Fig. 4.

URT commensal bacteria can hamper SARS‐CoV‐2 replication and infection. In healthy individuals, who show no symptom of COVID‐19 disease, the S‐protein‐binding proteins expressed by URT commensal bacteria trap viral particles. Consequently, the viral replication rate abolishes. Due to ageing or other conditions, the population of the URT commensal bacteria, which produce the S‐protein ‐binding proteins, reduces. As a result, either viral particles easily reach LRT causing severe disease (path 1) or viral particles infect the URT tissue (path 2), which increases replication rate and viral load. The resulting viral particles reach the LRT and cause severe disease.