SARS-CoV-2 transmission most likely to occur 2 days before to 3 days after symptom onset

Having a comprehensive understanding of the transmission patterns of SARS-CoV-2 is necessary for the successful implementation of these public health measures.

For example, obtaining information about when individuals with SARS-CoV-2 are most likely to spread the virus may guide decisions about the duration of contact tracing and the isolation period.

Transmission of the virus can occur before symptom onset. This is known as presymptomatic transmission. Similarly, people who do not show any symptoms can also transmit the virus. This is known as asymptomatic transmission.

A recently published cohort study reports the transmission patterns of the original SARS-CoV-2 strain in Zhejiang province in China between January and August 2020.

People with SARS-CoV-2 were most infectious between 2 days before and 3 days after symptom onset.
Individuals without symptoms were less likely to transmit the virus than those with mild or moderate illness.
Contacts who acquired a SARS-CoV-2 infection from an asymptomatic individual had a lower likelihood of developing symptoms.

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Key points

Question Is there an association between the timing of exposure to and severity of COVID-19 disease in close contacts of index patients with COVID-19?

Findings In this cohort study of 730 index patients with a COVID-19 diagnosis and 8852 close contacts, transmission potential was greatest in the first 2 days before and 3 days after onset of symptoms in the index patient.

When contacts received a diagnosis of COVID-19 infection, they were more likely to present asymptomatically if they had been exposed to an asymptomatic patient.

Meaning These results suggest that the quantity of exposure to a patient with COVID-19 may be associated with clinical presentation among close contacts that develop COVID-19.

Conclusions and Relevance This cohort study found that individuals with COVID-19 were most infectious a few days before and after symptom onset. Infected contacts of asymptomatic index patients were less likely to present with COVID-19 symptoms, suggesting that quantity of exposure may be associated with clinical presentation in close contacts.

Increased transmissibility of variants

This study analyzed data collected between January and August 2020, when the ancestral wild type strain of SARS-CoV-2 was circulating. However, more transmissible variants — such as Alpha and Delta — have subsequently emerged that might have different transmission patterns.

A recent study, which has not yet undergone peer review, suggests a higher risk of presymptomatic transmission due to the Delta variant than the wild type SARS-CoV-2 strain.