In a recent study posted to the medRxiv* preprint server, researchers determined the adherence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta variant to masks and respirators and the role of buccal protection in controlling SARS-CoV-2 transmission.
The results confirmed the effectiveness of facial protection in lowering SARS-CoV-2 transmission and infection.
Coronavirus disease 2019 (COVID-19), caused by the novel SARS-CoV-2 virus initially identified from Wuhan, China, has caused an unprecedented global health crisis during the last two years. COVID-19 is associated with aerosol transmission, and the use of masks and respirators during the COVID-19 pandemic has significantly helped reduce the SARS-CoV-2 transmission rate.
Different types of disposable masks such as N95 respirators and surgical masks are available. Surgical masks approved by the Food and Drug Administration (FDA) are intended for use in hazardous environments created by dangerous fluids, droplets, splashes, and sprays, according to the Centers for Disease Control and Prevention (CDC) and the National Institute for Occupational Safety and Health (NIOSH). However, respiratory protection by surgical masks against small airborne particles is very low.
On the contrary, N95 masks are described by the NIOSH as a filtration system that retains at least 95% of various sized airborne particles ranging from aerosols to large droplets and reduces the vulnerability of the user. Additionally, N95 masks have extra layers of protection compared to surgical masks.
Although there are previous studies on the filtration mechanism of aerosols containing SARS-CoV-2 in masks, detailed studies on the adherence of the SARS-CoV-2 virus on different layers of the masks are not yet available.
In the present study, the researchers evaluated SARS-CoV-2 adherence to masks and respirators by dissecting the layers of four N95 respirators and one surgical mask. The study included N95 respirators such as a closed-cell vinyl foam (North N95 7130), open-cell foam seal (North N95 5130), a standard respirator (3M 8210 N95), and a surgical respirator (3M 1860 N95), and a three-ply surgical mask.
The SARS-CoV-2 Delta variant was collected from the British Columbian Centre for Disease Control Public Health Laboratory, Vancouver, Canada. All experiments for the study were conducted in level three at the Facility for Infectious Disease and Epidemic Research in the British Columbian Centre for Disease Control Public Health Laboratory.
Vero cells were used for the replication and infection of the Delta variant and were grown in a minimum essential medium (MEM) Invitrogen supplemented with pyruvate, non-essential amino acids, and 5% fetal calf serum. To obtain viral stocks, Vero cells were infected with 1 ml of the virus at a 5x105 PFU/mL concentration. The viral titer was determined to be 5.9x105 pfu/mL using plaque assay.
The individual layers of the masks were decontaminated and stored in empty sterile 1.5 mL tubes until used. The layers were then exposed to the SARS-CoV-2 Delta variant in MEM and vortexed. The samples were subjected to 10-fold dilutions using Opti-MEM in 1.5 ml tubes.
The 5.0 x 105 Vero E6 cells were seeded into 12-well cell culture plates and incubated overnight. Further, the serial dilutions of the viral samples were added to the corresponding wells and incubated. The cells were incubated at 37oC supplemented with 5% carbon dioxide in an incubator. Later, the viral adherence was determined using a plaque assay.
The results showed that all layers of the masks retained SARS-CoV-2 at various degrees. Additionally, vortexing the samples did not release all adherent viruses from masks.
Log reduction of the layers had an accumulation ranging between 6.15-8.45. The open-cell foam seal respirator had superior retention of the virus with an increase of around one to two log reduction compared to other tested products.
At least one layer of polypropylene is present in respirators used in the study and augmented the viral retention by the electrostatic adsorption. According to the model of viral retention based on the electrostatic charges on the surface of the virus and the modified polypropylene, a strong electrostatic attraction arises between the receptor-binding domain (RBD)of the Spike (S) protein and the polypropylene electret, playing a major role in the retention of the viruses on the layers of the masks.
The study findings demonstrated the SARS-CoV-2 Delta variant’s adherence to several buccal protection methods such as masks and respirators. Vortexing did not release all adherent viruses, indicating the viruses have tightly adhered to the layers once they are dried. Strong electrostatic binding was suggested between the virus and the mask layers, and as a result, over six-fold cumulative viral log reduction was observed.
Overall, the study showed that masks and respirators successfully decrease the transmission of SARS-CoV-2.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.