How long is a Covid-19 (Coronavirus disease 2019) infected person contagious?

Duration of COVID-19 Contagiousness

A COVID-19 infected person is contagious for an average of 10 days from symptom onset in mild-to-moderate cases, but two-thirds remain infectious at day 5 and nearly one-quarter at day 7, with extended periods up to 15-20 days in severe or immunocompromised patients. 1

Timing of Peak Infectiousness

• Infectiousness begins 1-2 days BEFORE symptom onset, making pre-symptomatic transmission a critical feature distinguishing COVID-19 from many other respiratory infections 1, 2
• Peak viral shedding occurs within the first 7 days of illness, with the highest contagiousness in the 2 days before and 5 days after symptom onset 2, 1
• Live virus remains detectable in the respiratory tract for up to 9 days in most individuals 1

Duration by Disease Severity

Mild-to-Moderate Disease

• Self-isolation should be 10 days from symptom onset for patients with mild-to-moderate disease 2
• The median duration of infectious viral shedding is 5 days (range 3-7 days) from symptom onset 3
• However, 65% of cases continue shedding infectious virus at day 5, and 24% remain infectious at day 7 post-symptom onset 3
• Replication-competent virus is rarely cultured beyond 10 days after symptom onset 2, 4

Severe/Critical Disease

• Self-isolation should extend to 15-20 days for severely ill patients 2
• The risk of replication-competent virus is approximately 5% at 15 days after symptom onset and extremely rare at 20 days 2
• Viral shedding extends beyond 2 weeks in severe cases 1
• The longest documented interval with replication-competent virus is 20 days from symptom onset 4

Immunocompromised Patients

• Isolation should extend to at least 20 days or longer following symptom onset in severely immunocompromised patients 2, 1
• Infectivity may continue for extended periods in this population 2
• Specialist advice on duration of self-isolation may be needed 2

Critical Pitfalls to Avoid

• Do NOT use PCR testing to determine end of isolation - PCR positivity does not correlate with secretion of live virus and can remain positive for weeks to months after infectiousness has ended 2
• Viral RNA remains detectable in half of patients 21-30 days after symptom onset, but this does not indicate infectiousness 5
• Do NOT rely on symptom resolution alone - patients may remain infectious despite symptom improvement 3

Practical Isolation Guidance

• For mild-moderate symptomatic patients: Isolate for 10 days from symptom onset PLUS at least 24 hours fever-free without antipyretics PLUS improvement in other symptoms 2
• For severe cases: Extend isolation to 15-20 days from symptom onset 2
• For immunocompromised patients: Minimum 20 days isolation, potentially longer with specialist consultation 2, 1
• For asymptomatic cases who test positive: 10 days from positive test date 2

Asymptomatic Transmission Considerations

• Approximately 30-60% of patients shedding virus may have no symptoms 1
• Only 20% of cases shed infectious virus pre-symptomatically, though 63% have PCR-detectable virus before symptom onset 3
• This explains why symptom-based screening alone is insufficient for infection control 1

Role of Antigen Testing

• Nucleocapsid (N) antigen testing strongly correlates with culture positivity (relative risk 7.61) and is a better predictor of infectiousness than viral RNA or symptoms 5
• Lateral flow devices show poor sensitivity (67%) during viral growth phase but high sensitivity (92%) during decline phase 3
• N antigen may be more suitable than PCR or symptom assessment for determining safe deisolation within two weeks of symptom onset 5