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- A recently published cohort study reports the modes of transmission of the original strain of SARS-CoV-2 in Zhejiang province in China between January and August 2020.
- People with SARS-CoV-2 were most contagious between 2 days before and 3 days after symptom onset.
- People without symptoms were less likely to transmit the virus than those with mild or moderate illness.
- Contacts who acquired SARS-CoV-2 infection from an asymptomatic individual had a lower likelihood of developing symptoms.
As countries attempt to immunize a significant portion of their population, non-pharmaceutical interventions such as physical distancing and isolation after exposure to the virus remain essential to limit the transmission of SARS-CoV-2.
Having a comprehensive understanding of the modes of transmission of SARS-CoV-2 is necessary for the successful implementation of these public health measures.
For example, getting information about when people with SARS-CoV-2 are most likely to spread the virus can guide decisions about the length of contact tracing and the period of isolation.
Transmission of the virus can occur before symptoms appear. This is called presymptomatic transmission. Likewise, people who have no symptoms can also transmit the virus. This is called asymptomatic transmission.
Studies carried out at the start of the pandemic showed that presymptomatic and asymptomatic people could transmit the virus and make informed decisions about physical distancing and wearing face masks.
However, the extent of presymptomatic and asymptomatic transmission during the pandemic has been difficult to determine.
This lack of clarity is due, in part, to the monitoring of individuals for a limited period of time after their first positive test for COVID-19. As a result, many people who were initially asymptomatic later developed symptoms. This made it difficult to differentiate the roles of presymptomatic and asymptomatic transmission.
A recently published study examined patterns of transmission 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 symptoms appeared.
He also found that coming into contact with an asymptomatic individual was more likely to result in asymptomatic infection in exposed individuals.
Study co-author Dr Leonardo Martinez, epidemiologist at Boston University, said Medical News Today, “These data suggest that avoiding high exposure events may be important not only for preventing continued transmission, but also for disease severity.”
This study provides “further evidence that the original SARS-CoV-2 spreads primarily several days before and after symptom onset, and [it] stresses the need for universal control measures and early interventions to stop transmission, ”said Dr Eric Lau, infectious disease epidemiologist at the University of Hong Kong. Dr Lau was not involved in the study.
New research appears in the journal
To understand the transmission pattern of the wild-type strain of SARS-CoV-2, 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 first identified individuals, or indexes, mentioned above.
Index individuals and their close contacts underwent regular testing and daily screening for COVID-19 symptoms during times of isolation or quarantine.
Researchers monitored index individuals and close contacts with SARS-CoV-2 for presentation of any symptoms for at least 90 days after their initial positive test. This allowed them to determine whether people who had no symptoms at the time of their initial test remained asymptomatic or developed symptoms later.
To assess the risk of transmission, the researchers calculated the attack rate. This is the percentage of all exposed contacts who contracted SARS-CoV-2.
The researchers found that index individuals were more contagious between 2 days before and 3 days after the onset of symptoms.
In addition, the risk of transmission to close contacts was highest on the day of symptom onset.
“[These results show that] timing of exposure versus symptoms is an important transmission factor and provides further evidence that early detection and social isolation after symptom onset is key to controlling the epidemic, ”observed Dr Martinez. .
Researchers also assessed the influence of contact patterns and symptom severity on the transmission of SARS-CoV-2.
Household members and people with multiple contacts with the index individual had a higher probability of acquiring the infection than those who were exposed to the index individual during shared transport or in a shared confined space at home. outside the house.
Exposure to two index individuals also increased the risk of transmission compared to exposure to a single index individual.
Researchers found that the severity of COVID-19 symptoms was related to the risk of transmission, with asymptomatic individuals being less likely to transmit the virus than those with mild or moderate illness.
In addition, asymptomatic people were more likely to cause asymptomatic infection in contacts than those with mild or moderate illness.
Dr Martinez said these results suggest that “the level of exposure to a [person] with COVID-19 may be associated with clinical presentation in close contacts who develop COVID-19. “
“Thus, these results suggest that it is important to avoid exposure to someone with COVID-19; if you are exposed to someone with COVID-19, the intensity of the exposure is also an important factor. This study further highlights the need for vaccination, which reduces clinical severity in people [who] develop COVID-19, ”added Dr. Martinez.
“The main strengths of our study include the large sample size (8,852 people exposed to the 730 [individuals] with a diagnosis of COVID-19), the complete clinical history of index cases and the detailed schedule of exposure events, ”said Dr. Martinez.
Dr Lau added: “The study performed a detailed analysis of contact tracing data and carefully followed the [individuals] to identify truly asymptomatic COVID-19 cases. “
Dr Martinez acknowledged that the study had some limitations. He said: “A significant limitation is that although we have done large-scale screening and testing, it is possible that some exposed people who developed COVID-19 have been missed, including asymptomatic cases.”
Additionally, the researchers conducted the study in Zhejiang Province, when transmission of the virus was limited due to widespread implementation of public health measures. The authors add that the modes of transmission could vary depending on the health interventions implemented by the 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 – subsequently emerged that could have different transmission patterns.
Recognizing this limitation, Dr Martinez noted, “Our study was done before widespread vaccination and newer variants, like Delta. How the newer variants change this drivetrain dynamics – if at all – is unclear. Further studies are needed to replicate these findings in the context of the Delta variant and in people [who] have been vaccinated.
A recent study, which has not yet been peer reviewed, suggests a higher risk of presymptomatic transmission due to the Delta variant than the wild-type strain SARS-CoV-2.
In addition, infection with the Delta variant resulted in a high viral load 4 days before the onset of symptoms. Higher viral loads are associated with an increased risk of transmission, suggesting that infection with the Delta variant can make individuals contagious much sooner than they could with the wild-type strain.
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