#16,139
While it may not always prove correct, when a new pathogen appears on the scene, initial public health guidance is based mostly on what we know about previous outbreaks of similar pathogens.
In the case of COVID-19, in the first half of 2020 the closest analogue we had was SARS-CoV from 2002-2003, which turned out to be far less agile than this newly emerged coronavirus.
Despite growing anecdotal evidence suggesting we were underestimating the transmissibility of SARS-CoV-2 - without compelling scientific evidence to rely on - public health guidance was often slow to evolve.
This led to delays in the WHO declaring COVID a pandemic, delays before the CDC (and other public health entities) endorsed face masks in public spaces, and to a 6+ month debate over whether COVID was an `airborne' virus (vs. large droplets).
Until October of 2020, a `close contact' of a known or infected person - for the purposes of contact tracing or quarantine - was defined by the CDC as:
Someone who was within 6 feet of an infected person for at least 15 minutes starting from 2 days before illness onset (or, for asymptomatic patients, 2 days prior to specimen collection) until the time the patient is isolated.
This `15 minutes' exposure within `2-meters' was admittedly arbitrary, and was based on a short exposure during the SARS outbreak being unlikely to result in disease transmission.
Today, particularly with the more infectious COVID variants, even brief exposures are considered transmission risks (see Australia: Victoria Govt's Unusually Strong Statement on The Transmissibility of COVID Variant B.1.617.1 (Kappa))
The spread of COVID aerosols, via ventilation and air conditioning systems, was initially downplayed (see June 2020's ECDC Technical Report: Heating, ventilation and air-conditioning systems in the context of COVID-19), which reassured.
In conclusion, the available evidence indicates that:While there are still gaps in our knowledge, our understanding of COVID continues to evolve (albeit, not always as fast as the virus), as new studies are published.
- Transmission of COVID-19 commonly occurs in closed indoor spaces.
- There is currently no evidence of human infection with SARS-CoV-2 caused by infectious aerosols distributed through the ventilation system ducts of HVACs. The risk is rated as very low.
- Well-maintained HVAC systems, including air-conditioning units, securely filter large droplets containing SARS-CoV-2. It is possible for COVID-19 aerosols (small droplets and droplet nuclei) to spread through HVAC systems within a building or vehicle and stand-alone air-conditioning units if air is recirculated.
- Air flow generated by air-conditioning units may facilitate the spread of droplets excreted by infected people longer distances within indoor spaces.
- HVAC systems may have a complementary role in decreasing transmission in indoor spaces by increasing the rate of air change, decreasing recirculation of air and increasing the use of outdoor air.
After the epidemiological investigation suggested cross building contamination, a field experiment was conducted at that location using fluorescence microspheres with similar aerodynamic characteristics to the SARS-COV-2 spike pseudovirus as a proxy, to try to document its spread via A/C and ventilation systems.
I've only posted some excerpts from the article. Follow the link to read the full report.
Preplanned Studies: Field Simulation of Aerosol Transmission of SARS-CoV-2 in a Special Building Layout — Guangdong Province, China, 2021
Zhuona Zhang1;Xia Li1;Qin Wang1;Jin Xu2 ;Qinqin Jiang 3;Sili Jiang3;Jiayun Lyu3;Shiqiang Liu3;Ling Ye4;Jun Yuan 3; Wenru Feng 3; Dongqun Xu1, , View author affiliations
Summary
What is already known on this topic?
Aerosol transmission was one route for the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and usually occurred in confined spaces.
What is added by this report?
Aerosol transmission was found to exist between handshake buildings, i.e., buildings with extremely close proximity that formed relatively enclosed spaces. Transmission was mainly affected by the airflow layout caused by switching air conditioners on and off as well as opening and closing doors and windows.
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What are the implications for public health practice?
Centralized isolation and home isolation in handshake buildings creates a risk of SARS-CoV-2 aerosol transmission under certain conditions. Attention should be paid to the influence of air distribution layout on aerosol diffusion in isolation wards, and disinfection of isolation venues should be strengthened.
Transmission of coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), by aerosol has been confirmed in many studies (1-4), but transmission usually occurs in a confined space. In the epidemic that occurred in Guangzhou City of Guangdong Province in May 2021 (5), the index case and a close contact (later diagnosed as infected) arrived on international flights and were located in 2 different buildings in the hospital at the same time before diagnosis.
The buildings were close to each other and formed a relatively enclosed space due to the exterior ceiling between the two buildings; buildings in this layout are sometimes informally referred to as handshake buildings due to their extremely close proximity—in this case, approximately 50 cm separated the handshake buildings.
Figure 1 (B) Interior room layout of the buildings.
Epidemiological investigation and viral gene sequencing showed that there was a temporal and spatial crossover between the two individuals and their genetic sequences were highly homologous, so aerosol transmission may be likely.We used fluorescence microspheres with similar aerodynamic characteristics to the SARS-COV-2 spike pseudovirus to investigate the transmission path and influencing factors of the virus aerosol through field experiment simulation. The results showed that there was clear aerosol transmission path from the location of the close contact and the index case, and its transmission was mainly affected by the airflow layout that resulted from switching the air conditioner on and off as well as opening and closing doors and windows.In the future, more attention should be paid to the risk of aerosol transmission in close-proximity buildings and to the influence of air distribution layout on aerosol diffusion in isolation wards.
(SNIP)
Although some results were obtained, this study was subject to some limitations. First, real SARS-CoV-2 could not be used in real environments due to hazardous risk, so the virus aerosol could not be properly quantified. Second, there was no way to recover the meteorological conditions when the index case stayed in the hospital, so the analysis may be subject to some biases.
The above aerosol simulation experiments for tracing the index case not only supported aerosol transmission but also found key factors affecting transmission. Therefore, adequate space should be maintained between isolation wards and routine outpatient areas in hospitals, and air distribution layouts should be examined in isolation wards. Furthermore, disinfections in the isolation area need be strengthened. COVID-19 aerosol transmission risk exists in many handshake buildings in Guangzhou due to centralized isolation and home isolation and is highly concerning.
Despite this study, the absolute risk of spread of COVID via A/C and ventilation systems has yet to be quantified, and the CDC states: The risk of spreading SARS-CoV-2, the virus that causes COVID-19, through ventilation systems is not clear at this time.
CDC recommends a layered approach to reduce exposures to SARS-CoV-2, the virus that causes COVID-19. This approach includes using multiple mitigation strategies, including improvements to building ventilation, to reduce the spread of disease and lower the risk of exposure. In addition to ventilation improvements, the layered approach includes physical distancing, wearing face masks, hand hygiene, and vaccination.
Although the more transmissible Alpha and Delta variants are more formidable than the original `wild type' virus of last year, in 2020 we saw multiple examples of the rapid spread of COVID aboard cruise ships, in churches, restaurants, apartment complexes, nursing homes, and hospital wards.
MMWR: High COVID-19 Attack Rate Among Attendees at Events at a Church
MMWR Early Release: COVID-19 Superspreading Event In A Church Choir
MMWR: Asymptomatic & Presymptomatic SARS-CoV-2 Infections in Residents of a LTCF
The best evidence now suggests the old `6-feet-of-separation' rule is insufficient, particularly in indoor or enclosed spaces. The CDC's most recent scientific brief on COVID transmission allows that `Transmission of SARS-CoV-2 from inhalation of virus in the air farther than six feet from an infectious source can occur.'
All good reasons why, vaccinated or not, you should consider wearing a face mask when in public, particularly in enclosed spaces. Even if you are able to keep 6 feet of separation between you and others.
