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New research aims to establish when SARS-CoV-2 is most likely to transmit to others. Mario Tama/Getty Images
  • 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.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.

Vaccination against COVID-19 is the most effective tool available for preventing severe illness and protecting lives.

While countries attempt to vaccinate a substantial share of their populations, nonpharmaceutical interventions such as physical distancing and isolating after exposure to the virus remain critical to limit SARS-CoV-2 transmission.

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.

Studies conducted during the early part of the pandemic showed that presymptomatic and asymptomatic individuals could transmit the virus and have informed decisions about physical distancing and wearing face masks.

However, the extent of presymptomatic and asymptomatic transmission during the pandemic has been hard to pinpoint.

This lack of clarity is due, in part, to the follow-up of individuals for a limited time after their initial positive COVID-19 test. As a result, many initially asymptomatic individuals later developed symptoms. This has made it difficult to differentiate the roles of presymptomatic and asymptomatic transmission.

A recently published study investigated the transmission patterns in the population of Zhejiang province in China between January and August 2020.

The study reported that people with SARS-CoV-2 were more likely to transmit the virus a few days before and after symptom onset.

It also found that coming into contact with an asymptomatic individual was more likely to result in an asymptomatic infection in exposed individuals.

Study co-author Dr. Leonardo Martinez, an epidemiologist at Boston University, told Medical News Today, “This data suggests that avoiding high exposure events may be important not only for preventing ongoing transmission but also for the severity of the disease.”

This study provides “further evidence that the original SARS-CoV-2 mostly spreads several days before and after symptom onset, and [it] highlights the need of universal control measures and early interventions to stop transmission,” said Dr. Eric Lau, an infectious disease epidemiologist at the University of Hong Kong. Dr. Lau was not involved in the study.

The new research appears in the journal JAMA Internal Medicine.

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To understand the transmission pattern of the wild type strain of SARS-CoV-2, the researchers identified 730 people with SARS-CoV-2 between January and August 2020 in Zhejiang province.

They then used contact tracing to identify a total of 8,852 people who had come into close contact with the aforementioned first identified, or index, individuals.

The index individuals and their close contacts underwent regular testing and daily screening for COVID-19 symptoms during the isolation or quarantine periods.

The researchers monitored the index individuals and close contacts with SARS-CoV-2 for the presentation of any symptoms for a minimum of 90 days after their initial positive test. This allowed them to ascertain whether the people who showed no symptoms at the time of their initial test remained asymptomatic or developed symptoms later.

To evaluate the risk of transmission, the researchers calculated the attack rate. This is the percentage of all exposed contacts who acquired SARS-CoV-2.

The researchers found that index individuals were most infectious between 2 days before and 3 days after symptom onset.

Furthermore, the risk of transmission to close contacts was highest on the day of symptom onset.

“[These results show that] the timing of exposure relative to symptoms is an important transmission factor and provides further evidence that rapid testing and social isolation after symptom onset is critical to controlling the epidemic,” observed Dr. Martinez.

The researchers also assessed the influence of patterns of contact and the severity of symptoms on SARS-CoV-2 transmission.

Household members and individuals with multiple contacts with the index individual had a higher likelihood of acquiring the infection than those with exposure to the index individual on shared transportation or in a shared enclosed space outside the home.

Having exposure to two index individuals also increased transmission risk compared with having exposure to a single index individual.

The researchers found that the severity of COVID-19 symptoms was linked to transmission risk, with asymptomatic individuals being less likely to transmit the virus than those with mild or moderate illness.

Additionally, asymptomatic individuals were more likely to cause asymptomatic infection in contacts than those with mild or moderate illness.

Dr. Martinez said that these results suggest that “the level of exposure to a [person] with COVID-19 may be associated with clinical presentation among close contacts who develop COVID-19.”

“So, these results suggest that it’s important to avoid exposure to someone with COVID-19; if you are exposed to a person with COVID-19, the intensity of exposure is also an important factor. This study further emphasizes the need for vaccination, which reduces clinical severity among people [who] develop COVID-19,” added Dr. Martinez.

“Major strengths of our study include the large sample size (8,852 persons exposed to 730 index [individuals] with a COVID-19 diagnosis), the comprehensive clinical history of index cases, and the detailed timing of exposure events,” said Dr. Martinez.

Dr. Lau added, “The study performed a detailed analysis of contact tracing data and carefully followed up the [individuals] to identify truly asymptomatic COVID-19 cases.”

Dr. Martinez acknowledged that the study had a few limitations. He said, “An important limitation is that, although we performed widespread screening and testing, it’s possible that some exposed persons who developed COVID-19 were missed, including asymptomatic cases.”

Furthermore, the researchers conducted the study in Zhejiang province, when the transmission of the virus was limited due to the widespread implementation of public health measures. The authors add that the transmission patterns could vary depending on the health interventions implemented by local authorities.

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.

Acknowledging this limitation, Dr. Martinez noted: “Our study was performed prior to widespread vaccination and novel variants, like Delta. How novel variants change these transmission dynamics — if at all — is unclear. New studies are needed to replicate these results in the context of the Delta variant and among people [who] have been vaccinated.”

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.

Furthermore, infection with the Delta variant resulted in a high viral load 4 days before symptom onset. Higher viral loads are associated with increased transmission risk, suggesting that infection with the Delta variant may result in individuals becoming contagious much sooner than they might with the wild type strain.

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