What is the infectious window for COVID-19 (Coronavirus Disease 2019)?

Infectious Window for COVID-19

The infectious window for COVID-19 typically begins 1-2 days before symptom onset and extends for a median of 5 days after symptom onset in mild-to-moderate cases, though two-thirds of patients remain infectious at day 5 and nearly one-quarter remain infectious at day 7 post-symptom onset. 1, 2

Pre-Symptomatic Transmission Period

• Viral shedding begins 1-2 days before symptoms appear, making pre-symptomatic transmission a critical feature of COVID-19 spread 1
• However, only 20% of cases shed infectious (cultivable) virus pre-symptomatically, despite 63% having PCR-detectable viral RNA before symptom onset 2
• This pre-symptomatic transmission capability distinguishes COVID-19 and contributes to its rapid pandemic spread 1

Peak Infectiousness Timing

• Viral titers peak in the early phase of infection, typically within the first 7 days of illness 1
• Symptom onset occurs a median of 3 days before both peak viral RNA load and peak infectious viral load 2
• Live virus remains detectable in the respiratory tract for up to 9 days in most individuals 1

Duration of Infectiousness by Disease Severity

Mild-to-Moderate Cases

• Median duration of infectious viral shedding is 5 days (IQR 3-7 days) 2
• Viral shedding continues for 1-2 weeks in mild-moderate cases 1
• The CDC suggests contagiousness typically lasts an average of 10 days from symptom onset 3

Severe and Critical Cases

• Viral shedding extends beyond 2 weeks in severe cases 1
• For immunocompromised patients or those with severe/critical COVID-19, contagiousness can extend up to 15 days 3
• Viral viability has been documented up to 20 days from symptom onset in some cases 3

Critical Isolation Considerations

• Under a 5-day isolation period from symptom onset, 65% of cases would still be infectious when released into the community 2
• At day 7 post-symptom onset, 24% of cases continue to shed infectious virus 2
• Prolonged shedding of viral RNA has been observed in immunocompromised patients, indicating quarantine should extend to at least 20 or more days following symptom onset 1

Incubation Period Context

• The incubation period ranges from 1 to 14 days, with a median of 5.1 days 1, 4
• Most cases (97.5%) develop symptoms within 11.5 days of infection 4
• The typical incubation period is 3-7 days 1
• Symptoms usually appear between 2 and 14 days after exposure 1

Common Pitfalls and Caveats

• PCR tests may remain positive for extended periods (up to 30 days) even after a person is no longer infectious, due to detection of non-viable viral fragments 3
• The presence of ongoing symptoms along with a positive test suggests active infection rather than just residual viral RNA 3
• Approximately 30-60% of patients shedding virus may have no symptoms, making asymptomatic transmission a significant concern 1
• Lateral flow device (LFD) correlation with infectious viral shedding is poor during the viral growth phase (sensitivity 67%) but high during the decline phase (92%), supporting their use for safe deisolation but not for early diagnosis unless used daily 2

Practical Isolation Recommendations

• For symptomatic patients, isolation should continue until at least 3 days have passed since recovery (defined as resolution of fever without medications and clinically meaningful improvement in respiratory symptoms) 3
• Individuals with persistent symptoms beyond day 7 and positive testing should continue isolation, as this combination indicates active infection and potential infectiousness 3
• Immunocompromised patients require extended isolation of at least 20 days following symptom onset 1