Viral shedding is a natural biological process where an infected host releases viral particles into the environment. This occurs when a virus multiplies within the body’s cells, producing new copies that then exit the host. For COVID-19, viral shedding refers to the release of SARS-CoV-2 particles from an infected individual, which is important for understanding how the virus spreads.
Understanding COVID-19 Viral Shedding
COVID-19 viral shedding involves the expulsion of SARS-CoV-2 components, including viral RNA, proteins, or infectious virus particles. The virus enters the body, often through the respiratory tract, binds to cell receptors, and uses the cell’s machinery to replicate. These new viral copies then leave the infected cells.
The primary routes for SARS-CoV-2 shedding are through respiratory droplets and aerosols generated from the upper respiratory tract. These are released when an infected person talks, exhales, coughs, or sneezes. Viral shedding can also occur in stool, although its role in transmission is considered less significant.
It is important to distinguish between shedding infectious virus and shedding non-infectious viral fragments. While the body may continue to shed viral RNA for weeks or even months after infection, this does not always mean that viable, replication-competent virus is present. The detection of viral genetic material, particularly by highly sensitive tests, does not automatically equate to contagiousness.
Duration of Shedding and Contagiousness
The duration an individual sheds SARS-CoV-2 and remains contagious varies, influenced by several factors. People are most contagious in the days leading up to symptom onset and during the initial days of illness. Infectiousness can begin approximately 1 to 2 days before symptoms appear and peaks around symptom onset or shortly thereafter.
For individuals with mild to moderate COVID-19, contagiousness lasts up to 10 days after symptoms begin. Those with severe illness or compromised immune systems may shed infectious virus for a longer duration, sometimes up to 20 days or more.
While viral RNA can be detected for an extended period, sometimes up to 60 days or more by highly sensitive tests, the ability to isolate live, infectious virus generally declines significantly after 8 to 10 days from symptom onset for most people.
Implications for Testing and Public Health Guidelines
The understanding of viral shedding has direct implications for COVID-19 testing strategies. PCR (Polymerase Chain Reaction) tests, which are a type of Nucleic Acid Amplification Test (NAAT), are highly sensitive and detect SARS-CoV-2 RNA. Because these tests can detect even non-infectious viral fragments, a positive PCR result can persist for weeks or months after a person is no longer infectious.
In contrast, rapid antigen tests are more likely to detect the presence of active, infectious virus. These tests are generally less sensitive than PCR tests, especially in asymptomatic individuals, but can provide results quickly. A positive antigen test is usually reliable, particularly if symptoms are present, and can indicate the presence of enough viral protein to suggest contagiousness.
Public health organizations use the knowledge of viral shedding duration and contagiousness to formulate isolation guidelines. Recommendations often advise individuals to isolate until symptoms improve and they have been fever-free for at least 24 hours without medication. For an additional five days, precautions like mask-wearing and physical distancing are suggested to further reduce transmission risk.