Duration of covid-19 contagiousness
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COVID-19 Contagiousness Duration: Key Findings
Typical Duration of Infectiousness in COVID-19
Research shows that the duration of contagiousness for COVID-19 varies depending on the severity of the case and the methods used to measure it. For symptomatic cases, the average period during which patients can test positive for the virus is about 20 days from the onset of symptoms, with some patients—especially those aged 65 and older—remaining contagious for even longer (median 22 days vs. 19 days for younger patients) . In asymptomatic cases, the median infectious period is estimated to be between 6.5 and 9.5 days . The presymptomatic infectious period, when individuals can spread the virus before showing symptoms, typically lasts less than 1 to 4 days .
Viral Shedding and PCR Positivity
Viral shedding, as detected by RT-PCR tests, can persist for several weeks. In mild or asymptomatic cases, the median duration of viral shedding is around 19 days, but it can range from 6 to 37 days . However, a positive PCR test does not always mean a person is still contagious, as the test can detect non-infectious viral fragments Byrne2020Miyamae2020. Some patients may test positive again after initially testing negative, indicating intermittent viral shedding Xiao2020Miyamae2020.
Infectious Virus Shedding and Culture Studies
Studies using viral cultures, which better indicate actual contagiousness, show that the median duration of shedding infectious virus is about 8 days after symptom onset, with the likelihood of contagiousness dropping below 5% after about 15 days . Most mild cases are no longer contagious by day 10, as 90% of such patients have negative PCR tests by then . However, in severe cases or those receiving immunosuppressive treatments like tocilizumab and dexamethasone, isolation may need to be extended beyond 21 days if viral cultures remain positive or PCR cycle threshold (Ct) values are low (Ct <30) .
Cycle Threshold (Ct) Values and Contagiousness
Ct values from PCR tests can help estimate contagiousness. A Ct value above 30 is generally associated with a low risk of transmitting the virus, as only about 20% of samples with Ct = 30 are positive in viral culture, and this drops to less than 3% at Ct = 35 . Some experts suggest that patients with Ct values above 25–30 are unlikely to be contagious .
Special Considerations: Asymptomatic and Mild Cases
Asymptomatic and mild cases can still shed virus and contribute to transmission, especially early in the infection Byrne2020Wang2020. The duration of viral shedding in these cases is similar to or slightly shorter than in symptomatic cases, but the period of actual contagiousness may be shorter Byrne2020Miyamae2020. The incubation period for COVID-19 is about 5 days on average, with most people developing symptoms within 11.5 days of exposure .
Conclusion
The duration of COVID-19 contagiousness typically ranges from about 8 to 20 days after symptom onset, depending on the severity of illness and the method used to assess infectiousness. Most people are no longer contagious after 10–15 days, but some, especially those with severe illness or weakened immune systems, may remain infectious for longer. PCR positivity can persist beyond the period of contagiousness, so clinical context and, when available, viral culture or Ct values should guide decisions about isolation and return to normal activities Byrne2020Xiao2020Van Praet2020+4 MORE.
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Most relevant research papers on this topic
Dynamic profile of RT-PCR findings from 301 COVID-19 patients in Wuhan, China: A descriptive study
The average contagious period of COVID-19 patients is 20 days, with patients 65 years old needing longer observation and more than 2 series of negative viral tests.
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.
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