Study Shows That Higher Viral Loads And A Lower Minimal Infective Dose Increases Risk of Aerosol Transmission By SARS-Cov-2 Delta And Omicron Variants!
Omicron Aerosol Transmission
: A new Swiss led study involving researchers from the Swiss Centre for Occupational and Environmental Health-Switzerland, Universidad del Rosario-Colombia, Tokyo University of Science-Japan, Repubblica e Cantone Ticino-Switzerland, Centre for Primary Care and Public Health (Unisanté)-Switzerland and the State Secretariat for Economic Affairs (SECO)-Switzerland has found that higher viral loads and a lower minimal infective dose compared to the SARS-CoV-2 wild type strain increases the risk of aerosol transmission by the SARS-CoV-2 Delta and Omicron variants.
The study findings also importantly show that surgical masks are no longer sufficient in most public settings, while correctly fitted FFP2 respirators still provide sufficient protection, except in high aerosol producing situations such as singing or shouting.
Airborne transmission of SARS-CoV-2 is an important route of infection. For the wildtype (WT) only a small proportion of those infected emitted large quantities of the virus.
However, the currently prevalent variants of concern, Delta (B1.617.2) and Omicron (B.1.1.529), are characterized by higher viral loads and a lower minimal infective dose compared to the WT.
The Omicron Aerosol Transmission
study team aimed to describe the resulting distribution of airborne viral emissions and to reassess the risk estimates for public settings given the higher viral load and infectivity.
The study team reran the Monte Carlo modelling to estimate viral emissions in the fine aerosol size range using available viral load data.
The team also updated their tool to simulate indoor airborne transmission of SARS-CoV-2 by including a CO2 calculator and recirculating air cleaning devices. They also assessed the consequences of the lower critical dose on the infection risk in public settings with different protection strategies.
the study findings from their modelling suggests that a much larger proportion of individuals infected with the new variants are high, very high or super-emitters of airborne viruses: for the WT, one in 1,000 infected was a super-emitter; for Delta one in 30; and for Omicron one in 20 or one in 10, depending on the viral load estimate used. Testing of the effectiveness of protective strategies in view of the lower critical dose suggests that surgical masks are no longer sufficient in most public settings, while correctly fitted FFP2 respirators still provide sufficient protection, except in high aerosol producing situations such as singing or shouting.
From an aerosol transmission perspective, the shift towards a larger proportion of very high emitting individuals, together with the strongly reduced critical dose, seem to be two important drivers of the aerosol risk, and are likely contributing to the observed rapid spread of the Delta and Omicron variants of concern. Reducing contacts, always wearing well-fitted FFP2 respirators when indoors, using ventilation and other methods to reduce airborne virus concentrations, and avoiding situations with loud voices seem critical to limiting these latest waves of the COVID-19 pandemic.
The study findings were published in the peer reviewed journal: Swiss Medical Weekly.
Numerous past studies have indicated that the virus concentration in a room when infected humans exhale aerosols can be determined by combining the viral load in the lungs and throat with the known emissions of respiratory aerosols.
There are also other factors that play a key role in impacting the viral emission strength such as the size of the room, air exchange rate, and half-life of the virus when it is airborne.
The study team used a Monte Carlo model to describe the expected distribution of viral emission by an infected population of people who were either silent, speaking softly, or loudly. It also published a spreadsheet-based tool for the assessment of indoor airborne transmission of SARS-CoV-2 concerning room and ventilation parameters, different vocal and physical activities, and the types of masks worn by the emitter and the receiver.
The utilized tool was further updated by the addition of a recirculating air purification parameter and a CO2 simulator.
It should be noted however that these initial models were developed for the wild type (WT) of the virus. With time several variants of SARS-CoV-2 have emerged, the Delta (B1.617.2) variant and the recent Omicron (B.1.1.529) variant having higher transmissibility.
Existing data from previous studies suggests that the viral loads of these variants are higher than the WT.
Importantly the viral load of Omicron was reported to be ten- to one hundred-fold higher than that of Delta. Furthermore, the number of cells infected for a given number of ribonucleic acid (RNA) virus copies were found to be doubled and quadrupled for Delta and Omicron respectively.
Interestingly the critical dose of virus copies beyond which it is said to be infectious was reported to be 500 for the WT while it was 300 copies for Delta and 100 copies for Omicron. Also, the immune evading property of Omicron was found to be higher as compared to Delta and WT.
The study was aimed at determining the risk from higher viral loads and infectivity associated with the new variants of concern for SARS-CoV-2 by estimation of high, very high, and super-emitting individuals in a population along with the impact of a lower critical dose.
The research involved modeling the variants of concern using random sampling. A near-field/far-field well-mixed model was used for the estimation of emission rates for viruses in aerosols when a person is either silent, talking softly, or talking loudly.
A number of factors such as vocal loudness, room characteristics, average air velocity, air exchange rate, and degree of physical activity and mask types worn by the emitter are then combined with the half-life of the virus for determination of viral load after a given time along with the time required to reach the critical dose.
In the updated tool version, the single air exchange term was replaced by outdoor air supply and recirculating air-cleaning devices. Also, a CO2 calculator was added to the tool for the determination of CO2 concentration at quarter-hourly intervals as well as when leaving the room.
The study findings indicated that the emergence of the variants of concern led to an increase in the frequency of super-emitting individuals. During the circulation of WT, the super-emitters were reported to be 1 in 1000 infections while during Delta it was 1 in 30 and during Omicron it was 1 in 10 or 20.
Furthermore, the frequency of high and very high emitting individuals also increased for Delta and Omicron as compared to WT.
The study findings also indicated also that the Omicron and Delta variants had much higher infectivity and lower critical dose as compared to the WT and cannot be controlled by wearing surgical masks
Importantly, for the Delta and Omicron variants, FFP2 respirators were found to provide sufficient protection rather than surgical masks. However, even FFP2 respirators were found to be ineffective in case of prolonged exposure to extreme aerosols.
The study findings show that that higher viral load along with higher infectivity leads to the rapid spread of Omicron and Delta. However, there are other ways by which variants can affect transmission such as altered mucus viscosity and increased viral production near the vocal cords. Further research needs to be done to address such questions.
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