How soon after exposure are you contagious
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Contagiousness of SARS-CoV-2: How Soon After Exposure?
Early Transmission of SARS-CoV-2
Asymptomatic Transmission Within Days
Research indicates that individuals infected with SARS-CoV-2 can become contagious very quickly, even before showing any symptoms. A study analyzing a cluster of COVID-19 cases in Osaka, Japan, found that asymptomatically infected persons could transmit the virus as soon as 2 days after infection1. This rapid onset of contagiousness underscores the challenge of controlling the spread of the virus, especially in environments where people are in close contact.
Incubation Period and Symptom Onset
The incubation period of COVID-19, which is the time from exposure to the onset of symptoms, has been estimated to have a median of 5.1 days, with 97.5% of symptomatic cases developing symptoms within 11.5 days2. This period is crucial for understanding when an individual might start to be contagious. However, the contagious period can begin even before symptoms appear, making it difficult to identify and isolate infectious individuals promptly.
Duration of Infectiousness
Infectiousness in Mild to Moderate Cases
For individuals with mild to moderate COVID-19, the period of infectiousness typically does not extend beyond 10 days after symptom onset3. This finding is based on multiple studies that have cultured the virus and found that it is rarely isolated beyond this timeframe. However, this does not account for the pre-symptomatic phase, during which individuals can still spread the virus.
Prolonged Infectiousness in Severe Cases
In contrast, patients with severe or critical illness, as well as those who are immunocompromised, may shed the virus for a longer period. In some cases, the virus has been isolated up to 32 days after symptom onset3. This extended period of infectiousness in severe cases highlights the need for prolonged isolation and monitoring in these patients.
Implications for Public Health Measures
High Transmissibility Before Symptom Onset
The high transmissibility of SARS-CoV-2 before and immediately after symptom onset suggests that relying solely on isolating symptomatic patients is insufficient to control the spread of the virus. A study from Taiwan found that the secondary clinical attack rate was higher among contacts exposed within 5 days of symptom onset compared to those exposed later5. This indicates that more generalized measures, such as social distancing and widespread testing, are necessary to effectively interrupt transmission.
Importance of Early Detection and Isolation
Given the rapid onset of contagiousness, early detection and isolation of infected individuals, including those who are asymptomatic, are critical. Continuous efforts to avoid crowding, maintain personal hygiene, and implement social distancing are essential strategies for controlling the spread of COVID-191.
Conclusion
In summary, individuals infected with SARS-CoV-2 can become contagious as soon as 2 days after exposure, often before showing any symptoms. The median incubation period is around 5 days, but the infectious period can extend beyond 10 days in severe cases. These findings highlight the importance of early detection, isolation, and comprehensive public health measures to control the spread of the virus.
Sources and full results
Most relevant research papers on this topic
Cluster of SARS-CoV-2 infections linked to music clubs in Osaka, Japan: asymptomatically infected persons can transmit the virus as soon as 2 days after infection.
Asymptomatically infected individuals can transmit SARS-CoV-2 as soon as 2 days after infection, highlighting the need for continuous efforts to avoid crowding and maintain personal hygiene for effective COVID-19 control.
Observational StudyThe Incubation Period of Coronavirus Disease 2019 (COVID-19) From Publicly Reported Confirmed Cases: Estimation and Application
The median incubation period for COVID-19 is 5.1 days, similar to SARS, supporting current quarantine or active monitoring recommendations for potential exposed individuals.
Meta-Analysis
Rigorous Journal
Highly CitedThe duration of infectiousness of individuals infected with SARS-CoV-2
COVID-19 patients with mild-to-moderate illness are highly unlikely to be infectious beyond 10 days of symptoms, but those with severe-to-critical illness or immunocompromised may shed infectious virus for longer.
Systematic ReviewHighly CitedA generalized model of social and biological contagion.
Our model unifies existing contagion models, revealing three basic classes of contagion models based on memory length and infection probabilities, with implications for prevention and facilitation strategies.
Highly CitedContact Tracing Assessment of COVID-19 Transmission Dynamics in Taiwan and Risk at Different Exposure Periods Before and After Symptom Onset
COVID-19 is highly transmissible to close contacts, with higher attack rates in those exposed within 5 days of symptom onset.
Observational StudyRigorous JournalHighly CitedEmpirical evidence on the efficiency of backward contact tracing in COVID-19
Backward contact tracing in COVID-19 can identify more cases and reduce testing and quarantine time, but may identify infected contacts later in their infectious cycle.
Observational StudyDuration of 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.
Literature ReviewHighly CitedDetection of Measles Virus RNA in Air and Surface Specimens in a Hospital Setting.
Measles virus RNA was detectable in air, surface, and respirators in a hospital setting on days 5-8 after rash onset, suggesting that MeV-infected fomites may be present in healthcare settings.
Case ReportQuantitative assessment of the risk of airborne transmission of SARS-CoV-2 infection: Prospective and retrospective applications
This study presents a novel approach for quantifying airborne SARS-CoV-2 infection risk, highlighting that exposure times shorter than 20 minutes are necessary for an acceptable risk of 10-3 for exposed subjects in naturally ventilated indoor environments.
Rigorous JournalHighly Cited