In a recent study published on medRxiv* preprint server, researchers assessed the robustness and feasibility of a layered coronavirus disease 2019 (COVID-19) mitigation strategy involving the use of ionizers, masking, filtration, and ventilation to limit school transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
In the current scenario, elucidating the transmission mechanisms of SARS-CoV-2 is key to mitigating COVID-19. Studies have reported transmission of SARS-CoV-2 in schools that may accelerate viral evolution and thus further prolong the COVID-19 pandemic.
The study and the results
In the current study, researchers used a computational fluid dynamics (CFD) model to simulate the transmission of SARS-CoV-2 in schools and assess the effectiveness of the layered COVID-19 mitigation strategies required if the schools were to remain open. They evaluated the effect of infection control methods in several scenarios corresponding to variants with different viral loads.
The model was designed to assess viral transmission in indoor environments assuming aerosol transmission of airborne SARS-CoV-2 in enclosed spaces. The classroom was supposed to be a well-mixed container.
The team assessed the concentration of virions released into the classroom air by a viral transmitter (infected individual). The estimated concentration of emitted virions was then inserted into the model to determine the likelihood of uninfected individuals becoming positive for SARS-CoV-2.
The effects of air quality (ionizers, ventilation, and filtration) and masking on steady-state viral concentrations of virions emitted by infected virions and inhaled by the uninfected individual were assessed for Omicron, Delta and Wuhan strains with different viral loads to assess the effect of viral evolution on viral loads.
The model estimated that for a standard classroom (120 inches x 240 inches x 240 inches), the steady-state viral concentration emitted by an infected individual from Wuhan (low viral load) was approximately four virions / L, which would increase to hundreds and thousands when infected with Delta (high viral load) and Omicron (medium viral load) infections. Steady-state concentration of the virion would be reached if an emitter was present in the room for one to two hours in the case of the Wuhan-like variant and within minutes in the case of the Delta- or Omicron-like variant . This indicates that the concentration of emitted virions was directly proportional to the viral load of the SARS-CoV-2 strain.
If the transmitter were masked, the virion emission rate would decrease, decreasing the steady-state virion concentration. However, masking had no impact on the period during which the steady-state concentration of the virion would be reached. After the transmitter left the room, viral loads returned to almost zero within five hours.
By using air filters, the steady-state virion concentration would be lower since the virions would be eradicated from the classroom at a faster rate. For transmitters infected with the Wuhan strain, masking and air filtration would reduce the steady-state concentration of the virion to one virion/L of air. However, for those infected with the Delta or Omicron variants, the steady-state virion concentration would be consistently high despite the high rate and efficiency of virion shedding.
Additionally, virion emissions could also be reduced by ionizers to less than one virion per liter per hour in the event of slow virion emission (Wuhan strain infection or masked emitter). Ionizers inactivate virions and remove them from the air. However, if the emitter had a Delta infection or if virion emission continued for several hours (such as an eight-hour school day), viral concentrations could reach high levels despite the use of an ionizer. .
To acquire COVID-19, SARS-CoV-2 must be inhaled and deposited on the mucosal surfaces of the nasopharynx and respiratory tract. The amount of virion deposition in the nasopharynx was assessed by CFD airflow simulations which showed that 0.6% of breath-inhaled virions were deposited in the nasopharynx. This estimated value of nasopharyngeal virion deposition was used to assess the deposition of virions in the respiratory tract mucosa of an uninfected individual per hour.
High-quality masks that filter out almost all virions (>95%) could reduce viral spread in high-risk areas and increase transmission time by 10 times if the transmitter is masked (simple masking). If virion-emitting and inhaling individuals are masked (double masking), less effective masks such as tight-fitting surgical masks could achieve similar transmission time. Additionally, the team noted that if a transmitter was seated in the middle or back of the classroom, the aerosolized virions could be efficiently transmitted to uninfected individuals.
In conclusion, the study results showed that reopening schools without robust measures to mitigate COVID-19 such as masks, ionizers, air filters, widespread COVID-19 testing and school closures during times of high local viral transmission and restricted classroom capacity (by providing remote schooling) could accelerate the COVID-19 pandemic.
medRxiv preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behavior, or treated as established information.