For a virus to successfully transmit, it must first escape the body of the person carrying the infection. This escape route is known in epidemiology as the portal of exit. Identifying the portal of exit is the first step in creating effective public health interventions against Coronavirus Disease 2019 (COVID-19), which is caused by the SARS-CoV-2 virus.
Defining the Portal of Exit Concept
The portal of exit is an epidemiological term that identifies the specific path a pathogen takes to leave its current host. This concept is one of the six links in the “chain of infection,” and it is the point where the infectious material is released into the environment. The site of exit usually corresponds to the area of the body where the agent is localized or where it multiplies most effectively.
The portal of exit must be distinguished from the mode of transmission, which describes how the infectious material travels after it leaves the host. The portal is the source of the infectious material, while the mode of transmission is the method of travel, such as through direct contact or airborne particles.
The Respiratory System as the Primary Exit Route
For SARS-CoV-2, the respiratory tract serves as the main portal of exit. The virus replicates heavily in the cells lining these airways, generating a high concentration of viral particles. These particles are then expelled from the body through common mechanical actions like breathing, speaking, singing, coughing, and sneezing.
The process of exhalation creates a fine mist of respiratory fluid that carries the viral particles out of the body. These expelled fluids are categorized based on their size and behavior in the air. Larger, heavier respiratory droplets tend to settle rapidly onto surfaces or the ground within a short distance, typically less than six feet.
Smaller particles, known as aerosols, are less than 5 micrometers and are lightweight enough to remain suspended in the air for extended periods. These fine aerosolized particles can travel much farther than large droplets, especially in poorly ventilated indoor spaces. Both aerosols and droplets are composed of mucus and saliva from the respiratory lining, acting as the biological vehicle for the infectious SARS-CoV-2 virus.
Viral Shedding and the Infectious Period
The process by which the virus leaves the host cells and is expelled into the environment is called viral shedding. This shedding is directly linked to the infectious period, which is the time frame when a person is actively releasing enough viable virus to transmit the infection to others. Understanding the timing of shedding is crucial for determining effective isolation and quarantine measures.
For COVID-19, viral shedding often begins before a person develops any symptoms, a phase known as the pre-symptomatic period. The amount of virus shed, often measured as the viral load, typically reaches its peak around the time of symptom onset or shortly thereafter. This high viral concentration in the upper respiratory tract during the early stages contributes to the virus’s efficient spread.
For most people with mild to moderate COVID-19, the period of infectious viral shedding generally declines after the first five to ten days following symptom onset. However, in individuals with severe disease or those who are immunocompromised, infectious shedding can persist for a longer duration. The detection of viral RNA through PCR tests can last even longer, but this does not always indicate the presence of live, transmissible virus.