectious process has evolved and at the same time variants have become more transmissible and more severe. These factors must govern our future public health and personal care planning.
Initially we concentrated on the concept that an infected person passed on the virus primarily in large droplets by coughing and sneezing. These droplets were thought to be large enough that after coughing or sneezing they dropped to the ground or floor within two metres. This understanding drove the guidance to distance oneself at least two metres from others. Further, we felt that the large droplets could be prevented from leaving the infected person by them wearing a mask – or protect a non-infected person by wearing a mask. This is all correct.
We also thought that the virus could infect a person by being transferred from a contaminated surface to one’s hand and then to the mouth. However, this has rarely been documented and questions the validity of the ubiquitous practice of frequent sterilization of many “public” surfaces.
The newer information is that the virus is present in high concentrations in fine droplets often called aerosols, which are less than five microns in size. These droplets are usually formed deep in the lungs and are expelled by normal breathing. They are produced in great amounts by talking, singing and during heavy exertion. The concentration of virus in fine droplets is frequently greater than that in large droplets. Most concerning is the fact that these fine droplets can remain suspended in the air for hours and if indoors they will build up in concentration overtime. This build-up occurs in varying amounts depending upon the frequency of air exchange in the room and the quality of air filtration. There is robust evidence that the bulk of COVID-19 infections are due to fine droplet transmission rather than by large droplets. It is particularly telling that people have rarely been infected when outdoors, and further, spreader and super spreader events occur indoors.
Both the World Health Organization and the US Centers for Disease Control and Prevention (CDC) have acknowledged that at both long and short distances inhalation exposure to small droplets is the most important mode of spreading of COVID-19.
See Appendix for more details.
How Does the Use of Face Masks Protect Us from Droplet Transmission?
Although masks are very effective at decreasing large droplet transmission, the story is not as good for the effectiveness of masks in preventing the passage of small droplets. Fine droplet protection will depend upon the filtration quality of the mask and how snuggly the mask fits the face. To improve filtration of fine droplets, mask construction should use finely woven or spun materials in three layers.
Mask fit to the face is also important. A good quality face mask with gaps at the sides of the mask will allow significant amounts of air to escape or enter and will result in loss of much of the value of the mask. This gap problem is sometimes present with medical masks that commonly use ear loops. These leaks can easily reduce the efficiency of the mask to 50%. Double masking such as wearing a tight-fitting cloth mask on top of a blue “paper” medical mask can obliterate gap problem as can knotting the blue ties close to the mask. Both double masking and knotting are very effective.
The best masks are N95 or K95 — which by design prevent the transmission of at least 95% of airborne particles which are 0.3 microns or larger — as long as the mask fits snuggly to the wearer’s face. Most studies of N95 masks show a filtration efficiency of over 99%. Early in the pandemic these masks were in short supply, and were reserved for high-risk health care workers, but are now available on the internet. High quality masks are very necessary indoors or in enclosed spaces where the concentration of fine airborne droplets can build up.
In summary recent knowledge is that fine droplets are produced by normal breathing and talking. These droplets are heavily laden with virus. If the air is not moving, they can float about and build up concentration over time. Epidemiological evidence supports fine droplets as the cause of most transmissions of COVID-19 virus.
The basics for protection from fine droplet spread is being cautious when indoors, having frequent circulation of filtered indoor air and the use of efficient face masks.
Appendix : Additional Data on COVID-19 transmission.
ABOUT AIRBORNE DROPLETS
Fine droplets (aerosols) are smaller than 5 microns. The majority are less than 1 micron (for reference – a human hair is about 70 microns in diameter).
Depending on their size, fine droplets can remain floating in the air for ½ to many hours before dropping.
In laboratory studies the half-life of virus in aerosols in terms of retention of infectivity is 1-3 hours for Covid -19. This may vary for variants.
The concentration of virus in fine droplets is often 2 times that in large droplets.
ABOUT THE VIRUS
The infectiousness of a person with Covid-19 often peaks two days before symptom onset and extends for a variable time thereafter.
Commonalities among superspreading events include indoor settings, crowds, exposure durations of 1 hour or more, poor ventilation, vocalization, and lack of properly worn masks.
CDC currently recommends that all persons – both unvaccinated and vaccinated wear a mask indoors in areas of substantial risk of transmission.
For more details see: https://www.science.org/doi/full/10.1126/science.abd9149