Duration of contagiousness of coronavirus
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Duration of Contagiousness of Coronavirus: Key Findings
Peak Infectivity and Early Contagious Period
Research shows that people infected with SARS-CoV-2 are most contagious around the time their symptoms begin. Infectivity drops quickly after this point, especially in those with mild or moderate illness, with the risk of spreading the virus becoming very low after about 10 days. For people with severe illness or weakened immune systems, the period of potential contagiousness can last up to 15 days, and in rare cases, up to 20 days from when symptoms start 26.
Asymptomatic and Presymptomatic Transmission
Studies indicate that people can spread the virus even before they show symptoms, with the presymptomatic infectious period ranging from less than 1 day to about 4 days. Asymptomatic individuals (those who never develop symptoms) have a median infectious period of about 6.5 to 9.5 days 63.
PCR Testing, Viral Load, and Infectivity
PCR tests can detect viral RNA for weeks or even months after recovery, but this does not mean the person is still contagious. Most people stop shedding live, infectious virus within 10 days of symptom onset if their illness is mild or moderate. In severe cases, this period can extend, but rarely beyond 20 days. Studies using viral cultures show that the likelihood of contagiousness drops significantly as the PCR cycle threshold (Ct) value increases: at Ct values above 30, only about 20% of samples are still infectious, and above 35, less than 3% are infectious 254.
Special Cases: Immunocompromised and Treated Patients
Some patients, especially those who are severely ill or immunocompromised, may remain contagious for longer. In a case series, some patients treated with immunosuppressive drugs like tocilizumab and dexamethasone remained potentially infectious for more than 21 days, as indicated by positive viral cultures or low Ct values .
Environmental Persistence and Indirect Transmission
While the virus can survive on surfaces for days to weeks depending on the material and environment, the main risk of transmission is through respiratory droplets. The risk of catching the virus from surfaces is much lower than from direct contact with an infectious person, especially after the first 10–15 days of their illness 103.
Conclusion
Most people with COVID-19 are contagious for about 10 days after symptoms begin, with a longer period possible for those with severe illness or weakened immune systems. PCR tests may remain positive long after a person is no longer infectious. The highest risk of spreading the virus is in the days just before and after symptoms start, highlighting the importance of early isolation and preventive measures.
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Most relevant research papers on this topic
High Contagiousness and Rapid Spread of Severe Acute Respiratory Syndrome Coronavirus 2
The early spread of SARS coronavirus 2 in Wuhan, China, suggests that active surveillance, contact tracing, quarantine, and early social distancing efforts are needed to stop transmission.
Duration of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Infectivity: When Is It Safe to Discontinue Isolation?
SARS-CoV-2 infectivity rapidly decreases to near-zero after 10 days in mild-moderately ill patients and 15 days in severely-critically ill and immunocompromised patients, with the longest interval associated with replication-competent virus being 20 days from symptom onset.
DOI
Prevention of nosocomial COVID-19: Another challenge of the pandemic
Nosocomial COVID-19 prevention requires a multifaceted approach, involving respiratory droplets, respiratory contact, and hand-washing, with a focus on reducing respiratory contact and promoting hand-washing.
Correlation between 3790 qPCR positives samples and positive cell cultures including 1941 SARS-CoV-2 isolates
At Ct = 35, 3% of SARS-CoV-2 cultures are positive, suggesting a Ct value of 35 as a cutoff for determining contagiousness and eviction duration in COVID-19 patients.
DOI